HVAC Load Calculation Terms Explained
Don't let contractors dazzle you with jargon. Here is your cheat sheet.
To understand a Manual J calculation, you need to speak the language.
The Big Three
BTU (British Thermal Unit)
The amount of energy needed to heat or cool one pound of water by 1°F. It is the core unit of heat.
Ton (Tonnage)
12,000 BTUs/hour. Historically, the cooling power of melting one ton of ice in 24 hours.
Manual J
The protocol developed by ACCA (Air Conditioning Contractors of America) to calculate heat loss and gain.
Advanced Terms
SHGC (Solar Heat Gain Coefficient)
A number between 0 and 1 that measures how much solar heat comes through a window. Lower is better for cooling climates.
Latent Load vs Sensible Load
As explained in our Humidity article:
- Sensible: Temperature change (Thermometer).
- Latent: Moisture removal (Humidity).
ACH (Air Changes per Hour)
A measure of leakage. If your home has 0.5 ACH, half the air in your house leaks out and is replaced every hour.