HVAC Excellence Master LC-AC cheat sheet

Sensible heat ratio (SHR) = sensible heat / total heat; total heat = sensible + latent. Typical comfort SHR is 0.70–0.80.

Standard airflow rule: 400 cfm per ton of cooling capacity (range 350–450 cfm/ton). Low airflow drops suction pressure and can freeze the evaporator coil.

Superheat = actual suction line temperature minus saturated suction temperature. Target superheat for a fixed-orifice (piston) system depends on indoor wet-bulb and outdoor dry-bulb; typical 8–14°F.

Subcooling = saturated condensing temperature minus liquid line temperature. Target subcooling for a TXV system is typically 8–12°F; low subcooling indicates undercharge or a restriction.

Evaporator TD (air temperature drop) = return air dry-bulb minus supply air dry-bulb; typical 15–22°F. High TD suggests low airflow; low TD suggests overcharge, high airflow, or low load.

Condenser TD = outdoor ambient minus saturated condensing temperature; typical 15–25°F. High TD indicates dirty condenser, non-condensables, or overcharge.

Delta T (wet-bulb) across evaporator is typically 18–22°F; use wet-bulb for latent capacity checks, dry-bulb for sensible.

Static pressure budget: typical residential supply static 0.5–0.8 in. w.c.; total external static pressure (TESP) above 0.8 in. w.c. signals duct restriction.

Refrigerant glide matters for blends (e.g., R-410A has ~0.2°F glide; R-407C ~10°F). Use dew point for superheat and bubble point for subcooling on zeotropic blends.

Combustion air: 1 cubic foot of natural gas (≈1,000 Btu) requires ~10 ft³ of air for complete combustion; provide 50 ft³ per 1,000 Btu input for dilution and ventilation.

Furnace temperature rise = supply air temp − return air temp; must fall within the nameplate range (typically 35–65°F). High rise = low airflow; low rise = high airflow or overfiring.

Gas manifold pressure: natural gas typically 3.5 in. w.c.; LP typically 10–11 in. w.c. Verify at the manifold with the burner operating.

Flue draft: natural-draft Category I appliances require −0.02 to −0.04 in. w.c. at the flue collar; positive pressure indicates blocked vent or spillage.

Combustion analysis targets: O₂ 4–9%, CO₂ 8–10% (natural gas), CO < 100 ppm air-free, excess air 25–50%.

Heat output (Btu/h) = cfm × 1.08 × ΔT (standard air). Use 1.08 for sensible heating, 4.5 × cfm × Δh for total.

AFUE (Annual Fuel Utilization Efficiency) is the seasonal efficiency metric for furnaces and boilers; minimum 80% for standard, 90%+ for condensing.

Condensing furnace flue condensate is acidic (pH ~3–5); neutralize before drain disposal and use PVC/CPVC or polypropylene venting.

Electric heat: 1 kW = 3,412 Btu/h; resistance heat output = volts × amps × 3.412 (Btu/h) or kW × 3,412.

Pressure–temperature relationship: for a given refrigerant, saturation pressure corresponds to saturation temperature; use PT charts to convert between them.

Superheat at evaporator outlet ensures no liquid reaches the compressor; typical target 8–12°F at the evaporator, 20–30°F at the compressor (suction).

Subcooling at condenser outlet ensures solid liquid at the metering device; typical 8–15°F for TXV systems; zero or low subcooling signals undercharge.

Compression ratio = absolute discharge pressure / absolute suction pressure. Ratios above ~10:1 cause high discharge temperature and reduced capacity; use two-stage or economizer.

Refrigerant oils: mineral oil for CFC/HCFC (R-12, R-22); POE/PVE for HFC/HFO (R-410A, R-134a, R-32). POE is hygroscopic - keep sealed and evacuate to <500 microns.

Evacuation standard: 500 microns or lower with a standing vacuum test (rise <500 microns in 10 minutes) before charging.

Recovery cylinder fill limit: 80% by volume for safety; never fill to 100%. Use scale to verify weight.

Leak detection: electronic leak detector sensitivity ~0.1 oz/yr; nitrogen pressure test at 150–300 psig for lines; never use oxygen or air with refrigerant.

Refrigerant glide: zeotropic blends fractionate; charge as liquid and use dew/bubble points for superheat/subcooling calculations.

Ohm's law: V = I × R. Power: P (watts) = V × I × PF for AC; for resistive loads PF = 1.

Single-phase motor current: I = (HP × 746) / (V × efficiency × PF). Three-phase: I = (HP × 746) / (√3 × V × efficiency × PF).

Locked-rotor amps (LRA) ≈ 5–6 × running-load amps (RLA) for typical PSC compressors; verify against nameplate.

Capacitor check: measure microfarads with a meter; tolerance typically ±6% of nameplate. Bulged or leaking capacitors must be replaced.

Motor winding resistance: run-to-common plus start-to-common should equal run-to-start; open winding reads infinite, shorted reads near zero.

Control voltage: 24 VAC typical for residential thermostats and contactors; 120/240 VAC for line-voltage controls. Verify transformer VA rating vs. connected load.

Thermostat wiring: R = 24V power, C = common, W = heat, Y = cool, G = fan, O/B = reversing valve, AUX/E = emergency/aux heat.

Defrost controls: time-temperature defrost initiates on accumulated compressor run time and coil temperature; terminates on coil temperature or time limit.

NEC reference: disconnecting means within sight of equipment; GFCI protection for outdoor receptacles; conductor ampacity per Table 310.16.