HI HVAC cheat sheet

Sensible heat formula: Btu/h = 1.08 × CFM × ΔT for standard air at sea level; the 1.08 factor derives from 60 min/h × 0.075 lb/ft³ × 0.24 Btu/lb·°F.

Heating equipment output is rated in Btu/h input and output; steady-state efficiency = output ÷ input. AFUE is the seasonal rating that accounts for cycling and standby losses.

Gas furnace ignition sequence: call for heat → inducer proves draft via pressure switch → igniter energizes → gas valve opens → flame sensor proves flame → blower starts on time delay.

A thermocouple or flame rectification sensor proves main burner flame; a dirty or misaligned sensor causes short-cycling or lockout, not a no-heat condition with proven flame.

Condensing furnaces recover latent heat by cooling flue gas below the dew point (about 130°F for natural gas products); PVC venting is allowed because flue gas is cool and slightly acidic.

Hydronic boiler: 1 gallon of water weighs 8.33 lb; Btu/h = 500 × GPM × ΔT. A circulator moving 10 GPM at 20°F ΔT delivers about 100,000 Btu/h.

Electric resistance heat produces 3.413 Btu per watt-hour; a 5 kW strip heater delivers about 17,065 Btu/h at 100% efficiency.

Heat pump balance point is the outdoor temperature where capacity equals building load; below it, supplemental or aux heat (gas, electric strip) makes up the difference.

Combustion air: a Category I appliance needs roughly 50 ft³ per 1,000 Btu/h input of indoor volume, or two permanent openings sized per the mechanical code when using outdoor air.

Ventilation rate equation: Q = 60 × ACH × Volume ÷ 60 = ACH × Volume; for a 2,000 ft³ room at 0.5 ACH, Q = 1,000 CFM of outdoor air equivalent.

ASHRAE 62.1 uses the breathing-zone outdoor airflow: Vbz = Rp × Pz + Ra × Az, where Rp is people outdoor rate, Ra area rate, Pz zone population, Az zone floor area.

Demand-controlled ventilation uses CO2 sensors to modulate outdoor air; CO2 is a proxy for occupancy, not a direct measure of contaminant concentration.

MERV ratings: 8–13 for residential and light commercial filters; MERV 13+ needed for fine particles; HEPA (99.97% at 0.3 µm) for critical spaces.

Pressure relationship: negative pressure in a room means exhaust exceeds supply; positive means supply exceeds exhaust. Confine odors and contaminants with negative pressure.

Dilution ventilation reduces concentration by mixing outdoor air; local exhaust captures contaminant at the source and is generally more energy-efficient.

Makeup air must balance exhaust: kitchen hoods need 100% makeup air or a compensating supply; unbalanced exhaust depressurizes the building and can backdraft combustion appliances.

Relative humidity target 30–50% for occupied spaces; below 30% causes static and dryness, above 60% supports mold and dust mites.

Radon mitigation uses sub-slab depressurization; test result of 4 pCi/L or higher triggers mitigation per EPA guidance.

Refrigeration cycle order: compressor → condenser → metering device → evaporator → back to compressor. The compressor raises pressure and temperature of vapor.

Superheat = actual suction line temp − saturated suction temp at that pressure; typical target 8–12°F at the evaporator outlet for TXV systems.

Subcooling = saturated condensing temp − liquid line temp; typical target 8–12°F at the condenser outlet for TXV systems; low subcooling indicates undercharge.

Sensible heat ratio (SHR) = sensible capacity ÷ total capacity; typical residential cooling SHR is 0.70–0.80. Latent capacity handles moisture removal.

Total cooling capacity: Btu/h = 4.5 × CFM × Δh (enthalpy difference), where 4.5 = 60 × 0.075 lb/ft³.

Evaporator temperature must stay above 32°F to avoid coil icing; low airflow, low charge, or restricted metering device drops coil temperature below freezing.

Refrigerant pressure–temperature relationship: each refrigerant has a unique PT chart; R-410A operates at roughly 1.6× the pressure of R-22 at the same temperature.

