Heat loss & emitters
Fabric heat loss: one element
- where
- element heat loss (W)
- element U-value (W/(m²·K))
- net element area (m²)
- indoor–outdoor design difference (K)
Explain this equation
Works out the steady heat lost through one building element, such as a wall, roof, floor or window. Use it for each element when building up a room heat loss.
U is the U-value, the watts that pass through one square metre for each degree of temperature difference. A is the net area of the element. ΔT is the difference between design indoor and outdoor temperatures.
Heat flow through a layer rises with its area and with the temperature difference across it. The U-value bundles up the insulating effect of all the layers.
Sense check: 20 m² of wall at U 0.3 with a 23 K difference loses about 140 W. An old solid wall near U 2.0 would lose over 900 W. Double the area or ΔT and the loss doubles.
Watch out: deduct openings from the wall area and add them as separate elements. Thermal bridges and ventilation are not included.
Enter your values to see the result.
Good to know
One element only. Exclude openings from wall area and add their own losses; thermal bridges, ventilation and room-by-room design are separate.
Sources
Results are arithmetic only. Worked examples are not recommended design settings. This library does not provide automatic gas pipe selection, live gas tightness tests, purging procedures, combustion acceptance limits, installation approval or commissioning certification. Manufacturer data and the applicable current method remain essential.