How this calculator works
Envelope heat loss follows U x area x temperature difference. Known-area mode uses an area-weighted U-value that accounts for the complete surface. Room mode subtracts entered window and door areas from the exposed walls and adds their separate U-value losses. The main U-value applies to the remaining opaque envelope. Room mode treats the exposed walls in order: length, adjacent width, second length, second width. Floor and ceiling use the same effective U-value as a simplifying assumption. Air exchange adds sensible loss using an approximate air volumetric heat capacity of 1206 J/(m3 K), using 1.2 kg/m3 air density and 1005 J/(kg K) specific heat. Moisture, solar gains, heat recovery and warm-up are excluded.
Formula
Q = [sum of each surface U x area + 1206 x volume x ACH / 3600] x (Ti - To); Inverse opaque U subtracts window, door and ventilation conductance before dividing by opaque area.
Worked example
A 10 m2 surface with U = 2 W/(m2 K) across a 30 K difference loses 600 W without air exchange. A 900 W measured loss across the same surface and temperatures implies U = 3 W/(m2 K).
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Frequently asked questions
How do I include windows and doors?
Room mode uses repeated opening dimensions with separate window and door U-values. Known-area mode instead expects a combined area-weighted U-value.
Is this a whole-house equipment design?
It is a steady estimate. A room-by-room design must account for different assemblies, design weather, infiltration and distribution losses.
Can I solve for indoor temperature?
Yes. That mode finds the steady balance temperature from useful heating power under the entered envelope and ventilation assumptions.