The Complete Guide to Residential HVAC

Oct 27, 2026

As of 2026-10-28. Facts come from the research files and fetched pages in the source ledger. Anything unconfirmed reads "typical value, check the manufacturer manual or code". What changed: the guide now has drawn exhibits, such as the refrigerant loop, the split-system map, the furnace ignition sequence, thermostat wiring, no-cool and no-heat decision trees, the Manual J to S to D workflow, a pressure chart, the system choice flow, the duct loop, the A2L leak response and the startup order. It also has a Day 1 to master course, a safety table, one numbered installation list, a glossary and a table of what a master checks first. Every section links to the exact 3D scene it pairs with. Twelve new sections cover heat pumps, mini-splits, thermostats, ventilation, water heaters, venting and gas leaks, ductwork, A2L leak sensors, coastal heat, customers and warranty, rebates and careers. Corrections: the CO alarm statute is Health and Safety Code 17926, 91 FR 31284 supersedes the R-410A install cutoff, and the $627 furnace permit is a City of Santa Barbara figure, not a Montecito one. A 2026-10-28 audit fixed the rule-of-thumb overshoot (82% to 118% over the load), the 0.68 constant (1060 BTU/lb), the R-454B and R-22 pressure comparisons and the leak-repair exemption for residential equipment. It also linked the status chips to their sources. The 2026-10-28 local pass adds the section Montecito and Santa Barbara: your local rules (agency by address, County permits and fees, design review, Coastal Zone, noise, fire, flood, climate zone, utilities and open items), labels every City-only figure, and applies the local rules in the Installation, Diagnostics, Tools, Code, Business and Coastal sections.

Start here

Residential HVAC moves heat: out of a house in summer, into it in winter, and keeps the air clean and dry enough to live in. Every job comes down to three things, airflow, refrigerant and combustion or electricity, and a master checks them in that order.

How a master thinks. Measure before touching. Airflow problems cause most "refrigerant" complaints, so check filter, coil and static pressure first. Change one thing at a time, and write down the reading before and after.

The trade map.

How to use the package. Read a section here, then open the matching scene in HVAC Math in 3D and move the sliders until the numbers make sense. The tools and installation sections pair with the HVAC Tools in 3D page, and the math section pairs with HVAC Math in 3D.

How much to trust the numbers. Every fact in this guide was checked against a fetched web page where possible. The source ledger at the end of the guide lists what is confirmed and what is not. Some values differ by manufacturer: nitrogen test pressure and hold time, evacuation decay limit and add-on charge. For those, the guide shows a manufacturer table and sends you to the installation manual. Anything marked UNCONFIRMED is a typical value, not a rule.

Montecito and Santa Barbara: your local rules

Montecito is unincorporated, so the County of Santa Barbara, not the City, issues the mechanical permit, reviews how outdoor equipment looks and sets most of the local rules below. As of 2026-10-28. Anything marked Unconfirmed must be checked with the agency before you quote. Figures from the City of Santa Barbara are labelled CITY OF SANTA BARBARA ONLY wherever they appear in this guide, and they are not Montecito rules.

Which agency, by address

A ZIP code or mailing city does not decide the agency. No page read maps a ZIP code to Montecito, so confirm the jurisdiction, the fire district and the Coastal Zone by parcel. The Montecito Fire district map and Summerland's own zoning rules were not read.

Pre-job local checklist

• Confirm by parcel: County or City, and whether the parcel is inside the Montecito Fire Protection District.
• Look up the climate zone for the address in the CEC lookup. Zone 5 or 6 is expected, and it decides charge verification.
• Ask County Planning whether the parcel is in the Coastal Zone.
• Check the parcel against the flood map and the FEMA debris-flow Recovery Map before you pick a pad height.
• Open the County permit before you change a unit. A component swap is the only mechanical work the County Code exempts.
• Before you move a condenser, add a rooftop unit or run a visible line set, get a written Director determination from County Planning.
• Set the condenser on non-combustible ground. Keep wood pads, plastic covers, combustible screens and mulch out of the first 5 ft from the house.
• Place and buffer the condenser away from the neighbor's property line and bedroom windows.
• For a gas furnace, confirm the model is certified for the Santa Barbara County air district (Rule 352).
• For a heat pump, confirm the customer is on a 3CE account, keep proof of permit and quote the rebate as "may qualify".
• Quote County permit fees as the closest lines on the County schedule. Never use the City's $627 furnace figure.

Decision flow: which agency, which permit path

Read down the County column. Each yes sends you to a step you finish before the work, and the answers stack. The City box holds the figures that must not be reused in Montecito.

Permits and fees (Montecito, County of Santa Barbara)

The County permit is the first step in Montecito. The County fee schedule has no line named for an air conditioner, heat pump or change-out, so quote the closest lines and confirm with the County.

The County Code pages are read from the gocodebook.com republisher, not the County's own site, so they are secondary text.

Design review for outdoor equipment (Montecito)

HVAC is not on any exemption list that was read, so a condenser move, a rooftop unit or a visible line set may need design review. Get the answer in writing from County Planning before you order equipment.

Coastal Zone

Do not promise that a like-for-like swap is exempt. The Montecito parcel boundary is not confirmed, and no HVAC exemption wording was found.

Noise

No Montecito equipment noise limit was found. The County nighttime noise chapter is about amplified sound, so how it applies to a condenser is uncertain. Place and buffer the unit as if a limit applied.

Fire and wildfire zones (Montecito Fire Protection District)

Montecito Fire has its own Development Standards, separate from County Fire. Do not use County Fire's Standard 6 or the City's vent sheet on a Montecito job.

Flood, debris flow and water

Climate zone and Title 24 consequences

Montecito's coast and valley are in climate zone 6 and the upper foothills are in zone 5, and both are outside the zones where an air conditioner needs charge verification. A heat pump needs it in every zone. Confirm the zone by parcel.

Gas furnaces and the air district

Utilities and incentives for Montecito

Quote any rebate as "may qualify" and point to the live page, because the proof of permit is the County permit and funds run out.

Open items: confirm before you quote

None of these was confirmed. Tick each one when you have the answer from the agency.

• Climate zone of the parcel in the CEC lookup (the ZIP table was not read).
• Whether the parcel is in the Coastal Zone, and whether the Appendix C repair and maintenance guidance covers an HVAC swap.
• The County application checklist, submittal items, CF1R and HERS handling, and any over-the-counter path for a change-out.
• Which County fee line applies to an air conditioner, heat pump or change-out, and whether an energy review fee exists.
• Whether a condenser, rooftop unit or line set needs Board of Architectural Review, or only a Director determination.
• Whether a ground condenser may sit in a side setback.
• Montecito vent mesh and ember rules, duct openings, and clearance around gas meters, gas lines and condensers.
• Whether County Code Chapter 40 reaches a condenser, plus any County daytime equipment limit and construction hours.
• Whether a like-for-like change-out is "development" under the County floodplain chapter, and what Flood Control reviews.
• 3CE dollar amounts for 2026-27, SoCalGas rebates, and SCE panel and service guidance.
• County Energy Code amendments under Ordinance 5270, the status of the County reach code, and the current version of Rule 352.
• Whether a County business license or a staging and right-of-way permit applies to contractors.
• Summerland's own zoning rules, and the City's design review of HVAC (not researched here).

Safety first

Five hazards injure HVAC technicians: electricity, fire from brazing, flammable refrigerant, carbon monoxide, and falls. Learn these before touching a tool.

In the 3D tools page, every scene opens with its safety notes first.

Scenes to open: In HVAC Tools in 3D, open Protect your eyes, hands, lungs and ears, Ladder angle and fall arrest on a roof, Braze a copper joint safely and Lock out, then prove it is dead: tester, meter and 24 V.

How the systems work

Every system moves heat with a fluid. A refrigerant loop moves heat through a coil. Air carries it through ducts. A furnace makes heat by burning gas, and a heat pump moves heat instead of making it. Learn the loop once and every system becomes a variation.

The refrigerant loop. Four parts, in order:

1. The compressor raises the refrigerant's pressure and temperature.
2. The condenser rejects heat to the outdoor air and turns the gas into liquid.
3. The metering device drops the pressure, which chills the liquid.
4. The evaporator absorbs heat from the indoor air and boils the liquid back into gas.

The gauge readings you take, superheat and subcooling, tell you how full the evaporator and condenser are. The 3D math page animates this loop.

Read the loop clockwise from the compressor. Pressure and heat go up there. Heat leaves at the condenser, and pressure drops at the metering device. The evaporator then boils the liquid by absorbing house heat.

Watch a call for heat run down the left column, and when it stops, the step where it stopped points to the right-hand cause to test first.

Which refrigerant. R-410A has been the standard for residential systems. New equipment is moving to the mildly flammable A2L refrigerants R-32 and R-454B. California's limit on global warming potential for new ducted systems is under 750 (see the code section). A2L equipment needs A2L-rated tools and different handling.

Why matching matters. Compressor, coil, metering device and furnace are designed as a set. A mismatched coil changes pressures, airflow and capacity, and the manufacturer's rating no longer applies.

The suction line carries cool vapor back to the compressor. The liquid line carries warm liquid out to the metering device. The thermostat only sends 24 V calls, and the disconnect feeds the outdoor unit's line voltage.

