Using U times A times temperature difference to find the heat lost through a building element.
Heat loss calculations tell you how much heat a room loses so you can size emitters and the heat source. This example shows the loss through one element. The figures are chosen for the worked example.
The formula. Heat loss through an element = U x A x change in T, where U is the U-value in W per m2 per K, A is area in m2, and change in T is the temperature difference across the element.
The situation. An external wall has an area of 15 m2 and a U-value of 0.3 W/m2K. The inside design temperature is 20 °C and the outside design temperature is 0 °C, so the temperature difference is 20 K.
Step 1 — substitute. Heat loss = 0.3 x 15 x 20.
Step 2 — calculate. 0.3 x 15 = 4.5; 4.5 x 20 = 90 watts.
Reading the result. That wall loses about 90 W under those design conditions. A lower U-value (better insulation) would cut this in proportion: halve the U-value and you roughly halve the loss.
The bigger picture. A full room calculation repeats this for every wall, window, floor and roof element, then adds the heat needed to warm the air that leaks in and out (ventilation loss). Add them all up to get the room’s total heat loss. Use proper U-values and design temperatures, and current Part L guidance, for a real calculation.
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