How this calculator works
A furnace must deliver enough useful heat at the local winter design condition. Enter that load directly, use assessed W/m2, or multiply a whole-building heat-loss coefficient by the indoor-outdoor temperature difference. The coefficient must already include the intended envelope and air-exchange losses. Climate-zone labels, insulation descriptions and sunlight alone are not enough to assign a dependable load. Rated thermal efficiency estimates fuel input; seasonal AFUE is not an exact peak rating.
Formula
heat loss = area x assessed W/m2, or H x (indoor temperature - outdoor temperature); output = heat loss x (1 + allowance); fuel input = output / efficiency.
Worked example
100 m2 at 50 W/m2 needs 5 kW useful output before allowances. At 80% thermal efficiency that corresponds to 6.25 kW input. A 200 W/K building across 30 K loses 6 kW.
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Frequently asked questions
Should I use average winter temperature?
Use the local heating design temperature for equipment sizing. A seasonal average misses the cold conditions the equipment must handle.
Is nameplate input the delivered heat?
No. Input is the fuel rate. Useful output is lower because some heat leaves in exhaust or other losses.
Can I add a large safety margin?
Excess capacity can increase cycling. Use a documented design allowance and check staged or modulating operation with the installer.