For a heat pump in the mild coastal climate of Montecito and Santa Barbara (zone 6 on the CEC layer, so charge verification applies; confirm the zone by parcel), the defrost and backup heat questions matter less than in a cold climate. The heating-versus-cooling load balance still decides the size, so size from the load calculation, never from the old unit.

What a master checks first, by system

The order is always airflow, then electrical, then refrigerant or combustion last. Only the first and second checks change from system to system.

Scenes to open: In HVAC Math in 3D, open Airflow per ton and the temperature split, Superheat: how far past boiling, Subcooling: how far past condensed and Furnace input, output and temperature rise. In HVAC Tools in 3D, open Read the gauges: pressure to saturation temperature, Static pressure: supply plus return is the TESP and Delta T: supply and return temperature.

Heat pumps in practice: defrost, backup heat and balance point

Heat pumps lose capacity as outdoor air gets colder, so defrost, backup heat and thermostat setup decide how winter feels.

Choosing the system. Two facts pick the system: whether the house has usable ducts, and how cold winters get. The Department of Energy (DOE) says cold-climate heat pumps are designed for outdoor temperatures as low as 5 F, and that in a dual-fuel setup a furnace takes over in colder weather (DOE). Status: confirmed.

Start at the ducts: no usable ducts points to a mini-split, and with ducts the winter decides between a standard heat pump and a cold-climate or dual-fuel setup.

The DOE pages show no publication date and were fetched on 2026-10-28.

Mini-splits, multi-zone systems and zoning

A mini-split needs no ducts, so the install work is the line set, the wall hole, the drain and the commissioning checks.

The Department of Energy (DOE) says a ductless install needs only a three-inch hole through the wall (DOE). It also reports that about 32% of homes in colder zones lack ductwork (DOE). The table below comes from one real manual, the Daikin R-32 split installation manual (footers dated 2026-06-12). Other brands differ, so read the manual for the unit you install.

Multi-zone systems. One outdoor unit can feed several indoor heads. The manufacturer limits the total connected capacity and the line length to each head, so use that brand's multi-zone table rather than one head's manual. Status: practice-based, typical value, check the manufacturer manual or code.

Zoning a ducted system. Zoning uses motorized dampers to send air only to the rooms that call. It is easy to do badly:

• Bypass ducts. One expert writes that bypass ducts "should never be used", and a second says they can work only when no other option is feasible. In a test he cites, configurations with the bypass closed were 22%, 27% and 32% more efficient than with it open (GreenBuildingAdvisor, 2014). Status: confirmed as quoted, but opinion and one test, not code.
• Better design. Size each zone's ducts so it can carry the system's minimum airflow, and prefer multi-stage or variable-speed equipment that can slow down. The same article names ACCA's Manual Zr as the zoning protocol.
• Dampers leak. Closed residential dampers block most air but not all, so an off zone can still over-heat or over-cool (JLC).
• Wiring first. The author of that article finds wiring at the root of most zone-damper problems, such as a backward damper or one never connected. Set zone-size percentages to the real loads, because the 50/50 default can overheat or shut the system down.

Thermostats and smart controls

A thermostat is a set of switches that send 24 V calls, so most smart-thermostat problems are wiring and setup, not software.

The terminal jobs (R, C, W, Y, G, O/B) are in the Installation section's wiring diagram. This table covers what changes when the thermostat is smart, programmable or on a heat pump.

Ventilation, filtration, humidity and wildfire smoke

Clean, moderately humid air takes filters, fresh-air ventilation and humidity control together, and each one has limits you should explain to the customer.

ERV sizing example. AHRI sizes an ERV for at least 0.35 air changes per hour: volume divided by 60, times 0.35. A 2,000 ft2 house with 8 ft ceilings holds 16,000 ft3. That gives 16,000 / 60 x 0.35 = 93.3 CFM. The ASHRAE 62.2 example in the math section gives 90 CFM for the same floor area, so the two methods agree closely.

Wildfire smoke. EPA's guidance, updated in August and September 2026, is short:

• Switch the system to recirculate, or close the outdoor-air intake, so it stops pulling smoke in (EPA clean room page).
• Fit a MERV 13 or higher filter if the system allows it, set the fan to On instead of Auto, and replace filters more often than the maker says.
• When the air clears, open the fresh-air intake and run the kitchen and bath fans (EPA).
• Clean ducts only as needed, such as when dust or particles come out of the registers. Wet-mop ash instead of dry sweeping.

Status: confirmed. The DOE and EPA pages were fetched on 2026-10-28.

Water heaters and hydronic heat

Water heaters and hot-water (hydronic) heating belong in an HVAC guide only where they share air, venting, ducts or water with the heating system.

Combustion venting categories and gas leak response

A furnace's venting category tells you how its flue gas moves and what pipe can carry it, and a gas leak means leave first and fix later.

Status for the table: confirmed (Building America resource guide, PNNL, undated page, which cites NFPA 54). Vent and pipe sizing is table-driven. Use NFPA 54, the International Fuel Gas Code or the manufacturer's tables, never a guess.

If you smell gas or find a leak. These steps come from a gas utility's public safety page (SoCalGas). They apply if SoCalGas is the utility at the address, so confirm the utility first.

1. Leave the area at once and take everyone with you.
2. From a safe place, call the gas utility (SoCalGas lists 1-800-427-2200). Call 911 if there is damage that may endanger life.
3. Do not smoke, strike a match, switch lights or appliances on or off, or use any device that could spark.
4. Do not try to control the leak or repair a damaged pipe or meter.

The same page lists the warning signs: the smell of gas, a hissing or whistling sound, dirt or water blowing into the air, a dry patch of grass, or a damaged appliance connection. It also warns that odor can fade, so never rely on your nose alone. Status: confirmed.

On your own job. After any gas work, check every joint with leak solution or a gas detector, and run the CO test from the safety section. If you find a leak you cannot stop right away, treat it as the steps above say: leave and call. Status: practice-based. Follow your employer's written procedure.

The math

HVAC math is a small set of ideas used again and again. Heat moves because of a temperature difference. Air and water carry heat in proportion to their flow. Electricity follows Ohm's law. Learn the ideas and the constants stop looking like magic.

You can play with most of these formulas in HVAC Math in 3D. Every worked example below was re-checked in code.

A note on trust. Items marked typical value, check the manufacturer manual or code are standard textbook or industry numbers that the research could not confirm at the primary source. Use them to learn and to sanity-check. For a real job, the nameplate, the manufacturer's data and the adopted code win.

What each constant means

Each constant is just units multiplied together. Knowing where it comes from tells you when it stops being true.

The 0.075 lb/ft3 is the density of standard air at sea level. High up, air is thinner, so 1.08 and 4.5 shrink. At 5,000 ft the density ratio is about 0.862, so the sensible constant is about 0.93. Furnaces also lose capacity at altitude. A common rule is about 4% per 1,000 ft above 2,000 ft, which gives roughly 88% at 5,000 ft (typical value, check the manufacturer manual or code).

Manual J: how big is the load?

Manual J is the ACCA method for calculating how much heating and cooling a house needs. It adds up heat leaking through the building, heat from air leakage, and heat from people and appliances. Equipment is chosen after that, never before.

Never size from the old unit or from square feet per ton. The load comes first, and the equipment is chosen to it. The ducts are then sized to the equipment's airflow, and readings on site tell you whether to loop back.

Conduction. Heat flows through a wall, ceiling or window in proportion to its area and the temperature difference. Insulation (R-value) slows it. U is simply 1 divided by R.

Example: 1,000 ft2 of R-19 ceiling has U = 0.0526. At a 70 F difference it loses 3,684 BTU/h. Change it to R-38 and the loss halves to 1,842 BTU/h. Doubling R halves the loss.

Design temperatures. Manual J does not size for the worst day on record. It uses the outdoor temperature that is exceeded only 1% of the time in summer, and 99% of the time in winter. These come from ASHRAE weather data for the nearest city. Typical indoor settings are 70 F heating and 75 F cooling. In the example city, a 99% heating temperature of 30 F gives a 40 F difference. A 1% cooling temperature of 95 F gives a 20 F difference. Manual J also adjusts the cooling number for the daily temperature swing. (Typical value, check the manufacturer manual or code.)

Air leakage. Leaks bring in outdoor air. Convert air changes per hour (ACH) to CFM, then to load with the sensible formula.

Example: a 1,600 ft2 house with 8 ft ceilings is 12,800 ft3. At 0.5 ACH that is 106.7 CFM, which costs about 8,064 BTU/h at a 70 F difference. A blower door test gives ACH at 50 pascals. Divide by about 15 to 20 to estimate natural leakage. With 7 ACH50 and a divisor of 17, you get 0.41 ACH. (Typical value, check the manufacturer manual or code.)

People. Manual J assumes bedrooms plus one occupants. Each gives roughly 230 BTU/h sensible and 200 BTU/h latent. A 3-bedroom house has 4 occupants: 920 sensible plus 800 latent, or 1,720 BTU/h. The exact assumptions are in the paid manual. (Typical value, check the manufacturer manual or code.)

