How this calculator works
Start with a design heat loss, or multiply heated area by an assessed W/m2 allowance. Bedroom mode merely estimates area using your entered whole-home area per bedroom. Property age alone cannot determine heat loss after renovations. A combi with hot-water priority must meet the larger of heating and instantaneous water duties. A system boiler adds your cylinder recovery allowance. Input power is estimated using rated thermal efficiency; seasonal AFUE alone does not precisely describe peak performance.
Formula
space load = area x W/m2; hot-water duty = 1000 kg/m3 x 4186 J/(kg K) x flow (m3/s) x temperature rise; combi output = max(space load with allowance, hot-water duty); fuel input = output / efficiency.
Worked example
100 m2 at 60 W/m2 needs 6 kW before allowances, or 6.667 kW fuel input at 90% efficiency. Heating 10 L/min by 30 K requires about 20.93 kW useful output.
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Frequently asked questions
Can property age determine the boiler size?
No. Insulation upgrades, airtightness, glazing and exposed area matter. Enter a documented load or an allowance assessed for the actual building.
Why can combi output exceed the heating load?
Instantaneous hot water can require much more power than space heating. Check minimum modulation as well as maximum output.
Does bedroom mode replace a heat-loss survey?
No. Bedroom count is only a way to enter an approximate floor area. Final selection needs a building load calculation and manufacturer checks.