BPI AC/Heat Pump cheat sheet

Sensible heat formula: Q_s = 1.08 × CFM × ΔT (BTU/h, sea level); latent heat: Q_l = 0.68 × CFM × ΔW (grains/lb).

Total heat formula: Q_t = 4.5 × CFM × Δh (enthalpy difference in BTU/lb).

COP = cooling or heating output (BTU/h) ÷ electrical input (BTU/h); EER = BTU/h ÷ watts; SEER and HSPF are seasonal efficiency ratings.

Refrigerant cycle: compressor raises pressure/temperature, condenser rejects heat, metering device drops pressure, evaporator absorbs heat.

Superheat = suction line temperature − evaporator saturation temperature (measured at bulb); target 8–12°F for fixed orifice, 5–10°F for TXV.

Subcooling = condenser saturation temperature − liquid line temperature; target 8–12°F for TXV systems, 5–10°F for fixed orifice.

Heat pump reversing valve swaps evaporator and condenser roles; in heating, outdoor coil is evaporator and indoor coil is condenser.

Defrost cycle initiates when outdoor coil temperature falls below ~30°F and coil sensor or pressure differential triggers; terminates on time/temperature.

Latent heat of vaporization for R-410A at 40°F evaporator is approximately 125 BTU/lb; use manufacturer PT charts, not generic tables.

Manual J (ACCA) calculates room-by-room heating and cooling loads using orientation, insulation, windows, infiltration, and internal gains.

Sensible heat ratio (SHR) = sensible load ÷ total load; equipment selected must have SHR close to structure SHR to avoid humidity problems.

Manual S selects equipment using manufacturer expanded performance data at design conditions, not nominal capacity.

Manual D sizes ductwork: friction rate = available static pressure ÷ total effective length; typical residential supply friction 0.08–0.10 in. w.c. per 100 ft.

Return duct sizing: face velocity at filter grille should not exceed 300–400 fpm to avoid pressure drop and noise.

Refrigerant line sizing: suction line sized for pressure drop ≤ 2–3 psi equivalent; liquid line ≤ 1–2 psi; follow manufacturer tables.

Heat pump balance point: outdoor temperature where heat pump capacity equals building load; below it, auxiliary heat supplements.

Ventilation: ASHRAE 62.2 requires mechanical ventilation when infiltration is below 0.35 ACH natural; calculate CFM = 0.03 × floor area + 7.5 × (bedrooms + 1).

Braze with nitrogen purge at 1–2 psig to prevent oxidation inside tubing; use silver-phos or 15% silver alloy depending on joint type.

Evacuate to 500 microns or below and hold 500 microns for 10 minutes with pump isolated; decay indicates leak or moisture.

Charge by weight when line set length differs from factory charge; adjust per manufacturer table (typically 0.6 oz per foot of 3/8" liquid line for R-410A).

Commissioning checks: static pressure, temperature split (18–22°F cooling), superheat/subcooling, amp draw vs nameplate, refrigerant pressures vs ambient.

Electrical: verify supply voltage within ±10% of nameplate, compressor locked-rotor amps, and correct breaker/wire size per NEC 440.

Condensate drain must have trap (depth ≥ total static pressure) and slope 1/8" per foot; secondary pan with float switch required.

Airflow verification: measure external static pressure and compare to fan performance table; target 350–450 CFM/ton in cooling, 400–450 CFM/ton in heating.

Document startup readings on commissioning sheet: ambient, return/supply temps, pressures, superheat, subcooling, amps, and filter size.

Cooling temperature split = return air dry-bulb − supply air dry-bulb; normal 18–22°F; low split indicates low airflow or charge issue.

High superheat + low suction pressure = undercharge or restriction; low superheat + high suction pressure = overcharge or metering overfeed.

High subcooling + high head pressure = overcharge or condenser airflow restriction; low subcooling + bubbles = undercharge.

Compressor amp draw above nameplate RLA with high head = mechanical or electrical fault; check capacitor, windings, and contactor.

Heat pump heating mode: check outdoor fan operation, defrost termination, reversing valve solenoid, and auxiliary heat staging.

Use a multimeter with clamp to measure capacitor microfarads; replace if outside ±10% of rated value.

Leak detection: electronic leak detector, UV dye, or nitrogen pressure test at 150–300 psig; never use oxygen or acetylene.

Verify airflow with a balometer at registers; total measured CFM should match design within ±10%.

Thermostat diagnostics: check R–Y, R–G, R–W call voltages (24 VAC ±10%), and verify anticipator or cycle rate settings.

Lockout/tagout: disconnect power at breaker, verify with meter, and apply lock before servicing; never rely on thermostat off.

Refrigerant safety: use gloves and eye protection; R-410A is high pressure (≈1.6× R-22); never heat a sealed system or mix refrigerants.

EPA Section 608: technicians must be certified to purchase or handle refrigerant; recovery required before opening system; venting prohibited.

Electrical safety: verify 24 VAC control voltage and line voltage separately; treat all conductors as live until tested.

Documentation: record model/serial, refrigerant type and charge, filter sizes, startup readings, and any deficiencies on invoice.

Customer communication: explain findings in plain language, provide written estimate before work, and confirm satisfaction at completion.

Personal protective equipment: safety glasses, gloves, and steel-toe boots; use fall protection on roofs above 6 ft.

Warranty documentation: retain proof of installation date, maintenance records, and refrigerant charge verification for manufacturer claims.