Why rules of thumb fail. "One ton per 500 to 600 ft2" ignores insulation, glass, sun, ducts and leakage. For a 2,000 ft2 house it says 3.3 to 4 tons, or 40,000 to 48,000 BTU/h. A tight, well-insulated house may calculate 22,000 BTU/h, which is 1.83 tons, so a 2-ton unit. The rule would overshoot by 82% to 118%. Oversizing causes short cycling, poor humidity control and uneven rooms.

Manual S: picking the equipment

Manual S matches real equipment to the Manual J load. You do not use nominal tons. You use the manufacturer's expanded performance data at your actual indoor wet-bulb and outdoor dry-bulb temperature. The sensible capacity must cover the sensible load, and the latent capacity must cover the latent load.

Manual S limits how far equipment may exceed the load. Only two figures were found, both from a 2009 ACCA-staff article: 115% of the cooling load for an air conditioner and 140% of the heating load for a furnace. Limits for heat pumps are unconfirmed from ACCA's own pages. (Typical value, check the manufacturer manual or code.)

Example: with a 30,000 BTU/h cooling load, an air conditioner may go up to 115%, which is 34,500 BTU/h or 2.88 tons. A 3-ton unit whose real capacity at your conditions is 34,500 BTU/h sits exactly at 115%. That is the top of the limit, not over it. It passes, but it leaves no margin. A 2.5-ton unit also fits. Never round up just to be safe.

The sensible heat ratio (SHR) is the share of the cooling load that is temperature rather than moisture.

Example: 21,000 sensible plus 9,000 latent is 30,000 total, so SHR = 0.70. Comfort cooling usually lands between 0.70 and 0.80. Pick a coil whose SHR matches the house.

Manual D: sizing the ducts

Manual D sizes ducts so the blower can push the right airflow. The blower has a fixed pressure budget. Spend part of it on the filter, coil and grilles. Spread what is left over the longest duct run.

FR is the friction rate in inches of water column (in wc) per 100 ft. ASP is the available static pressure. TEL is the total effective length: straight duct plus the equivalent length of every fitting, on supply and return, for the longest run.

Example: a blower rated for 0.50 in wc, with 0.20 used by components, leaves 0.30. If the longest run has TEL = 300 ft, then FR = 0.30 x 100 / 300 = 0.10 in wc per 100 ft. Fitting lengths come from the Manual D tables. A sample of 50 ft straight, three elbows at 30 ft and two boots at 15 ft totals 170 ft (typical values, check the manufacturer manual or code).

Speed and size. Air speed is CFM divided by duct area. Pressure from motion is the velocity pressure.

Example: 400 CFM in an 8 in round duct. Area is 0.3491 ft2, so V is 1,146 fpm and VP is 0.0819 in wc. Typical residential targets are 700 to 900 fpm in trunks and 500 to 700 fpm in branches (typical values, check the manufacturer manual or code).

Friction in round duct. A common fit for galvanized steel:

Q is CFM and D is diameter in inches. For 400 CFM in 8 in, friction is 0.281 in wc per 100 ft. To hold 0.08, you need D = 10.27 in. Notice D sits to the fifth power. A slightly bigger duct cuts friction a lot. This fit is widely quoted but not checked against the handbook (typical value, check the ASHRAE Fundamentals handbook).

Rectangular duct. A rectangle and a round duct are equal when they have the same friction at the same airflow, not the same area.

Example: a 12 x 6 in duct has De = 9.14 in. The matching round duct has an area of 65.6 in2, which is less than the rectangle's 72 in2. The round shape moves air more efficiently, so it needs less area.

Fan laws. Change the fan speed and airflow changes in proportion. Pressure changes with the square, and power with the cube.

From 800 to 1,000 rpm: airflow goes from 1,200 to 1,500 CFM, pressure rises 1.56 times and power rises 1.95 times. Restrictive ducts cost airflow fast, and extra speed costs a lot of power.

Measured pressure. Total external static pressure (TESP) is the return suction plus the supply pressure, both as positive numbers.

Example: supply 0.38 and return -0.32 gives 0.70 in wc. If the unit is rated for 0.50, the duct, filter or coil is too restrictive.

The airflow and heat equations

All the air formulas share one shape: a constant, times CFM, times a change.

• Sensible: 1,200 CFM warmed by 20 F needs 1.08 x 1,200 x 20 = 25,920 BTU/h. Check the constant: 60 x 0.075 x 0.24 = 1.08.
• Latent: 1,200 CFM that loses 14 grains per lb (W drops 0.0020) removes 0.68 x 1,200 x 14 = 11,424 BTU/h.
• Total: 1,200 CFM with return enthalpy 29.0 and supply 20.0 BTU/lb (a 9 BTU/lb drop) gives 4.5 x 1,200 x 9 = 48,600 BTU/h, which is 4.05 tons.
• Water: 10 GPM cooled by 20 F moves 500 x 10 x 20 = 100,000 BTU/h. Antifreeze mixes carry less per gallon, roughly 450 to 485 depending on percent.

Common mistakes: using 1.08 for moisture, using the room temperature difference instead of the supply-minus-return difference, and forgetting altitude. These constants are typical values, check the manufacturer manual or code.

Temperature split

The split is return air temperature minus supply air temperature on cooling. It follows from airflow and the sensible load. Design airflow is about 400 CFM per ton, and California verification uses 350.

Example: a 3-ton system at 400 CFM per ton moves 1,200 CFM. At SHR 0.75, sensible capacity is 0.75 x 36,000 = 27,000 BTU/h, so the split is 20.8 F. At 350 CFM per ton (1,050 CFM) the split rises to 23.8 F. Less air means a bigger split and a colder coil. (Typical value, check the manufacturer manual or code.)

Superheat and subcooling

Refrigerant boils at a temperature that depends on pressure. A pressure-temperature (PT) chart converts a gauge reading to that saturation temperature. Add 14.696 to gauge pressure to get absolute pressure. Superheat and subcooling measure how far the real line temperature is from saturation.

These PT values were computed with the open-source CoolProp library:

At the same temperature R-32 sits 2 to 3% above R-410A, while the whole R-454B range sits below it. A chart for the wrong refrigerant therefore gives a wrong saturation temperature.

Example, superheat: R-410A suction at 118 psig saturates near 40 F. A suction line at 52 F gives 12 F of superheat. Example, subcooling: R-410A at 317 psig saturates at 100 F. A liquid line at 90 F gives 10 F of subcooling.

Use superheat to charge fixed-orifice systems and subcooling to charge TXV or EEV systems. Targets (one Daikin Comfort chart lists 8 F, and Carrier lists 5 to 11 F by size) vary by model, so follow the manufacturer's chart or nameplate. R-454B was not in the library, so check its chart from the manufacturer. Blends such as R-410A glide slightly.

Psychrometrics

Psychrometrics is the math of air plus water vapor. Three numbers matter most.

Dew point is the temperature where air becomes saturated. A surface colder than that sweats. The Magnus formula (T and dew point in C; a = 17.625, b = 243.04):

At 75 F and 50% RH, the dew point is 55.1 F.

Wet-bulb is the temperature of a wet thermometer in moving air. Equipment ratings are listed against indoor wet-bulb, with 67 F as the standard. The Stull fit (T in C, RH in percent):

At 75 F and 50% RH, wet-bulb is 62.7 F.

Enthalpy is the total energy in a pound of dry air, sensible plus latent.

At 75 F and 50% RH, the humidity ratio is W = 0.0092 lb water per lb dry air. That is about 64.4 grains (7,000 x 0.0092). Then h = 0.240 x 75 + 0.0092 x (1061 + 33.3) = 28.1 BTU/lb. The coil's real job is to move enthalpy, not only temperature. These formulas are standard fits (typical values, check the ASHRAE handbook or a psychrometric chart).

Furnace firing and temperature rise

Input is the gas energy burned each hour. Output is the heat delivered to the air. AFUE is the efficiency averaged over a season.

Example: an 80,000 BTU/h input at 96% AFUE delivers about 76,800 BTU/h. An 80% unit delivers 64,000. Steady-state efficiency differs a little from AFUE.

The nameplate lists an allowed temperature rise, such as 25 to 55 F. Rise is supply minus return on heating.

Example: 76,800 BTU/h of output into 1,200 CFM gives a 59.3 F rise. That is high, so airflow is too low. For a 50 F rise you need 1,422 CFM. Too high a rise means too little air. Too low a rise means too much air or low fire.

Checking the firing rate. Time the gas meter dial.

Example: 1 ft3 in 36 seconds is 100 CFH. At 1,030 BTU/ft3 that is 103,000 BTU/h. Compare with the nameplate, and set manifold pressure to the nameplate value. Natural gas is commonly about 3.5 in wc and propane about 10 in wc (typical values, check the manufacturer manual or code).

Gas piping and venting. Pipe sizing comes from the code tables for the longest run and the total load. Do not guess it from a formula. Vent sizing is also table driven (NFPA 54 or the IFGC). Condensing furnaces use sealed PVC pitched back toward the furnace. Use the adopted code and the manufacturer's instructions for both. Combustion air has its own code rules too.

Electrical

Ohm's law. Voltage pushes current through resistance, and power is voltage times current.

