HVAC Excellence cheat sheet

Superheat = suction line temperature − saturation temperature at the measured suction pressure; read it at the evaporator outlet.

Subcooling = saturation temperature at liquid-line pressure − liquid-line temperature; read it at the condenser outlet.

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

R-410A saturation pressure at 70°F is about 201 psig; R-22 at 70°F is about 121 psig.

R-410A is a near-azeotropic blend, so it must be charged and recovered as a liquid to prevent composition shift.

Recovery cylinders are filled by weight and never above 80% of their water capacity.

Evacuate to 500 microns and hold a standing vacuum test; a rising reading indicates moisture, a leak, or both.

EPA 608 leak-repair thresholds for appliances of 50 lb charge or more: 10% comfort cooling, 20% commercial refrigeration, 35% industrial process.

E = I × R; P = E × I for resistive loads; single-phase P = E × I × power factor.

Locked-rotor amps are typically 5 to 6 times rated-load amps on a single-phase compressor.

Voltage imbalance on three-phase supply must not exceed 2%; current imbalance should not exceed 10%.

Run capacitor tolerance is ±10% of nameplate microfarads, measured after the capacitor is discharged.

Thermostat terminals: R = 24 V power, W = heat, Y = cooling, G = indoor fan, O/B = reversing valve.

Compressor winding check: run-to-common plus start-to-common equals run-to-start resistance.

Circuit conductors are sized at 125% of RLA; the disconnect must be rated at least 115% of RLA.

A 24 V control circuit is normally protected by a 3 A or 5 A fuse at the transformer.

CH4 + 2O2 → CO2 + 2H2O; roughly 10 ft³ of air per ft³ of natural gas for stoichiometric combustion.

Manifold pressure: about 3.5 in w.c. for natural gas, 10–11 in w.c. for propane.

Inlet gas pressure: about 7 in w.c. natural gas, 11–13 in w.c. propane.

Typical flue readings on an atmospheric gas burner: 4–9% O2 and about 8–10% CO2.

Air-free CO in undiluted flue gas should stay below roughly 400 ppm.

Natural-draft flue pressure runs about −0.02 to −0.04 in w.c., measured with a digital manometer.

Flame sense current is typically 1–6 µA DC; most controls drop out below about 0.5 µA.

Temperature rise = supply air − return air; compare with the nameplate range (commonly 35–65°F) and adjust blower speed.

Sensible heat: BTU/h = 1.08 × CFM × ΔT.

Total heat: BTU/h = 4.5 × CFM × Δh; latent heat: BTU/h = 0.68 × CFM × Δgrains.

Nominal cooling airflow is 400 CFM per ton; about 350 CFM/ton in humid climates and 450 CFM/ton in dry climates.

Standard air density is 0.075 lb/ft³ at 70°F and sea level.

Velocity pressure VP = (V ÷ 4005)²; velocity V = 4005 × √VP.

Total pressure = static pressure + velocity pressure.

Common duct sizing friction rate is 0.1 in w.c. per 100 ft of equivalent length.

Fan laws: CFM varies with RPM, static pressure with RPM², and brake horsepower with RPM³.

Blower door test pressure is 50 Pa; ACH50 = CFM50 × 60 ÷ conditioned volume in ft³.

Duct blaster test pressure is 25 Pa; leakage is reported in CFM25.

Total duct leakage minus leakage to outside equals leakage to conditioned space.

High-performance duct leakage target: about 6 CFM25 per 100 ft² of conditioned floor area, or about 10% of system airflow.

A tight new-construction envelope target is 3 ACH50 or lower.

Rough natural infiltration estimate: CFMnatural ≈ CFM50 ÷ 20.

Zone pressure diagnostics compare house-to-zone and zone-to-outside pressures to locate duct leakage paths.

Smoke pencils and infrared imaging locate envelope bypasses that pressure testing only quantifies.

Evaporator air temperature drop is typically 15–22°F; a low drop with correct charge points to low airflow.

Condenser approach = saturated condensing temperature − outdoor ambient; typically 10–15°F at design conditions.

Airflow from temperature rise: CFM = output BTU/h ÷ (1.08 × ΔT).

Sensible heat ratio = sensible capacity ÷ total capacity; typically 0.70–0.75 for residential cooling.

AFUE = annual heat output ÷ annual energy input; about 80% non-condensing, 90–98% condensing.

COP = refrigeration effect ÷ work input; EER = BTU/h ÷ watts.

Verify airflow within ±10% of design before judging refrigerant charge.

Charge verification methods: weigh-in, subcooling (TXV), or superheat (fixed orifice).

High head pressure causes: dirty condenser, overcharge, non-condensables, high ambient, failed condenser fan.

Low suction pressure causes: undercharge, restricted liquid line or metering device, dirty evaporator, low load.

High superheat plus low subcooling = undercharge; low superheat = overcharge or an overfeeding TXV.

High head pressure with high subcooling and a bubbly sight glass = non-condensables in the system.

Ice on the suction line indicates low charge, airflow restriction, or a restricted metering device.

90%+ AFUE furnaces condense flue gas and vent in PVC; flue gas must cool below about 130°F to condense.

SEER2 uses higher external static pressure test conditions than legacy SEER, so SEER2 values run roughly 4–5% lower.

No-cooling diagnostic order: verify airflow, then electrical, then refrigerant charge.