Heat Load Formulas
The three air-side heat equations every load calculation rests on — sensible, latent, and total heat — with their standard-air constants and where those constants come from. Sensible heat changes temperature, latent heat changes moisture, and total heat is the sum, read off enthalpy. All three assume standard air density.
Sensible, latent & total heat
| Heat | Formula (BTU/hr) | Constant derivation |
|---|---|---|
| Sensible | Qs = 1.08 × CFM × ΔT(°F) | 60 min/hr × 0.075 lb/ft³ × 0.24 BTU/lb·°F |
| Latent | Ql = 0.68 × CFM × ΔW(grains/lb) | 4.5 × 1,060 BTU/lb ÷ 7,000 grains/lb |
| Total | Qt = 4.5 × CFM × Δh(BTU/lb) | 60 min/hr × 0.075 lb/ft³ |
Three heats, one air stream
Every BTU an air stream carries is either sensible or latent. Sensible heat is what a thermometer reads — it changes the dry-bulb temperature, and its formula keys on ΔT. Latent heat is hidden in the moisture: it does not change temperature, only the humidity ratio, so its formula keys on ΔW in grains. Total heat is the sum of the two, and because enthalpy already bundles temperature and moisture together, the total formula keys on the enthalpy difference Δh. The three constants — 1.08, 0.68, 4.5 — are just the same standard-air density and specific heat repackaged for each driving variable.
For example, 1,000 CFM across a 20°F temperature drop removes 21,600 BTU/hr of sensible heat. To turn that into a system, pair it with the airflow from the CFM calculator and the saturation temperatures from the psychrometric reference.
Common questions
What is the sensible heat formula in HVAC?
Sensible heat (the heat that changes air temperature) is Qs = 1.08 × CFM × ΔT, where ΔT is the dry-bulb temperature difference in °F. The 1.08 constant is 60 minutes/hour × 0.075 lb/ft³ (standard air density) × 0.24 BTU/lb·°F (the specific heat of air). So 1,000 CFM across a 20°F rise is 1.08 × 1,000 × 20 = 21,600 BTU/hr.
What are the 1.08, 0.68, and 4.5 constants?
They are the standard-air constants for the three heat equations: 1.08 for sensible heat (per °F), 0.68 for latent heat (per grain of moisture), and 4.5 for total heat (per BTU/lb of enthalpy). All three assume standard air density of 0.075 lb/ft³ — 4.5 = 60 × 0.075, 1.08 = 4.5 × 0.24, and 0.68 = 4.5 × 1,060 ÷ 7,000.
Do these formulas work at high altitude?
Not directly. All three constants are built on standard air density (0.075 lb/ft³, roughly sea level at 70°F). Above about 2,000 feet the air is thinner, so you apply a density correction factor — the constants scale down with the reduced density.
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