Example: a 240 V supply across a 12 ohm element draws 20 A. Power is 4,800 W, which is 16,378 BTU/h. A 10 kW strip heater makes 34,120 BTU/h and draws 41.7 A at 240 V.

Three phase. Commercial units use it. Power uses the square root of 3 and line-to-line voltage.

Example: 208 V, 20 A, power factor 0.85 gives 6,125 W. Legs at 205, 208 and 213 V average 208.7 V. The worst leg is 4.3 V off, so imbalance is 2.08%. Keep it under 2% (typical value, check the manufacturer manual or code).

MCA and MOCP. MCA is the minimum circuit ampacity, the smallest wire. MOCP is the maximum overcurrent protection, the largest breaker or fuse. Both are printed on the nameplate, so use those.

Example: compressor RLA of 18 A and a fan at 1.5 A give MCA = 1.25 x 18 + 1.5 = 24.0 A. MOCP is 1.75 x 18 + 1.5 = 33.0 A, and you round down to a 30 A breaker. Never round up, and never size the breaker to MCA. The 125% and 175% factors were confirmed in a trade source, Electrical Construction and Maintenance, not in the NEC text. Check the nameplate and the adopted NEC.

Capacitors. The microfarad value can be estimated from current and voltage.

Example: 7.2 A measured at 370 V gives 51.6 uF. Compare with the rating on the can. Tolerance is usually plus or minus 6% (typical value, check the manufacturer manual or code).

Efficiency ratings

COP, EER, SEER2 and HSPF2 are the same idea, useful heat or cooling out divided by electricity in, at different test conditions.

• COP 3.5 equals an EER of 11.9.
• HSPF2 of 8.1 BTU/Wh is a seasonal COP of 8.1 / 3.412 = 2.37.
• A 36,000 BTU/h unit at EER 11 draws 36,000 / 11 = 3,273 W.

SEER2, EER2 and HSPF2 replaced SEER, EER and HSPF under a newer federal test method (Appendix M1), confirmed in the Federal Register notice. Do not compare an old SEER number directly with a SEER2 number. A COP of 3 is not 300% efficient. It moves three units of heat for one unit of electricity. Use EER2 for peak power and SEER2 for yearly energy. The exact conversion factors between old and new ratings are typical values, check the manufacturer manual or code.

Fuel cost comparison: divide the price per unit by the energy delivered.

With made-up prices: gas at $1.80 per therm and 96% AFUE costs $18.75 per MMBtu. Resistance heat at $0.28 per kWh costs $82.06. A heat pump with COP 3 costs $27.35. Prices are illustrative only.

ASHRAE 62.2 ventilation

The standard sets whole-house mechanical ventilation for indoor air quality. It has a floor-area term and an occupant term.

Example: 2,000 ft2 with 3 bedrooms gives 0.03 x 2,000 + 7.5 x 4 = 90 CFM. An infiltration credit may lower it. The formula is confirmed by ACCA's 62.2 update post. Local exhaust rates, such as 100 CFM intermittent for a kitchen and 50 CFM for a bath, are typical values, check the manufacturer manual or code.

California Title 24 numbers

California requires measured, third-party (HERS) verification of ducts, airflow, fan power and charge on many installs. These are pass or fail limits. The numbers below were confirmed at energy.ca.gov. The 2025 code cycle takes effect Jan 1, 2026.

• New ducts: leakage at or below 5% of fan flow (6% for ducts in a garage and on the nonresidential form).
• Altered or extended ducts: at or below 10% total, or 7% to outside.
• Airflow: at least 350 CFM per ton.
• Fan efficacy: at most 0.45 W/CFM (gas furnace) or 0.58 W/CFM (other).

Example: a 3-ton unit needs at least 1,050 CFM. At 0.45 W/CFM the fan may use 472.5 W, or about 473 W. At 0.58 W/CFM it may use 609 W. Common errors: skipping the HERS rater, testing only the supply side, and using values from the wrong code year.

Quick recap

• Heat moves as constant x flow x change. The constant is just unit conversion.
• Size the load first (Manual J), then the equipment (Manual S), then the ducts (Manual D).
• Test with measurements: temperature split, superheat, subcooling, temperature rise and static pressure.
• Follow the nameplate, the manufacturer and the adopted code over any number on this page.

Scenes to open. Each part of the math section has matching scenes in HVAC Math in 3D:

Tools and instruments

Every tool below does one job in the install, service or test chain. Each group has a table: the tool, what it does, the one master secret, and what to buy. Prices are rough. Where a number could not be confirmed, it says "typical value, check the manufacturer manual." The manufacturer's manual and local code always win.

Tubing and flaring

Flare nut torque depends on brand and size, so read the unit's manual. Across 13 brands, 3/8 in flares mostly list 24-31 ft-lb. The 1/4 in size runs about 10-13 ft-lb, 1/2 in about 36-46, and 5/8 in about 45-60. That reading comes from an unlabeled sticker, so treat it as an inference. Lubricant changes torque, so stay on the low side if the threads are oiled. Follow the manual on flare lubricant: the Daikin R-32 manual says no refrigeration oil on the flare or threads. Minimum bend radius of about 5 times the tube diameter is a typical value, check the manufacturer manual.

Brazing and nitrogen

BCuP-5 melts between 1,190 and 1,475 F. Acetylene may not be used above 15 psig, and the cylinder valve opens no more than 1.5 turns, preferably 3/4. Leave the cylinder wrench on the stem. The flow tool is fed at 50-60 psi, but that is not the pressure inside the copper.

Vacuum and micron gauge

The usual target is 500 microns or lower. Water boils at 1 F at 1,000 microns and at -12 F at 500 microns. Pump size of 5-6 CFM two-stage for homes is a typical value, check the manufacturer manual. Manifold gauges cannot read this range. Each manufacturer sets its own target and decay limit (see the manufacturer table in Installation).

Gauges and charging

Reference points: R-410A saturates at 118.4 psig at 40 F and 317.6 psig at 100 F. R-454B runs about 107-112 psig at 40 F and 291-301 psig at 100 F.

Recovery

Leak detection

Electrical meters

Flame sensor current of about 0.5 to 7 microamps is normal on one brand's tech tip, typical value, check the manufacturer manual. The Fluke 116 reads down to 0.1 microamp.

Airflow and pressure

Total external static pressure target is 0.5 in. w.c. or less. Trouble starts at 0.8 or higher on most newer homes. Run the blower on high, wait about 3 minutes, then read.

Combustion

Air-free CO equals measured CO times 20.9, divided by (20.9 minus O2). So 100 ppm at 14% O2 is over 300 ppm air-free. Do not tune to the 400 ppm certification limit. A healthy furnace reads under 50 ppm air-free. Replace sensors on schedule: O2 cells about 2-3 years, CO sensors 3-5 years (one source).

Ducts

IECC 2012 allows total duct leakage of 4 CFM25 per 100 sq ft after construction. Later editions need checking.

Drains

Power tools

PPE and ladders

The corrections that matter

Several figures in older notes were wrong. These are the fixed versions.

Recovery level. For a high-pressure appliance under 200 lb of charge, the required recovery level is 0 in. Hg. The 10 in. Hg figure applies only at 200 lb or more, and only for equipment made after November 15, 1993. Older equipment at 200 lb or more is 4 in. Hg. The rule is 40 CFR 82.156 Table 1. If leaks prevent it, recover to the lowest level you can reach.

Nitrogen flow. Purge flow is about 20-50 SCFH. Brazing flow is about 2-3 SCFH. One source gives a wider 2-5 SCFH range. Too much flow cools the joint and hurts penetration. A full nitrogen cylinder reads 2,000 to 2,640 psig at 70 F depending on cylinder type, so always use a regulator (Nippon Sanso). One pipe volume of nitrogen is not proof the air is gone, so time the purge from the real flow.

Flare torque. For 3/8 in flares, most brands list 24-31 ft-lb. An older note of 20-30 ft-lb was too low. Use the manual for your unit.

Capacitors. A new run capacitor is held to plus or minus 6% of its rating. In service, many shops replace one that reads more than 10% low under load. Some shops use 5-6%. Always replace the run capacitor when you replace a PSC motor.

A2L recovery machine. The A2L-rated machine to name is the Fieldpiece MR45. It is rated for R-32, R-454B and R-1234yf, and costs about $1,150. It auto-stops at 0 psig by default.

Temperature and nitrogen tests. A pressure change of about 0.3 psi per degree F is normal at 150 psig. An older rule of 1 psi per 5 F was off at that pressure.

Unconfirmed items. Anything labeled "typical value, check the manufacturer manual" was not checked against a manufacturer document. Model numbers marked unverified in the research notes were left out of the picks.

Local compliance kit (practice-based). Carry a tape or laser measure for the 5 ft Zone 0 and the setback checks, and a level for the pad. Add a sound-level meter set to dBA to record the condenser at the property line: the County has no confirmed equipment limit, and the City's 53 dBA is CITY OF SANTA BARBARA ONLY. Take before and after photos of the pad and screening, keep the County permit card on site, and check the parcel's climate zone in the CEC lookup on your phone.

Sources used: Fieldpiece for meters and recovery, ACCA blog on 500-micron vacuum, HVAC School on target superheat, and the DOE duct leakage FAQ.

