HVAC Formula Reference
Every formula used in HVAC field work and system design — organized by workflow. Each entry includes the formula, variable definitions, typical values, and a link to the free calculator that does the arithmetic for you.
BTU Cooling Load (rule of thumb)
Variables
- sqft
- — Conditioned floor area
- ceiling_factor
- — Ceiling height ÷ 8 (e.g. 10 ft ceilings → 1.25)
- climate_factor
- — 0.85 (cool) · 1.0 (moderate) · 1.15 (hot/humid)
- sun_factor
- — 0.95 (shaded) · 1.0 (average) · 1.10 (sunny, large windows)
- insulation_factor
- — 0.90 (well insulated) · 1.0 (average) · 1.15 (poor)
Normal range / typical values
~20 BTU/sqft moderate climate; ranges from 14 to 25+ BTU/sqft
Tons of Cooling
Variables
- BTU/h
- — Calculated cooling load or equipment capacity
- 12,000
- — BTU/h per ton (heat absorbed melting 1 ton of ice in 24 h)
Normal range / typical values
Residential: 1.5–5 tons; commercial: 5–hundreds of tons
Furnace Input BTU
Variables
- Output BTU/h
- — Heat delivered to the building (from load calculation)
- AFUE
- — Annual Fuel Utilization Efficiency as a decimal (e.g. 0.95)
Normal range / typical values
80% AFUE furnace requires 25% more input than output
Heat Loss (simplified Manual J)
Variables
- BTU/sqft/zone
- — Zone 1: 25 · Zone 2: 35 · Zone 3: 45 · Zone 4: 55 · Zone 5: 70 · Zone 6: 85 · Zone 7: 100
- insulation_factor
- — 0.75 (well insulated) · 1.0 (average) · 1.3 (poor)
- window_factor
- — 0.9 (few windows) · 1.0 (average) · 1.15 (many windows)
Normal range / typical values
Zone 4 home: ~55 BTU/h per sqft at design conditions
CFM from Room Volume
Variables
- Volume
- — Length × width × ceiling height in cubic feet
- ACH
- — Required air changes per hour (2–6 for residences, 8–12 for kitchens)
- 60
- — Minutes per hour
Normal range / typical values
~400 CFM per ton of cooling for a split system
Duct Friction Rate
Variables
- FR
- — Friction rate in in.w.c. per 100 ft
- ASP
- — Available static pressure = equipment rated ESP − component losses
- TEL
- — Total equivalent length of the duct run (ft of straight duct + fitting equivalents)
Normal range / typical values
0.05–0.10 in.w.c./100 ft residential; 0.08–0.12 commercial
Fan Laws
Variables
- N
- — Fan speed in RPM
- CFM
- — Airflow
- SP
- — Static pressure
- BHP
- — Brake horsepower (shaft power)
Normal range / typical values
Doubling fan speed doubles CFM, quadruples SP, and requires 8× the power
Delta T (Cooling)
Variables
- T_return
- — Return air dry-bulb temperature at the air handler (°F)
- T_supply
- — Supply air dry-bulb temperature at the supply plenum (°F)
Normal range / typical values
16–22°F at proper charge, airflow, and load. <14°F → low airflow or overcharge. >22°F → low charge or low airflow
Total External Static Pressure
Variables
- SP_supply
- — Positive pressure in supply plenum (in.w.c.)
