AR HVAC cheat sheet

1 ton of refrigeration = 12,000 BTU/h = 200 BTU/min = 3.517 kW.

Sensible heat (standard air): BTU/h = 1.08 × CFM × ΔT dry bulb.

Latent heat (standard air): BTU/h = 0.68 × CFM × Δgrains-per-pound.

Total heat: BTU/h = 4.5 × CFM × Δh (enthalpy difference).

Superheat = actual vapor temperature − saturation temperature at suction pressure; subcooling = saturation temperature − liquid line temperature.

Standard air density is 0.075 lb/ft³; 1 kW = 3,412 BTU/h; 1 boiler horsepower = 33,475 BTU/h.

Evacuation target is 500 microns, holding below 1,000 microns after the pump is isolated; a manifold gauge is not a vacuum instrument.

Pressure testing is done with dry nitrogen only - oxygen or shop air can explode when mixed with refrigerant oil.

EPA Section 608 prohibits knowingly venting refrigerant during service, maintenance, or disposal; liquid refrigerant contact causes frostbite, so gloves and eye protection are required.

Cycle order: compressor → condenser → metering device → evaporator → back to compressor.

Compressor raises vapor pressure and temperature; condenser rejects heat and subcools liquid; metering device drops pressure and flashes part of the refrigerant; evaporator absorbs heat and superheats vapor.

Fixed-orifice target superheat is commonly 10–15°F; a TXV holds roughly 5–10°F superheat; target subcooling is commonly 8–12°F, with manufacturer values always controlling.

Low superheat with high suction pressure indicates overcharge or overfeed; high superheat with low suction pressure indicates undercharge or a restriction.

R-410A is a near-azeotropic blend of 50% R-32 and 50% R-125, operates roughly 50–70% above R-22 pressures, and must be charged as a liquid.

ASHRAE safety groups: A1 = low toxicity, no flame propagation (R-22, R-410A); A2L = mildly flammable (R-32, R-454B); B1 = higher toxicity (R-123).

ODP ranking: CFC highest, HCFC lower, HFC zero. Approximate GWP: R-410A 2,088; R-22 1,810; R-134a 1,430; R-32 675.

Accessory functions: filter drier removes moisture, acid, and particulate; sight glass shows flash gas and moisture; accumulator blocks liquid floodback; crankcase heater prevents refrigerant migration; heat pumps use bi-flow driers.

Superheat is measured at the evaporator outlet; subcooling is measured at the condenser outlet.

E = I × R; P = E × I; single-phase P = E × I × power factor; balanced three-phase P = 1.732 × E × I × power factor.

Discharge capacitors before testing; a measured microfarad value within about ±10% of the rated value is normally acceptable.

PSC motor uses a run capacitor only; CSCR adds a start capacitor with a potential relay; CSIR uses a start capacitor with a current relay.

Compressor terminals: highest resistance is across common-to-start, lowest across common-to-run, and run-to-start equals the sum of the other two.

Three-phase voltage imbalance should not exceed 2%; greater imbalance causes motor overheating and shortened life.

NEC Article 440 governs air-conditioning and refrigeration equipment; branch-circuit conductors must be at least 125% of the compressor rated-load current plus other loads.

Residential low-voltage control is 24 V from a step-down transformer; high-pressure, low-pressure, float, and freeze stats break that 24 V circuit to stop the unit.

Standard electrical diagnostics: supply voltage, amperage versus nameplate RLA/FLA, capacitor microfarads, contactor pitting, overload continuity, and winding resistance to ground with a megohmmeter.

A compressor drawing locked-rotor amps without starting points to a seized rotor, failed capacitor, or open start circuit.

ACCA Manual J = room-by-room load calculation; Manual S = equipment selection; Manual D = duct design; Manual T = air distribution.

Nominal cooling airflow is 400 CFM per ton; reducing to roughly 350 CFM per ton improves latent (moisture) removal in humid climates.

Total external static pressure is the sum of supply and return static; residential air handlers are commonly rated at 0.5 in. w.c., checked against the fan performance table.

Residential duct sizing typically uses a friction rate of about 0.1 in. w.c. per 100 ft of equivalent length.

The condensate drain requires a trap, must slope at least 1/8 in. per foot, and must not be lifted above the drain pan outlet.

After nitrogen pressure testing and leak repair, evacuate to 500 microns; maintain a slow nitrogen purge during brazing to prevent internal oxidation.

Charging methods: weigh-in using a scale and nameplate charge is most accurate; superheat is used for fixed-orifice systems and subcooling for TXV systems.

Recovery cylinders must not be filled beyond 80% of capacity by weight; EPA Section 608 certification is required to purchase and handle refrigerant.

Condenser clearances follow the manufacturer (commonly 12–18 in. at the sides and 5 ft above); efficiency is rated by SEER2 for cooling, HSPF2 for heat pump heating, and AFUE for furnaces.

ACCA Manual J = room-by-room load calculation; Manual S = equipment selection; Manual D = duct design; Manual T = air distribution.

Nominal cooling airflow is 400 CFM per ton; reducing to roughly 350 CFM per ton improves latent (moisture) removal in humid climates.

Total external static pressure is the sum of supply and return static; residential air handlers are commonly rated at 0.5 in. w.c., checked against the fan performance table.

Residential duct sizing typically uses a friction rate of about 0.1 in. w.c. per 100 ft of equivalent length.

The condensate drain requires a trap, must slope at least 1/8 in. per foot, and must not be lifted above the drain pan outlet.

After nitrogen pressure testing and leak repair, evacuate to 500 microns; maintain a slow nitrogen purge during brazing to prevent internal oxidation.

Charging methods: weigh-in using a scale and nameplate charge is most accurate; superheat is used for fixed-orifice systems and subcooling for TXV systems.

Recovery cylinders must not be filled beyond 80% of capacity by weight; EPA Section 608 certification is required to purchase and handle refrigerant.

Condenser clearances follow the manufacturer (commonly 12–18 in. at the sides and 5 ft above); efficiency is rated by SEER2 for cooling, HSPF2 for heat pump heating, and AFUE for furnaces.