Scenes to open. Every group of tools above is animated step by step in HVAC Tools in 3D: Safety: Protect your eyes, hands, lungs and ears; Ladder angle and fall arrest on a roof. Hand tools and drilling: Panel, terminal, wire and valve: the hand tools; Scan, pilot, cut and sleeve: drilling a line set penetration. Tubing: Cut, ream, swage, bend and pinch copper tube; Make a flare, then torque the nut. Brazing and nitrogen: Braze a copper joint safely; Nitrogen purge: purge flow versus braze flow; Nitrogen pressure test and temperature drift. Vacuum: Set up the vacuum: pump, large hoses, cores out; Micron gauge: pull down, isolate, watch the rise. Gauges, recovery and leaks: Read the gauges: pressure to saturation temperature; Recover refrigerant: machine, cylinder and scale; Find a leak: electronic, bubbles or UV dye. Gas and combustion: Gas pipe: wrench, back-up wrench and leak check; Combustion analyzer: air-free CO, O2 and stack temperature. Electrical: Lock out, then prove it is dead: tester, meter and 24 V; Clamp meter: amps against the nameplate; Capacitor, megohm and burnout checks. Air side: Static pressure: supply plus return is the TESP; Measure airflow: pitot, anemometer and flow hood; Delta T: supply and return temperature. Duct work: Snips, crimper and brake: cut and shape duct; Seal the duct, then test the leakage. Drain: Clear and pump the condensate drain.

Installation

This is a split-system install (outdoor condenser or heat pump, indoor coil or air handler) in the order a careful crew does it. Each step has the reason behind it. Where values differ by brand, a table is shown. In every such case, use the manufacturer's installation manual. Safety first: lock out power, discharge capacitors, and hold an EPA 608 certification to buy or handle refrigerant. Never use oxygen or compressed air to test pressure. A2L refrigerants (R-454B, R-32) are mildly flammable, so use A2L-rated tools and keep ignition sources away.

1. Set the equipment. Place the outdoor unit on a level pad with the clearances in the manual. Set the indoor unit level, or sloped as the manual says for the drain. Check that the breaker and disconnect match the unit's MCA and MOCP numbers.
   • Why: a tilted unit drains badly and strains the compressor. Poor clearance starves the coil of air and makes every later reading harder to trust.
   • Local rules (Montecito). Open the County permit before you change a unit, because the County exempts only a swap of component parts. Set the condenser on a non-combustible pad and keep wood pads, plastic covers, combustible screens and mulch out of the 5 ft next to the house. Raise the pad if the parcel has flooded, and place or buffer the unit away from neighbors. Moving the condenser, adding a rooftop unit or running a visible line set needs a written Director determination first. See Montecito and Santa Barbara: your local rules.
2. Run the line set. Size the suction and liquid lines per the manufacturer. A common 3-ton example is 3/4 in suction and 3/8 in liquid, but this is a typical value, check the manufacturer manual.
   • Use clean, capped ACR copper and keep tube ends capped until you braze.
   • Insulate the suction line. Support it every 4-5 feet, avoid kinks, and keep it off metal framing with isolators.
   • Keep total length within the maximum.
   • Why: dirt and moisture inside the tube ruin a system. Bare suction line sweats. Metal contact wears through the copper over time.
3. Braze with nitrogen flowing. Confirm the system is open to atmosphere through a port or open end, never a sealed pressure.
   • Flow regulated nitrogen through the line while you braze. The brazing flow is about 2-3 SCFH, low enough not to cool the joint. A purge before lighting runs about 20-50 SCFH.
   • Clean the pipe with abrasive cloth. Use 15% silver rod (BCuP-5) without flux for copper to copper. Use silver rod with flux for copper to brass or steel.
   • Heat the tube first, then the fitting, and touch the rod to the joint away from the flame.
   • Protect valves and the expansion valve with a heat shield or wet rag, and follow the manual on removing Schrader cores.
   • Why: without nitrogen, black scale forms inside the tube and later clogs the metering device and fouls the oil.
4. Pressure test with nitrogen. Set the regulator so you never exceed the lowest-rated part on the system. Raise pressure in steps and check joints with bubble solution. Then hold and watch the gauge. The numbers differ by manufacturer, so use the manufacturer's installation manual.
   • Test pressure is not universal. Manufacturer manuals run from 160 psig (Lennox) to 604 psi (Daikin R-32 mini-split). See the manufacturer table after this list.
   • The hold time runs from 15 minutes (Goodman, Rheem) to 1 hour (Trane, Carrier) to 24 hours (Fujitsu).
   • Use the manufacturer's installation manual for the test pressure and hold. Never exceed the lowest-rated part.
   • Correct for temperature: P2 = T2 x P1 / T1, with pressure in psia and temperature in degrees Rankine. As an example, 150 psig at 80 F drops to about 146.95 psig at 70 F with no leak, so about 0.3 psi per degree F. A digital gauge with a probe does this for you.
   • Why: finding a leak now costs minutes. Finding it after charging costs refrigerant and a return trip.
5. Evacuate. Remove the Schrader cores with a core removal tool. Use 3/8 in or larger hoses and a pump rated for the refrigerant (A2L-rated if you work with R-32 or R-454B). Put the micron gauge on a separate port, ideally at the far end of the system, and use fresh pump oil. Pull to 500 microns or lower. ACCA confirms 500, and HVAC School and EPA GreenChill pull to 300, so deeper is better. Then close the valve to the pump and watch the decay. The pass line differs by manufacturer, so use the manufacturer's installation manual:
   • ACCA: stay under 1,000 microns for 5 minutes.
   • Building Science Corporation: 700 microns or less in 10 minutes, from a 300 micron start.
   • EPA GreenChill: 100 microns or less drift over 24 hours.
   • Manufacturers set their own pass line: Trane allows a rise of 500 microns in 1 minute and 1,500 in 10 minutes, Carrier 1,500 in 10 minutes, and Lennox no rise above 500 within 20 minutes. See the manufacturer table.
   • A rise that levels off means moisture is still boiling off, so keep pumping. A steady climb to atmosphere means a leak.
   • For a wet or contaminated system, use a triple evacuation: pull down, break the vacuum with nitrogen to a few psig, then pull again.
   • Why: water boils at 1 F at 1,000 microns, so only a deep vacuum removes moisture. Moisture and air inside the system make acid and block the metering device.
6. Charge. Start with weigh-in, then verify.
   • Weigh-in. The condenser nameplate gives a factory charge for a set line length (often 15 ft, check the nameplate). Add refrigerant by weight for extra line. The oz-per-foot figure depends on the liquid line size and brand. For a 3/8 in liquid line, manufacturers list 0.47 oz/ft (Trane) to 0.60 oz/ft (Carrier, Rheem), so use the manufacturer's installation manual. One mini-split brand (Pioneer, R-454B) lists 0.11 oz/ft beyond 25 ft. Use a scale that resolves half an ounce.
   • Superheat. This is for fixed-orifice or piston systems. Take suction pressure, convert it to saturation temperature, and subtract it from the suction line temperature measured about 6 in from the compressor. The target comes from the manufacturer's chart using indoor wet bulb and outdoor dry bulb (confirmed by HVAC School). A "typical 8-20 F" range is unconfirmed, so it is a typical value, check the manufacturer manual. The California Energy Commission passes a unit within plus or minus 5 F of the chart target.
   • Subcooling. This is for TXV systems. Subtract the liquid line temperature from the condensing saturation temperature. The target is on the nameplate. A "typical 8-12 F" is unconfirmed, so it is a typical value, check the manufacturer manual. California passes a unit within plus or minus 3 F.
   • Conditions. Run the system until stable, about 15 minutes, with clean coils and correct airflow. California charge verification needs condenser air between 55 and 115 F dry bulb and return air above 70 F, or you use the weigh-in alternative.
   • Heat pump charge verification. California requires it for heat pumps in every climate zone. An air conditioner in zone 5 or 6 does not need it, but confirm the zone by parcel, because the CEC ZIP table was not read.
   • Pressure references. R-410A saturates at 118.4 psig at 40 F and 317.6 psig at 100 F (CoolProp). R-32 saturates at 121.0 and 325.7 psig at the same temperatures (CoolProp). R-454B runs about 107-112 psig at 40 F and 291-301 psig at 100 F. That is a range from secondary sources, and the spread comes from bubble versus dew points, so use an app set to R-454B. R-454B reads about 5 to 10% below R-410A. R-410A also reads about 131 psig at 45 F and 417 psig at 120 F (interpolated from the BPA chart).
   • Why: weighing gets the amount right. Superheat and subcooling then prove the charge is right for this coil, this airflow and this weather.
7. Ducts and mastic. Design with Manual D. Fasten joints with screws first, then seal every seam and joint.
   • Use water-based mastic (UL 181 listed), with fiberglass mesh over gaps wider than 1/4 in. UL 181 foil tape also works. Cloth duct tape is not UL 181 approved.
   • Insulate ducts in unconditioned space (R-8 on supply ducts and R-6 on return ducts, per IECC 2009 as cited in the Building America guide).
   • Seal the air handler cabinet, the return platform and the boot-to-drywall gaps, which are the biggest leakers.
   • Test with a duct blaster at 25 Pa. The 2012 IECC allows total leakage of 4 CFM25 per 100 sq ft after construction, and a 2021 jurisdiction form shows the same number. Rough-in limits are 4 with the air handler installed and 3 without.
   • California Title 24 sets leakage as a percent of fan flow. The limit is 5% or less for an entirely new residential duct system, and 6% for ducts in a garage and on the nonresidential form. Duct testing applies above 25 ft of new or replacement duct. For altered systems, airflow verification uses 300 CFM per ton or more (CEC airflow form).
   • Why: leaky ducts waste conditioned air in the attic and pull in dust.
8. Plenum and return. Build the supply plenum at least as wide as the coil, with turning vanes in sharp transitions. Size the return for low friction.
   • Size filters for about 300 feet per minute of face velocity, which is 1 sq ft per 300 CFM (typical value, check the manufacturer manual). A larger media filter lowers pressure drop.
   • Never draw the return from a garage or mechanical room. Give closed bedrooms a return path, such as jump ducts or transfer grilles.
   • Aim for total external static pressure of 0.5 in. w.c. or less, and expect trouble at 0.8 or higher.
   • Why: a starved return raises static pressure, cuts airflow, and freezes coils.
9. Condensate. Use a 3/4 in PVC primary drain sloped 1/4 in per foot. The International Mechanical Code minimum is a 1% slope.
   • Add a trap as the manufacturer requires. Never glue the removable trap connection.
   • Install a secondary drain or an auxiliary pan with a float switch, and a vent and cleanout tee downstream of the trap.
   • Where the coil is below the drain, use a condensate pump with a safety switch. Finish with a pour-water test.
   • In Montecito the County storm drain code lists air conditioning condensation as an exempt discharge if it is properly managed and not a nuisance. Sewer connection rules were not confirmed.
   • Why: water damage is among the costliest callbacks, and algae clogs a flat drain within a season or two.
10. Low-voltage wiring. Use 18-gauge thermostat wire with enough conductors, such as 18/5 or 18/8. Land each wire on the terminal below (the wiring diagram follows this list).
    • R (Rc, Rh): 24 VAC hot from the transformer. Rc is cooling and Rh is heat, jumped if there is one transformer.
    • C: 24 VAC common. Smart thermostats need it.
    • W (W1, W2): heat stages, or auxiliary heat on a heat pump.
    • Y (Y1, Y2): compressor cooling stages. On a heat pump the compressor runs in both modes.
    • G: blower fan.
    • O/B: reversing valve. O energizes in cooling on most brands. B energizes in heating on Rheem and Ruud.
    • Check 24 to 28 VAC between R and C. To isolate the thermostat, jump R to Y for cooling, R to W for heat, and R to G for fan, right at the air handler.
    • Why: one wrong terminal on a heat pump runs it backwards, and a missing C wire makes a smart thermostat reboot.
11. Commission and document. Work down the checklist, write the readings down, and leave a copy.
    • Unit level, clearances kept, breaker and disconnect sized to MCA and MOCP.
    • Line set sized and insulated; nitrogen hold passed.
    • Evacuation reached target and the decay test passed.
    • Charge weighed in and verified by superheat or subcooling.
    • Static pressure at or under 0.5 in. w.c. (or per the blower data) and airflow about 350-450 CFM per ton. California requires at least 350.
    • Cooling temperature split about 16-22 F (typical value, check the manufacturer manual). Heat rise within the nameplate range.
    • Furnace: manifold pressure, gas inlet pressure, flame microamps and a CO test. Trane lists natural gas manifold at 3.3-3.5 in. w.c. and inlet 4.5 minimum to 14.0 maximum. Read the nameplate for your unit.
    • Condensate: pour-water test and float switch test.
    • Thermostat set up (O/B, stages, balance point) and 24 V verified.
    • Duct leakage tested, filters in.
    • Customer walkthrough, warranty registration.
    • Permit finaled, and the proof of permit kept for the County record and for a 3CE rebate (Montecito).
    • Why: the readings prove the job and protect you if the system fails later.

