AL HVAC cheat sheet

Superheat = suction line temperature minus saturation temperature at suction pressure; measured at the evaporator outlet.

Subcooling = saturation temperature at liquid line pressure minus liquid line temperature; typical target 8–12°F on TXV systems.

Compression ratio = absolute discharge pressure ÷ absolute suction pressure; ratios above roughly 10:1 raise discharge temperature and stress compressor valves.

The four core components: compressor raises pressure, condenser rejects heat, metering device drops pressure, evaporator absorbs heat.

At saturation, a pure refrigerant boils and condenses at a single temperature for a given pressure; PT charts give that relationship for R-410A, R-32, and R-454B.

R-410A is a near-azeotropic 50/50 blend of R-32 and R-125 and must be charged as a liquid from the cylinder to avoid fractionation.

The evaporator absorbs mostly latent heat; the condenser rejects desuperheating sensible heat plus the latent heat of condensation.

When a system is off, refrigerant migrates to the coldest point; crankcase heaters and pump-down cycles limit migration and liquid floodback.

In zeotropic blends with glide, use the dew point for superheat and the bubble point for subcooling calculations.

Natural gas manifold pressure is typically 3.5 in. w.c.; LP gas is typically 10–11 in. w.c.; always confirm against the rating plate.

Temperature rise = supply air temperature minus return air temperature, and it must fall inside the range printed on the rating plate.

1 watt of electric resistance heat = 3.412 Btu/h, so a 5 kW heater delivers about 17,060 Btu/h.

Unconfined-space rule: 50 ft³ of indoor volume per 1,000 Btu/h of total input is required for combustion, draft, and dilution air.

AFUE is annual fuel utilization efficiency; 80% units are non-condensing while 90%+ condensing furnaces recover latent heat and require a condensate drain.

Heat pump balance point = outdoor temperature where capacity equals building heat loss; below it, auxiliary or supplemental heat stages run.

Defrost is initiated by outdoor coil temperature and/or demand; the reversing valve shifts to cooling mode and auxiliary heat tempers supply air.

No. 2 heating oil delivers roughly 138,000–140,000 Btu per gallon, so nozzle size in GPH × 140,000 approximates input Btu/h.

COP = heat output (Btu) ÷ energy input (Btu), and HSPF ≈ COP × 3.412.

1 ton of refrigeration = 12,000 Btu/h; nominal airflow for standard residential cooling is 400 CFM per ton.

Sensible heat ratio (SHR) = sensible capacity ÷ total capacity, where total capacity = sensible + latent.

A normal air temperature split across an operating evaporator coil is 16–22°F.

Design evaporator temperature difference (entering air dry bulb minus evaporating temperature) is about 35°F.

Design condenser temperature difference (outdoor ambient to condensing temperature) is about 15–20°F at design conditions.

Chilled water coils commonly use 44°F supply and 54°F return water, a 10°F delta-T.

Fixed-orifice systems are charged by superheat; TXV systems are charged by subcooling.

Low-ambient operation requires head pressure control such as fan cycling, fan speed control, or a flooding valve to keep condensing pressure up.

Coil bypass factor decreases as coil surface area increases or as airflow through the coil decreases.

Ohm's law: E = I × R; single-phase power = E × I × power factor.

Three-phase power = 1.732 × line voltage × line current × power factor.

Locked rotor amps (LRA) is roughly 5–6 times rated load amps; a compressor held at LRA trips the overload.

Run capacitors should read within ±6% of rated microfarads; start capacitors are short-time rated and include bleed resistors.

Residential control circuits are 24 VAC; the C terminal supplies the common return needed by smart thermostats.

Motor service factor, such as 1.15, permits a small overload above nameplate amps at rated ambient conditions.

PSC blower speed is selected by choosing motor taps; ECM blower speed is set by low-voltage programming taps.

Contactor pitting or welded contacts cause single-phasing; verify line-to-line voltage on all three legs under load.

A 2% voltage imbalance in a three-phase supply can produce roughly 8–10 times that percentage in motor winding heating.

Sensible airflow: CFM ≈ sensible Btu/h ÷ (1.08 × temperature difference).

Duct velocity in feet per minute = CFM ÷ free area in square feet.

Equal-friction residential duct design uses about 0.1 in. w.c. static loss per 100 ft of equivalent length.

A typical residential air handler has 0.5 in. w.c. of external static pressure available, split between supply and return.

Flex duct must be pulled tight; a sag equal to 1.5 times the duct diameter can substantially cut delivered capacity.

Return grilles are sized for lower face velocity (about 500 FPM) than supply diffusers to reduce noise.

Seal duct joints with mastic or UL 181 tape; leakage in unconditioned space adds directly to the building load.

Each 90° elbow is roughly equivalent to 30–40 ft of straight duct in equivalent-length calculations.

