HVAC Formulas Reference
Complete collection of HVAC formulas for service techs and installers. From air-side heat and superheat to duct sizing and the electrical math you use in the field.
Get Formulas in AppAir-Side Formulas
The core heat-transfer and airflow relationships for the air side of any system.
Sensible heat (BTU/hr)
Total heat, enthalpy-based (BTU/hr)
Latent heat (BTU/hr)
Sensible heat ratio (~0.75 typical AC)
Airflow from sensible heat
Airflow from velocity
Velocity (FPM) from velocity pressure
Capacity in tons (1 ton = 12,000 BTU/hr)
Where: CFM = airflow (cubic feet/min), ΔT = dry-bulb temperature difference (°F), Δh = enthalpy difference (BTU/lb), ΔW = humidity ratio difference (grains/lb), VP = velocity pressure (in. w.c.). The 1.08 constant = 0.075 lb/ft³ × 60 min/hr × 0.24 BTU/lb·°F.
Refrigerant-Side Formulas
Charge verification and compressor health, read straight off your gauges and pipe clamps.
Superheat
TXV target 8–14°F. Below 5°F = flooding/overcharge risk; above 20°F = starved/undercharged.
Subcooling
Target 10–18°F (verify nameplate). Low SC = undercharge; high SC = overcharge.
Target Superheat (Fixed Orifice)
Carrier method. WB = indoor wet bulb, ODB = outdoor dry bulb (°F). Varies by manufacturer.
Compression Ratio
Uses absolute pressures. Healthy is typically ~2.5:1 to 3.5:1.
Saturation temps come off the PT chart for the specific refrigerant (R-410A, R-454B, R-22, R-32, R-407C) at your measured suction and liquid/discharge pressures.
Hydronic (Water-Side) Formulas
Heat transfer for chilled- and hot-water systems on the commercial side.
Water Heat Transfer
BTU/hr. 500 = 8.33 lb/gal × 60 min/hr × 1.0 BTU/lb·°F.
GPM per Ton
At a 10°F ΔT this works out to ≈ 2.4 GPM/ton.
Where: GPM = flow rate (gallons/min), ΔT = supply-to-return water temperature difference (°F).
Duct Sizing Formulas
Friction rate, area, and velocity for laying out supply and return duct.
Friction Rate
iwc per 100 ft. ASP = available static pressure, TEL = total equivalent length.
Round Duct Area
Square inches. D = duct diameter (in).
Equivalent Round
a, b = rectangular duct dimensions (in).
Velocity
FPM. Area in square inches. Target ~700–900 FPM supply, ~600–700 return.
Electrical Formulas (HVAC-Related)
The electrical math you actually use on compressors, motors, and capacitors.
Power (Single-Phase)
PF = power factor (typically 0.85–0.95).
Power (Three-Phase)
1.732 = √3 for three-phase equipment.
Ohm's Law
I = current (amps), R = resistance (ohms).
Capacitor Test
Replace if measured microfarads are more than 6% off the rated value.
Always size wire and breaker from the nameplate — MCA (Minimum Circuit Ampacity) for wire gauge and MOCP (Maximum Overcurrent Protection) for breaker. Compressor inrush is the exception to normal sizing rules.
Field Target Ranges
The benchmarks techs check against every service call.
Cooling Delta-T (supply vs return)
Concern below 14°F or above 22°F
Superheat (TXV system)
Below 5°F flooding, above 20°F starved
Subcooling
Varies by manufacturer — check nameplate
Total External Static Pressure
Concern above 0.80 iwc
Airflow Rule of Thumb
Standard airflow target for cooling
Condenser Split (ambient vs cond sat)
Above 30°F: dirty condenser or overcharge
Estimates for reference only — always verify against the manufacturer's specs, the equipment nameplate, and local code. A full Manual J is required for real load sizing.
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