Heat loss calculator

A house loses heat two ways and most calculators count one

Fabric loss is the one everybody does: the area of each wall, roof, floor, window and door, times its U-value, times the temperature difference between inside and out.

Ventilation loss is the one almost nobody does. Warm air leaves the house and cold air arrives to replace it, through trickle vents, extractor fans, an open chimney and every gap in the building. That air has to be heated from outside temperature to inside temperature, continuously.

The sum is short and it is not an estimate:

0.33 × air changes per hour × volume × temperature difference

The 0.33 is the volumetric heat capacity of air, in watt hours per cubic metre per kelvin. It is a physical constant, not a safety factor, which matters because it means the ventilation figure is as solid as the fabric one. On a leaky older house at two air changes an hour it comes out at comfortably a third of the total.

Drop it and you undersize everything downstream at once: the boiler, the radiators, and the case for insulating anything.

The air change rate is the number to be honest about

The constant is fixed and the volume is measurable. The uncertain input is how leaky the house actually is, so resist the temptation to be optimistic.

A draughty older house with suspended floors, an open chimney and original windows can be two air changes an hour or more. A well sealed modern build is well under one. If you are somewhere in between and unsure, run the calculation twice and see how much the answer moves, because that spread is the real uncertainty in your figure.

Thermal bridging, the third piece

Heat takes a short cut through junctions, lintels, window reveals and the joints between elements. None of that appears in an area-times-U-value sum, because those sums assume each element is uniform and independent.

A percentage added to the fabric loss is a working allowance rather than a measurement. A proper assessment uses psi values calculated for each junction type. It is included here because leaving it out entirely is worse than approximating it, and it is flagged as an approximation so nobody mistakes it for a measured figure.

Why there are no U-values built in

This is the same refusal as in the boiler sizing and radiator output tools, for the same reason.

Your wall's U-value depends on its construction, whether it was ever insulated, how well that was done and what condition it is in now. The tables of typical values circulating online are not traceable to anything, and they are the mechanism by which a heat loss calculation becomes a confident number that is wrong. People then order a boiler against it.

Three places to get real ones:

• Your EPC. It states the assumed construction for every element, which is where a surveyor started too.
• A survey. The only way to get measured rather than assumed values.
• The building regulations. Anything replaced since they applied to it has a limiting U-value it could not legally exceed, so a window fitted last year has a known ceiling.

This is a design day, not a typical day

The external temperature in this calculation is the cold end, not an average. That is deliberate and it is how heating systems are sized: you size for the worst day, not the usual one.

It gets misread constantly. A house with a 9kW design heat loss does not need 9kW in October; it needs 9kW on the coldest day of a cold year and considerably less the rest of the time. That is precisely why a boiler which can only modulate down to a third of its output spends most of the winter cycling, which is the whole subject of the boiler sizing tool.

Where the money goes

The tool names your single largest fabric loss, so look at that before spending anything on insulation.

Windows feel like the problem because they are cold to stand next to and you can feel the draught. But a large area of poorly insulated wall usually loses far more in total, simply because there is so much more of it. Cold to touch and large in total are different things, and only one of them is on your bill.