Boiler Size Calculator

This boiler size calculator estimates the heating output your home needs, so you can match it to a boiler, heat pump or heater rather than guessing. It works from three things: the floor area you are heating, the ceiling height, and how well the home holds heat. Larger and taller spaces lose more heat, and a draughty, single-glazed home loses far more than a modern, well-insulated one, so all three feed into the answer. The calculator multiplies your floor area by a heat factor in watts per square metre, chosen from your insulation level, then scales the result for ceiling height above the standard 2.4 metres, and returns the required output in kilowatts and BTU per hour. Enter the total floor area you want to heat, pick the insulation band that best fits your home, and adjust the ceiling height if your rooms are unusually high. The result is the space heating requirement: the steady output needed to hold a comfortable indoor temperature on a cold day. Remember that combi boilers, which also heat tap water on demand, are usually sized larger so they can supply hot water at a good flow rate, so treat this as the heating side of the sizing. For an exact specification before you buy, get a professional heat loss survey, since real homes vary with orientation, window area and air tightness.

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A 120 m² home with average insulation and a 2.4 m ceiling needs about 7.8 kW of heating output. A combi boiler that also heats tap water would be sized larger for hot water flow.

Space heating estimate only, not including hot water demand. Real needs vary with window area, orientation and air tightness, so get a heat loss survey before buying. Estimate only.