Glossary · Heating, cooling & ventilation
Load calculation
Also called: load calc · heating load calculation · cooling load calculation · room-by-room load calculation
An engineering calculation of how much heating and cooling a house needs, room by room, used to size equipment correctly.
What it means
It uses your home's insulation levels, windows, air leakage, size, orientation and local design temperature to estimate peak heat loss and heat gain in BTU/h. In the US the standard method is ACCA Manual J, with Manual S for choosing equipment and Manual D for duct design. Audit data such as a blower door result make it more accurate.
Why it matters in an audit
Correct sizing avoids oversizing and short cycling and improves comfort, and redoing it after envelope upgrades shows how much smaller a system you may need.
Typical numbers
Reference values with their sources. Your energy advisor’s report gives the figure for your own house.
| What | Value | Source |
|---|---|---|
| Rule of thumb vs. calculation (ACCA blog, 2016) | Rules of 400–600 ft² per ton vs. a 1,431 ft² per ton average from 40 Manual J calculations; practitioners cited 15%–40% excess capacity even with Manual J | hvac-blog.acca.org ↗ |
In Canada
In Canada the equivalent method is CSA F280 (see heat-loss calculation), which NRCan recommends for sizing heat pumps.