Furnace Size Calculator

One matched to the design heat loss plus a modest margin for warm-up. That means doing a heat loss calculation rather than sizing from floor area, which cannot account for how the house is built.

Because an undersized system produces immediate complaints and an oversized one produces gradual inefficiency nobody attributes to it. The incentives push one way, and the result is a system that never runs at its best.

The ability to run at partial output rather than only on or off. A modulating boiler matches its output to the load, which largely removes the short cycling penalty of a large maximum output.

Marginally, and mostly during warm-up from cold. Once at temperature the system only needs to replace the losses, which is what the design load measures.

Heat pump output falls as outside temperature drops, so sizing has to consider capacity at the design temperature rather than the nominal rating. Oversizing is also less harmful for an inverter-driven unit, which modulates.

If insulation or glazing upgrades are planned, size for the improved fabric. A system sized for the current fabric becomes substantially oversized once the losses drop.

One that recovers latent heat by condensing water vapour from the flue gases, which requires a low return water temperature. Run with a high return temperature it never condenses and loses most of its efficiency advantage.

Lower is more efficient, particularly for a condensing boiler or a heat pump, and it requires larger emitters to deliver the same heat. Weather compensation lowers it automatically in milder weather.

Yes — hot water demand usually sets the size rather than the heating load, because instantaneous hot water needs far more output than space heating. That is why combis are commonly much larger than the heat loss requires.

The burner firing and stopping repeatedly rather than running steadily. Frequent brief cycles in mild weather are the classic sign of an oversized boiler.

Commonly 10 to 15 years with servicing, less if it short-cycles or runs with poor water quality. System water treatment and a magnetic filter make a substantial difference.

Rarely on efficiency alone — the payback on replacing a functioning modern boiler is long. Replacing an old non-condensing one is a different calculation entirely.