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 auditor’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 the United States

ACCA describes Manual J as the national ANSI-recognized standard for producing equipment sizing loads, and says national building codes and most state and local jurisdictions require a load calculation done to it.

Sources

Learn more here

Sizing heating and cooling: the load calculation →

Related terms

Heat-loss calculation A calculation of how fast a home loses heat on the coldest days it's designed for, used to size heating equipment correctly.
Design temperature The cold (or hot) outdoor temperature a heating or cooling system is sized for; colder than most winter days, but not the record low.
Oversizing Installing heating or cooling equipment with much more capacity than the house needs, usually from rules of thumb, not a load calculation.
Short cycling When heating or cooling equipment turns on and off frequently in short bursts instead of running in steady, longer cycles.
BTU and BTU/h A unit of heat energy; BTU per hour (BTU/h) measures how fast a furnace, heat pump or air conditioner delivers heating or cooling.
Ton (of cooling) A unit of heating and cooling capacity: one ton equals 12,000 BTU per hour (3.5 kW), based on the cooling from melting a ton of ice in a day.
Manual J (ACCA load calculation) The US industry standard for calculating how much heating and cooling a house needs, room by room, so equipment can be sized correctly.
Manual S (ACCA equipment selection) The US industry standard for choosing heating and cooling equipment whose real capacity, from the maker's data, fits the home's calculated loads.
Manual D (ACCA duct design) The US industry standard for designing and sizing a home's ducts so each room gets the airflow the heating and cooling equipment is meant to deliver.

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