Read the top row left to right, then the bottom row right to left. A reading that is out of range sends you back to repeat that step.

Manufacturer values for the nitrogen test, evacuation and add-on charge. These differ by brand and model, so use the manufacturer's installation manual. Each row links to the manual it came from. Where a value was not found, the cell says so. Trane, Lennox, Rheem and Daikin Comfort require a refrigerant detection sensor on their A2L systems.

Wire to this picture and every terminal has one job: R and C carry the 24 V power, and a call on W, Y, G or O/B closes the circuit for heat, the compressor, the blower or the reversing valve.

Rules that changed. Since January 1, 2026, the AIM Act leak-repair rule applies to HFC appliances with 15 lb or more of charge. The old threshold was 50 lb. Residential and light commercial AC and heat pumps are exempt (EPA fact sheet). The old January 1, 2026 install cutoff for R-410A systems built from pre-2025 inventory is superseded by 91 FR 31284, effective July 27, 2026. It removes the installation deadline for qualifying legacy-inventory systems, and only post-2025 R-410A components stay restricted.

Sources: ACCA blog on 500-micron vacuum, Building Science Corporation startup guide, EPA leak guidelines, HVAC School on target superheat, California Energy Commission RA3.2, DOE duct leakage FAQ, Trane gas pressure article, and ServiceMag on the R-410A rule.

Scenes to open. In HVAC Tools in 3D, open Scan, pilot, cut and sleeve: drilling a line set penetration, Cut, ream, swage, bend and pinch copper tube, Make a flare, then torque the nut, Braze a copper joint safely, Nitrogen purge: purge flow versus braze flow, Nitrogen pressure test and temperature drift, Set up the vacuum: pump, large hoses, cores out, Micron gauge: pull down, isolate, watch the rise, Static pressure: supply plus return is the TESP, Seal the duct, then test the leakage and Clear and pump the condensate drain. In HVAC Math in 3D, open Subcooling: how far past condensed, Pressure and temperature go together and Evacuation and pressure test: the method.

Ductwork practice, attics and crawlspaces

Most comfort and efficiency losses hide in the ducts, so how you hang, seal and insulate them matters as much as the equipment.

Why it matters. A Building America guide says duct losses "can be up to 30% or more" when ducts are leaky, poorly insulated and sit in an uninsulated attic or crawlspace. It recommends putting ducts in conditioned space, such as a dropped hallway chase or an insulated, air-sealed crawlspace. Ducts outside conditioned space should be insulated to R-8 for supply and R-6 for return under the 2009 IECC (Building America guide, 2011). Status: confirmed, but the guide is older, so check the current code. California's leakage limits are in the code section.

The four numbered checks apply along the whole loop: seal joints and boots, support the flex, and insulate wherever ducts leave the conditioned space.

Flex duct practice

Attics and crawlspaces

Sizing and design mistakes

A2L refrigerants on the job: leak sensors and mitigation

On an A2L system the factory leak sensor is a safety device, so you wire it, test it and never bypass it.

A refrigerant detection system (RDS) is a sensor plus a control board that reacts to a leak. ACHR News describes how three makers build it (ACHR News, 2024-09-06).

A sensor trip turns the blower on and stops cooling, and the system stays that way until the sensor reads clear.

Diagnostics: the top 25 failures

These are the 25 service calls you will see most. Numbers marked "typical value, check the manufacturer manual" were not confirmed against a manual. The nameplate and the manual always win.

Local rules when a diagnosis ends in a replacement. In Montecito the County Code exempts only a swap of component parts, such as a capacitor, contactor or motor. Replacing a whole condenser, furnace or air handler needs a County permit before the work, and moving a condenser can need a written Director determination. Tell the customer that the permit comes first and that the County fee line for HVAC is unconfirmed. See Montecito and Santa Barbara: your local rules.

The master routine (use it on every call)

1. Safety first. Lock out and tag the power, discharge capacitors, and wear PPE. Use regulated nitrogen only for pressure tests. A2L refrigerants (R-454B, R-32) are mildly flammable, so ventilate and remove ignition sources.
2. Airflow before anything else. Check the filter, the blower wheel, the coil face, the registers, and total external static pressure (design 0.5 in. w.c. or less, typical value, check the nameplate).
3. Electrical next. Measure 24 VAC at the control, line voltage at the disconnect, capacitor microfarads, and amp draw against the nameplate.
4. Refrigerant last. Only after airflow and electrical are proven good. Run 10 to 15 minutes, then read superheat or subcooling and compare with the manufacturer chart.
5. Document your readings. Write down before and after numbers, outdoor and indoor temperatures, and the part you replaced. Then ask why the part failed.

No-cool decision tree

Answer the four questions top to bottom and stop at the first no. Each no names the cause to test first and the table rows that cover it.

No-heat decision tree

Prove the 24 V call first. Split off the heat pump at question two. Then follow the furnace sequence in order, because the first step that fails is the fault.

