Room & Heat Sources
Total Heat Load
Heat load is the total quantity of heat, measured in BTU per hour, that flows into a room and must be removed by the air conditioner to maintain a comfortable temperature. Unlike a simple area-based estimate, a heat load calculation identifies each separate source of heat so you can see what is driving your cooling requirement. This is valuable because it tells you not just what size AC you need, but also which improvements would reduce that need.
| Source | Typical share | What drives it |
|---|---|---|
| Walls and roof conduction | 40 to 60% | Outdoor temperature, wall thickness, roof exposure and insulation |
| Solar gain through windows | 15 to 30% | Window area, orientation, and external shading |
| People | 5 to 20% | Number of occupants and their activity level |
| Lighting and equipment | 3 to 15% | Total watts of electrical devices running in the room |
| Air infiltration | 10 to 25% | Gaps around doors and windows, and outdoor humidity |
A 14 by 12 ft top-floor bedroom with an uninsulated roof, medium west-facing windows, 2 occupants and 250 W of equipment, in a hot climate.
The breakdown immediately shows the two biggest opportunities: insulating the roof would cut the largest component, and sealing gaps would cut the second largest. Together those could drop the load enough to use a 1.5 ton unit instead.
What is heat load in air conditioning?
The total heat in BTU per hour entering a room that the AC must remove to hold the set temperature. It is the sum of conduction through walls and roof, solar gain through windows, internal gains from people and equipment, and infiltration of outside air.
What is the difference between heat load and cooling load?
They are usually used interchangeably. Heat load is the rate heat enters; cooling load is the rate the AC removes it. At steady state they are equal. This tool breaks the load down by source so you can target reductions.
Which source contributes the most heat?
For most rooms, walls and roof conduction, typically 40 to 60%. Solar gain is usually second, especially for west windows in hot climates. Rooms with many people or heavy equipment can be dominated by internal gains.
How can I reduce my room's heat load?
Target the largest components: external shading for solar gain, roof insulation for top-floor conduction, sealing for infiltration, and LED lighting for internal gains.
Does heat load change through the day?
Yes. It peaks when outdoor temperature and solar angle combine, usually mid to late afternoon. West-facing rooms peak around 3 to 5 pm. Size the AC for this peak, not the average.
What heat does a person add?
A resting adult adds roughly 450 to 500 BTU per hour, split between sensible heat (about 250) and latent moisture heat (about 200 to 250). Active occupants add more.
Heat load estimates are based on standard residential methods and the conditions you select. Actual values vary with specific construction and local weather. For commercial or critical installations, consult a licensed HVAC engineer.