Heat Loss Calculator
How to estimate heat loss
- Enter the inside and outside design temperatures and the room volume and air changes.
- Enter the areas and U-values of the walls, windows, roof and floor.
- Read the heat loss in watts and BTU per hour to size a heater or radiator.
Frequently asked questions
Where do I find U-values?
From building specifications or typical values: an insulated modern wall is about 0.2 to 0.35 W/m²K, double glazing about 1.2 to 2.8, and an uninsulated solid wall 2 or more.
How is air leakage counted?
As 0.33 × air changes per hour × volume × temperature difference, in watts.
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