ASHRAE 62.2 whole-house ventilation rate ≈ 0.03 × conditioned floor area + 7.5 × (bedrooms + 1) CFM.

Dry nitrogen must flow through the tubing during brazing to displace oxygen and prevent internal oxide and carbon scale.

Copper-to-copper joints use phosphorus-bearing brazing filler that self-fluxes; copper to brass or steel requires a separate flux.

Suction lines are sized for about 1–2°F equivalent pressure drop; liquid lines for about 1°F.

ACR tubing is cleaned, dehydrated, and nitrogen-charged and is the only grade acceptable for refrigerant piping.

Evacuate to 500 microns or lower and hold with the pump isolated; a rising reading indicates moisture or a leak.

Oil traps at the base of a suction riser, and a trap every 15–20 ft of riser, return oil to the compressor.

Soft copper minimum bend radius is roughly 3–5 times the tube diameter; a bender prevents kinking and flattening.

Flare fittings use a 45° flare and must be torqued to manufacturer specification to avoid leaks.

Triple evacuation with nitrogen sweeps between pulls removes moisture from large or wet systems.

ACCA Manual J = residential load calculation; Manual S = equipment selection; Manual D = duct design; Manual T = diffuser selection.

Winter heat loss = U × A × temperature difference summed over all envelope surfaces, plus infiltration and ventilation load.

Cooling load components: conduction through envelope, solar gain through glazing, internal gains, infiltration, and duct gains.

Use 1% summer dry-bulb and 99% winter design temperatures for the location rather than record extremes.

Lower solar heat gain coefficient (SHGC) glazing reduces solar load; low-E coatings cut both solar and long-wave radiation.

Ducts located in unconditioned attics gain or lose heat; apply duct gain/loss multipliers from Manual J.

Infiltration latent load (Btu/h) = 0.68 × CFM × humidity ratio difference in grains.

Oversizing causes short cycling, poor latent removal, and elevated indoor humidity, so equipment is sized to the calculated load.

As altitude rises, air density falls, so the 1.08 sensible heat factor becomes proportionally smaller.

Pressure test with dry nitrogen at the manufacturer's specified test pressure, commonly 150–300 psig for R-410A systems.

Acceptable evacuation is 500 microns or below with the pump valved off and a 10–15 minute decay hold.

Weigh-in charging is used for new systems or when the line set differs from the factory charge, adding the per-foot adjustment.

Startup documentation records final superheat or subcooling, temperature split, static pressure, voltage, and amp draw.

Air balance: set blower speed for design CFM, then adjust branch dampers; total airflow within ±10% of design.

Refrigerant must never be used to leak-test or purge lines; nitrogen serves both purposes.

Before startup, verify the condensate trap, drain slope, and secondary pan float switch.

Supply voltage must be within 10% of nameplate, phase balance acceptable, lugs tight, and breaker and wire size correct.

Confirm the nameplate refrigerant type and charging method before adding refrigerant; mixing refrigerants is prohibited.

High superheat with low suction pressure = starved evaporator (undercharge, restriction, or failed metering device).

Low superheat with high suction pressure = overcharge or overfeeding metering device.

Low superheat with low suction pressure = restricted evaporator airflow or a flooded coil.

High head pressure with high subcooling suggests non-condensables or overcharge; high head with normal subcooling suggests a dirty condenser or high ambient.

Frost or ice on the suction line and evaporator indicates low charge, restricted airflow, or a blockage.

Compressor short cycling causes include high/low pressure switches, internal overload, thermostat faults, and control board failures.

Capacitor diagnosis: bulge, oil leak, or an out-of-tolerance microfarad reading means replacement; discharge before testing.

A low temperature split with normal airflow indicates low charge or poor compressor performance.

High amp draw on a motor points to high head pressure, low supply voltage, bad bearings, or shorted windings.

EPA Section 608 certification is required to service, maintain, or dispose of appliances containing regulated refrigerants.

Knowingly venting refrigerant to the atmosphere is prohibited; recovery equipment must be used.

Recovery cylinders are filled by weight and never above 80% of capacity, leaving room for liquid expansion.

R-410A has zero ODP and a GWP near 2,088; R-22 has an ODP of 0.055 and has been banned from US production and import since 2020.

A2L refrigerants such as R-32 and R-454B are mildly flammable, so charge limits, ignition-source clearance, and leak detection apply.

EPA carbon monoxide limits are 9 ppm for 8 hours and 35 ppm for 1 hour; CO alarms are installed per manufacturer instructions.

Liquid refrigerant causes frostbite and eye injury; wear safety glasses and insulated gloves and never trap liquid between valves.

Lockout/tagout: lock the disconnect, verify with a meter, and discharge capacitors before servicing.

Section 608 leak repair rules apply to appliances containing 50 or more pounds of refrigerant, with allowable leak rates that vary by appliance type.