- SP_return
- — Negative pressure in return plenum (in.w.c., taken as absolute value)
Normal range / typical values
Equipment rated at 0.5 in.w.c.; above 0.8 in.w.c. degrades airflow significantly
Superheat (Suction Side)
Variables
- T_suction_line
- — Suction line temperature measured 6 in. from compressor (°F)
- T_saturation_suction
- — Saturation temperature from PT chart at suction pressure (°F)
Normal range / typical values
TXV system: 8–12°F. Fixed orifice: use target superheat formula
Target Superheat (Fixed Orifice)
Variables
- WB_indoor
- — Indoor wet-bulb temperature at the return (°F)
- DB_outdoor
- — Outdoor dry-bulb temperature (°F)
Normal range / typical values
Typically 5–30°F depending on conditions
Subcooling (Liquid Line)
Variables
- T_saturation_liquid
- — Saturation temperature from PT chart at high-side pressure (°F)
- T_liquid_line
- — Liquid line temperature measured at the service valve (°F)
Normal range / typical values
TXV system: 10–20°F. Fixed orifice: 5–10°F
Dew Point (Magnus Approximation)
Variables
- T
- — Air temperature (°C for full formula; °F for simplified)
- RH
- — Relative humidity (%)
Normal range / typical values
At 75°F / 50% RH, dew point ≈ 55°F
Sensible Heat
Variables
- 1.08
- — 0.075 lb/ft³ (air density) × 0.24 BTU/(lb·°F) (specific heat) × 60 min/h
- CFM
- — Airflow rate
- ΔT
- — Temperature difference across coil or duct (°F)
Normal range / typical values
~1,000 BTU/h per 100 CFM per °F
Latent Heat
Variables
- 0.68
- — 0.075 lb/ft³ × 1,061 BTU/lb (latent heat of vaporization) × 60 min/h ÷ 7,000 grains/lb
- Δgrains
- — Difference in humidity ratio (grains of water per pound of dry air)
Normal range / typical values
Moisture removal is typically 20–30% of total cooling load in humid climates
Mixed Air Temperature
Variables
- CFM_OA / T_OA
- — Outdoor air flow and temperature
- CFM_RA / T_RA
- — Return air flow and temperature
Normal range / typical values
With 20% OA at 95°F and 80% RA at 75°F → MAT = 79°F
ASHRAE 62.2 Ventilation Rate
Variables
- floor_area
- — Conditioned floor area in square feet
- bedrooms + 1
- — Estimated occupancy from number of bedrooms
Normal range / typical values
Typical 2,000 sqft 3-bedroom: 60 + 30 = 90 CFM continuous ventilation
HRV/ERV Sensible Effectiveness
Variables
- T_supply_out
- — Temperature of fresh air leaving the core (after heat exchange)
- T_supply_in
- — Outdoor fresh air temperature entering the core
- T_exhaust_in
- — Indoor exhaust air temperature entering the core
Normal range / typical values
Good HRV/ERV: 70–85% sensible effectiveness; premium units 80–90%
SEER / SEER2 Conversion
Variables
- 0.9524
- — Official DOE M1→M2 conversion factor for split-system AC and heat pumps
Normal range / typical values
Federal minimums (2023): South 14 SEER2, North 13.4 SEER2, heat pumps 14.3 SEER2
SEER Savings (Annual)
Variables
- BTU/season
- — Estimated seasonal cooling BTU = tons × 12,000 × cooling_hours
- SEER_old / SEER_new
- — Efficiency ratings of existing and replacement units
- kWh_rate
- — Local electricity cost ($/kWh)
Normal range / typical values
Going from SEER 14 to SEER 20 saves ~30% on cooling electricity
AFUE Savings (Annual)
Variables
- heating_cost_old
- — Current annual fuel cost for heating ($)
- AFUE_old / AFUE_new
- — Efficiency of existing and replacement furnace as decimals
Normal range / typical values
Upgrading 80% to 96% AFUE saves ~16.7% on gas bills
Operating Cost
Variables
- BTU/h
- — Equipment cooling capacity
- hours
- — Annual run hours (1,000–2,500 for most climates)
- EER
- — Energy efficiency ratio (SEER2 × 0.875 approximation)
- kWh_rate
- — Local electricity rate ($/kWh)
Normal range / typical values
3-ton 16 SEER2 system, 1,500 hours, $0.13/kWh: ~$390/year
Let the calculator do the arithmetic
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