Read this before the table

• Superheat target. It comes from a chart using indoor wet bulb and outdoor dry bulb (HVAC School). The "8 to 20 F superheat" and "5 to 12 F subcooling" figures below are typical values, check the manufacturer manual.
• Igniters. A silicon-carbide igniter should not draw more than 4.75 A. Cold resistance runs about 40 to 75 ohm on fast models and 45 to 400 ohm on others. A used igniter reading more than twice its new value is weak (ACHR News). Silicon-nitride ranges are unverified; one blog says about 80 to 300 ohm.
• Flame sensor. One brand's tech tip lists normal as about 0.5 to 7.0 microamps DC, with 0.5 as the minimum. Check the control board manual for your brand.
• Pressure switches. Closing setpoints run roughly -0.20 to -1.0 in. w.c. depending on the model. The value is printed on the switch.
• Gas pressure. Natural-gas manifold is 3.5 in. w.c. and LP manifold is 10 in. w.c. Trane shows natural-gas inlet at 4.5 minimum and 14.0 maximum (Trane). That is one brand, so check your furnace's rating plate.
• Evacuation. The target is 500 microns or lower, and deeper is better (ACCA). Decay limits disagree between sources: ACCA says under 1000 microns in 5 minutes, and Building Science says 700 or less in 10 minutes. Use the manufacturer manual.
• Leak repair rule. The 50 lb threshold is outdated for HFC systems. The EPA leak-repair fact sheet applies the 15 lb threshold to HFC appliances from 1 January 2026. Residential and light commercial AC and heat pumps are exempt.

The top 25 table

Master secrets and common mistakes

• Find why the part died. Capacitors and contactors fail from low voltage, ants, high head pressure, or short cycling. Fix that or the new part fails too.
• Check the 30-second items first. The 24 V fuse, the float switch, and the flame sensor solve many calls. Most shorts are staples, cracked jackets, or line-set rub points.
• Fix airflow before touching refrigerant. Never add refrigerant to a dirty or iced coil. Never charge by suction pressure alone.
• Never top off without finding the leak. Leak repair rules apply. Recover refrigerant and never vent it.
• Take readings on a stable system. Run 10 to 15 minutes. Record indoor wet bulb and outdoor dry bulb. Charge by weigh-in, superheat or subcooling per the manufacturer.
• Never jump out a safety. Limit, rollout, and pressure switches protect people. Jumping one "to test" and forgetting it is a classic mistake. Resetting a rollout and leaving is another.
• Handle igniters by the base. Skin oil creates hot spots and early failure. Bright orange-white glow is healthy. Dull or slow glow means age or low voltage.
• Discharge capacitors through a resistor. Never replace a capacitor with a different uF to "try it". Match uF and use equal or higher VAC.
• Never upsize a breaker or fuse. Check the nameplate maximum overcurrent protection and the 24 V fuse rating instead.
• Most bad TXVs and reversing valves are not the part. Bulb mounting and insulation cause most TXV calls. Solenoids fail more often than valves. A tap on the valve is a diagnostic, not a repair.

Scenes to open. In HVAC Tools in 3D, open Read the gauges: pressure to saturation temperature, Combustion analyzer: air-free CO, O2 and stack temperature, Lock out, then prove it is dead: tester, meter and 24 V, Clamp meter: amps against the nameplate, Capacitor, megohm and burnout checks, Static pressure: supply plus return is the TESP, Measure airflow: pitot, anemometer and flow hood, Delta T: supply and return temperature and Clear and pump the condensate drain. In HVAC Math in 3D, open Airflow per ton and the temperature split, Static pressure: what the blower pushes against, Superheat: how far past boiling, Subcooling: how far past condensed and Is the run capacitor still good?.

Coastal air, heat waves and the work day

Salt air corrodes coils and cabinets, and heat waves put you in the attic at the worst hour. This section covers both for Montecito and Santa Barbara work, with the local pad, fire, flood and noise rules that bite on outdoor units.

Code, licensing and permits (California)

Three rules decide whether a job is legal: your license, your refrigerant certification, and the Title 24 energy code. Facts below link to the page they came from. Items marked unconfirmed could not be checked.

Title 24 HERS verification. A third-party rater tests duct leakage, airflow, fan watts and refrigerant charge.

Check this for Montecito and Santa Barbara. The Energy Commission lists refrigerant charge verification for air conditioners only in climate zones 2 and 8 to 15, and for heat pumps in all zones (CEC). The CEC Building Climate Zones layer puts Montecito's coast and valley, the City of Santa Barbara, Carpinteria, Summerland and Goleta in zone 6, and upper Montecito above about 34.455 N in zone 5. Neither is in zones 2 or 8 to 15, so a heat pump needs charge verification and an air conditioner does not. The CEC ZIP table was not read, so confirm the zone by parcel with the CEC lookup; a 2016 CAPCOA table lists 93108 as zone 8, which conflicts with the CEC layer, and in zone 8 an air conditioner would need it.

Permits. Mechanical permits plus Title 24 forms with HERS testing before final are the general process. In Montecito the permit comes from the County of Santa Barbara Planning and Development, not the City. A whole-unit replacement needs a County permit, and the County fee schedule has no air conditioner, heat pump or change-out line, so quote the closest lines and confirm with the County (see the section Montecito and Santa Barbara: your local rules). CITY OF SANTA BARBARA ONLY: the FY26 fee schedule (effective 2025-09-01) lists $627 for a furnace new or replacement permit and $315 for a wall heater, with no separate line for an air conditioner or heat pump (City fee schedule). Like-for-like replacements can use the City's On-Demand portal, and Santa Barbara Clean Energy covers up to $820 of heat pump permit fees (City sustainability page). The City's HVAC permit checklist says a HERS duct leakage test is required when a furnace is replaced and the ducts stay, and it lists a 53 dBA condenser limit at the adjacent residential property line and 5 ft side and 10 ft front setbacks. Those are City figures: do not quote them for a Montecito job. The Coastal Zone exemption for HVAC was not confirmed.

Montecito, Santa Barbara and coastal considerations

Rows marked Unconfirmed or practice-based were not confirmed against a fetched county, city or manufacturer page, so check them locally before you quote them to a customer. The confirmed County rules for permits, design review, noise, fire and flood are in the section Montecito and Santa Barbara: your local rules.

Scenes to open: In HVAC Math in 3D, open COP, EER and HSPF2: heat for every watt, Whole-house ventilation, ASHRAE 62.2 and California Title 24: airflow, fan watts, leakage.

Business: contracts, liens and pricing

A job is not finished until you are paid, and in California the paperwork protects you as much as the customer. This section gives the sourced rules. Hours, prices and margins are not sourced anywhere and are marked as such.

Home improvement contracts (B&P 7159).

• Put it in writing when the price is over $500.
• The down payment is capped at $1,000 or 10% of the price, whichever is less.
• The customer can cancel within 3 business days (5 days for seniors, 7 for emergencies).
• Include your license number, a "Home Improvement" heading, start and completion dates, and the lien and CSLB notices.
• Never take payment for work you have not finished.

Preliminary notice and liens (Civil Code 8200; deadlines from the CSLB guide).

A contractor working directly for the owner does not send a preliminary notice but must give the owner the statutory notice. These deadlines come from the CSLB homeowner guide to mechanics liens, which cites the 2016 code edition. Confirm them against the Civil Code or a construction attorney before relying on them.

Warranties. California allows suit for latent construction defects for 10 years from substantial completion (CCP 337.15). Manufacturer warranties vary by brand and by whether the unit is registered. Read each one.

Pricing math you can trust. Margin is profit divided by price. Markup is profit divided by cost. A 40% margin needs a 66.7% markup, so adding 40% to your cost gives only a 28.6% margin. Use the margin formula to set prices:

Labor hours per job, overhead and burden percentages, price ranges for a system and the HERS test fee are rules of thumb, not sourced. Build your own numbers from your last ten jobs rather than copying anyone else's.

Quoting in Montecito. Quote the County permit as the closest lines on the County fee schedule (heating appliance, cooling unit add-on, furnace or air-handling unit, plus a $313 hourly plan check, a $156 issuance fee and 6.00% and 3.70% surcharges) and say the County sets the final fee. Never carry over the City's $627 furnace figure. A whole-unit replacement needs a County permit, so the $1,000 unlicensed-work exemption in B&P 7048 does not apply. Add schedule time for a written Director determination and, if design review applies, a Board of Architectural Review meeting (every other Thursday, applications due three weeks before). Tell the customer a rebate needs proof of permit and may not apply. See Montecito and Santa Barbara: your local rules.

Customers, upsell ethics, documentation and warranty registration

A trusted tech sells what the house needs, writes down what was done, and registers the equipment. The rows below cover most of it.

California rebates and incentives for heat pumps, as of 2026-10-28

Most heat pump money is now reserved or expired, so check the live pages before you promise a customer anything.

To join TECH as a contractor, use the TECH Contractor Enrollment form or email TECH.contractor@energy-solution.com. Never promise a rebate in a quote. Say "may qualify" and point to the live page.

Day 1 to master: a staged course

This course takes you from your first day to a service and design master in nine stages. Move to the next stage only when you can do the practice alone and answer the "explain" line without notes. The time per stage is a practice-based suggestion for someone working at it most days, not a requirement from any code or license. Each stage links to scenes by name in HVAC Math in 3D (35 scenes) and HVAC Tools in 3D (25 scenes, 70 tools; set its Level menu to Beginner for the first stages).