Compression ratio = absolute discharge pressure ÷ absolute suction pressure; ratios above about 10:1 on single-stage systems risk high discharge temperature.

R-410A is a zeotropic blend requiring liquid charging; vapor charging can fractionate the blend and alter composition.

Duct sizing by equal friction: total equivalent length × friction rate (in. w.c. per 100 ft) gives available static; typical residential supply trunk uses 0.08–0.10 in. w.c./100 ft.

Velocity pressure: VP = (V ÷ 4,005)² in. w.c.; total pressure = static + velocity pressure. Duct static is measured with a manometer.

Equivalent length accounts for fittings: a 90° radius elbow equals roughly 30 ft of straight duct; a hard 90° tee can equal 60–100 ft.

Friction loss in round duct: ΔP varies with velocity squared and inversely with diameter; doubling diameter reduces friction by roughly 16× at the same flow.

Duct leakage: seal all joints with mastic or UL 181 tape; flex duct must be fully extended and supported every 4–5 ft, with no sharp bends.

Return air path: return grille free area should be about 2 ft² per ton; undersized returns raise static pressure and reduce airflow.

Piping: refrigerant lines should be sized by equivalent length; a suction line velocity below about 500 ft/min risks oil return failure; above 4,000 ft/min causes noise.

Water piping: 1 psi of head equals 2.31 ft of water column; a pump must overcome friction plus static lift.

TXV and EEV both maintain superheat by modulating refrigerant flow; a clogged screen or stuck valve causes low suction pressure and high superheat.

Ohm's law: V = I × R; power P = V × I = I²R. A 240 V, 30 A circuit draws 7,200 W.

A thermostat's R terminal is 24 V hot; W calls heat, Y calls cooling, G calls fan, O/B reverses valve on heat pumps, C is common for power.

A contactor coil is typically 24 V; a shorted coil blows the low-voltage fuse. Measure coil resistance and compare with manufacturer spec.

Sequence of operation for a call for cooling: thermostat Y and G → contactor pulls in → compressor and condenser fan start → indoor blower starts after delay.

A capacitor stores energy; discharge before testing. A bulged top or reading below 10% of rated µF indicates replacement.

Locked rotor amps (LRA) are much higher than running load amps (RLA); a compressor that trips on overload within seconds often has a seized rotor or shorted winding.

Superheat and subcooling are the primary charge diagnostics; static pressure and temperature rise diagnose airflow. Target temperature rise matches the data plate ±10%.

Voltage imbalance across three-phase legs should stay under 2%; current imbalance under 10%. Higher imbalance derates motor capacity.

A blower motor with a failed run capacitor humps and overheats; the motor may spin freely by hand but not start under load.

Manual J heating load: sum of transmission (U × A × ΔT) and infiltration (1.08 × CFM × ΔT) losses; cooling load uses Manual J's radiant and internal gain methods.

Manual S selects equipment within 100% of the calculated load; oversizing causes short cycling, poor dehumidification, and higher energy use.

Manual D sizes ducts from the equipment airflow and available static pressure; Manual T sizes registers and grilles.

Combustion safety: check for spillage at the draft hood, measure CO in flue gas (target under 100 ppm air-free), and verify draft pressure.

Gas piping: size by Btu/h input and length using the code's capacity table; add the pressure drop of fittings and the appliance regulator.

Clearance to combustibles for a Category I furnace is typically 1 in. to the vent connector and 6 in. to single-wall flue pipe; always follow the listing label.

Refrigerant safety: ASHRAE 15/34 classify refrigerants; A2L refrigerants (e.g., R-454B, R-32) are mildly flammable and require ignition-source controls and leak detection in occupied spaces.

Electrical: branch circuits must be sized per NEC Article 440 for hermetic motor compressors; nameplate minimum circuit ampacity and maximum fuse size govern wire and breaker selection.

Working clearance at electrical panels: 30 in. wide, 36 in. deep, 6.5 ft high per NEC 110.26.