Your first-day tool kit

Buy these first. Each has a scene or tool entry in HVAC Tools in 3D.

• Safety glasses, work gloves, a headlamp and a CO monitor.
• A nut driver set (1/4 and 5/16 hex), an insulated screwdriver set, and pliers with wire strippers.
• A digital multimeter, a non-contact voltage tester and a clamp meter.
• An analog manifold gauge set with a PT chart or app, and a digital or clamp thermometer.
• A bubble leak solution, a flashlight, and a ladder you have learned to set at the right angle.
• Not on day 1: the torch, vacuum pump, recovery machine and gas tools. Learn those beside a mentor at stages 4 to 6.

Stage 0, day 1: safety and names

• Study: Safety first, and this tool kit.
• Practice: Name every tool in your bag. Lock out and tag a disconnect with a mentor. Test your meter on a known live source. Set a ladder at the correct angle.
• Explain: Why you prove a circuit dead, why a run capacitor still holds charge, and what A2L means.
• Open: Tools: Protect your eyes, hands, lungs and ears, Ladder angle and fall arrest on a roof, Panel, terminal, wire and valve: the hand tools and Lock out, then prove it is dead: tester, meter and 24 V.

Stage 1, week 1: how the system works

• Study: How the systems work, the refrigerant loop diagram and the split-system map.
• Practice: Trace the refrigerant path on a real unit. Pull a filter and clean a coil. Read a manifold gauge and say what it means before checking the chart.
• Explain: The four parts of the loop in order, what superheat and subcooling tell you, and why airflow comes first.
• Open: Math: Superheat: how far past boiling, Subcooling: how far past condensed and Pressure and temperature go together. Tools: Read the gauges: pressure to saturation temperature and Delta T: supply and return temperature.

Stage 2, weeks 2 to 3: electricity and measuring

• Study: The math section on electrical, and the thermostat wiring diagram.
• Practice: Measure 24 VAC from R to C. Jump R to Y, W and G at the air handler. Measure amps against the nameplate. Test a capacitor.
• Explain: Why the C wire matters, what MCA and MOCP are for, and why you never upsize a breaker or fuse.
• Open: Math: Volts, amps, watts and BTU: a heat strip, Wire and breaker: MCA and MOCP and Is the run capacitor still good?. Tools: Lock out, then prove it is dead: tester, meter and 24 V, Clamp meter: amps against the nameplate and Capacitor, megohm and burnout checks.

Stage 3, weeks 4 to 6: airflow, ducts and static pressure

• Study: The math on airflow and ducts, and installation steps 7 to 9.
• Practice: Measure total external static pressure and the supply and return temperatures. Calculate CFM from a temperature split, then measure it and explain the difference. Seal a duct sample with mastic. Run a pour-water test on a drain.
• Explain: Where the 1.08 comes from, why 0.5 in. w.c. is the design target, and why a starved return freezes a coil.
• Open: Math: How much heat the air carries, Airflow per ton and the temperature split, Duct speed, friction and size, Rectangular duct, round equivalent, Static pressure: what the blower pushes against and Fan laws: speed up a little, pay a lot. Tools: Static pressure: supply plus return is the TESP, Measure airflow: pitot, anemometer and flow hood, Delta T: supply and return temperature, Snips, crimper and brake: cut and shape duct, Seal the duct, then test the leakage and Clear and pump the condensate drain.

Stage 4, weeks 7 to 10: copper, nitrogen and vacuum

• Study: The tools section on tubing, brazing and vacuum, and installation steps 2 to 5.
• Practice: Braze ten joints with nitrogen flowing, cut them open and look for scale. Make flares and torque them. Run a nitrogen pressure test with temperature correction. Pull a vacuum on a micron gauge and find a leak.
• Explain: Purge flow versus braze flow, why manufacturers differ on test pressure and hold time and where you find the right number, and what a rising micron reading means.
• Open: Tools: Cut, ream, swage, bend and pinch copper tube, Make a flare, then torque the nut, Braze a copper joint safely, Nitrogen purge: purge flow versus braze flow, Nitrogen pressure test and temperature drift, Set up the vacuum: pump, large hoses, cores out, Micron gauge: pull down, isolate, watch the rise and Find a leak: electronic, bubbles or UV dye. Math: Evacuation and pressure test: the method.

Stage 5, months 3 to 4: charge and service

• Study: Installation step 6, and the whole diagnostics section.
• Practice: Weigh in a charge, run 15 minutes, then verify by superheat or subcooling. Run the master routine on ten service calls and write down every reading.
• Explain: Why you weigh in first and verify second, which metering device uses superheat and which uses subcooling, and why you never top off without finding the leak.
• Open: Math: Superheat: how far past boiling, Subcooling: how far past condensed, Pressure and temperature go together, Dew point and wet bulb and Humidity ratio and enthalpy. Tools: Read the gauges: pressure to saturation temperature and Recover refrigerant: machine, cylinder and scale.

Stage 6, months 5 to 6: gas, combustion and furnaces

• Study: The furnace ignition diagram, and the math on furnaces and gas.
• Practice: Walk a furnace through its ignition sequence and name the step where it fails. Check temperature rise against the nameplate. Clock a gas meter. Run a combustion analyzer and a CO test after every service. Do gas piping only under licensed supervision.
• Explain: Why the pressure switch must prove draft before the igniter glows, why a flame sensor fails, and what 400 ppm air-free means.
• Open: Math: Furnace input, output and temperature rise, Clocking the gas meter, Combustion air for a closet and Venting: rise to the chimney, fall to the furnace. Tools: Gas pipe: wrench, back-up wrench and leak check and Combustion analyzer: air-free CO, O2 and stack temperature.

Stage 7, months 7 to 12: design and code

• Study: The math on loads, the Manual J to S to D workflow, and the code section.
• Practice: Walk a house and guess the load, then run the calculation and note the error. Choose equipment to the load. Watch a HERS verification and read the form.
• Explain: Why a rule of thumb fails, how J feeds S and S feeds D, what Title 24 asks of airflow, fan watts and duct leakage, and why new-duct leakage is 5% for an entirely new residential duct system but 6% for garage ducts and the nonresidential form.
• Open: Math: Heat leaking through a ceiling, Air leaking in and out, Rule of thumb vs a real load calculation, COP, EER and HSPF2: heat for every watt, Whole-house ventilation, ASHRAE 62.2 and California Title 24: airflow, fan watts, leakage.

Stage 8, year 2 and on: master and contractor

• Study: The business section, the code section and the diagnostic habits in the master routine.
• Practice: Find why every failed part failed. Leave a commissioning sheet on every job. Write an estimate and a contract. Train a helper through stage 1.
• Explain: Your price from cost and target margin, what the contract must say, and what you check first on each system (the table in the systems section).
• Open: Math: What a million BTU costs by fuel and California Title 24: airflow, fan watts, leakage. Browse the Find a tool list in HVAC Tools in 3D for the pro-level tools.

Common mistakes of beginners

• Judging a temperature split before you know the airflow. A weak blower can make a good-looking split on a starved system.
• Adding refrigerant before checking the filter, coil and static pressure, or charging by suction pressure alone.
• Topping off a leaking system without finding the leak.
• Brazing without nitrogen, so scale forms inside the tube and later clogs the metering device.
• Reading gauge pressure as absolute, or using the chart for the wrong refrigerant. R-410A runs about 60 to 70% higher than R-22.
• Measuring the suction line temperature far from the service valve, or reading before the system has run 10 to 15 minutes.
• Jumping out a safety "to test" and forgetting it, or resetting a rollout and leaving.
• Swapping a capacitor for a different uF, or upsizing a breaker or 24 V fuse.
• Forgetting the flare nut before flaring, or over-torquing the nut.
• Using a standard recovery machine or ignition-prone tools on an A2L system.
• Sizing from the old unit or from square feet per ton instead of running the load calculation.

Self-check questions

Cover the answer column and try each question aloud.

Careers, certifications and training

Only the EPA 608 card is legally required to handle refrigerant. The rest raise pay and trust.

Mastery path, glossary and source ledger

The skill ladder. Move up only when you can do the step alone and explain why.

1. Helper: safety, tools by name, reading a manifold gauge, pulling a filter, cleaning a coil.
2. Install tech: flare and braze a line set with nitrogen, pressure test, evacuate to a micron gauge, land low-voltage wires.
3. Service tech: run the diagnostic routine, measure static pressure and temperature split, set charge by superheat and subcooling.
4. Lead tech: run a load calculation, pick equipment, design and test a duct system, pass a HERS verification.
5. Contractor: license, estimate, contract, permit, lien notices, warranty.

Practice drills.

• Tape up three different gauge readings and say what each means before checking the chart.
• Calculate CFM from a temperature split and a heat input, then measure it and explain the difference.
• Braze ten joints with nitrogen flowing, cut them open, and look inside for scale.
• Pull a vacuum with a bad hose, find the leak with the micron gauge, and write down how long it took.
• Walk a house and guess the load, then run the calculation and note the error.

Glossary.

Source ledger. What each part of this guide rests on:

The manufacturer table in the Installation section cites the manuals that were fetched. Where this guide and a manual disagree, the manual wins.