Heat Loss Calculator
Floor area alone cannot tell you the answer.
Work out Heat Loss. Floor area alone cannot tell you the answer. Free, with no account and nothing to install.
0.3 for a very airtight build, 1.0+ for an older leaky one
Design heat loss
3.19 kW
10,877 BTU/h · 32 W per m² of floor
Rules of thumb based on floor area alone ignore insulation, glazing and air-tightness, which are exactly what decide the answer — a well-insulated house and a poorly insulated one of identical size can differ threefold. Ventilation loss is 29% of the total here, and it is the part that responds to draught-proofing rather than to insulation. The margin added to the recommendation covers warm-up after a setback rather than being a safety factor; oversizing beyond that causes short cycling and lower seasonal efficiency.
How the Heat Loss Calculator works
Design heat loss from fabric U-values plus ventilation, with the split between them shown. Rules of thumb based on floor area ignore insulation, glazing and air-tightness — which are exactly what decide the answer.
Also known as: house heating load calculator · how many kw of heating do i need · heat loss from u values · btu heat loss for a house
Why floor area cannot answer the question
Two houses of identical floor area can differ threefold in heat loss, depending on insulation, glazing and air-tightness. A rule of thumb in watts per square metre cannot know any of that — it is averaging over the exact variables that decide the answer.
The honest calculation is area times U-value times temperature difference for each element, plus ventilation loss. It needs more inputs precisely because the answer depends on them, and gathering those inputs is what a heat loss survey does.
Where U-values are unknown, published defaults by construction era and type give a reasonable start. A thermal camera is what reveals where those assumptions are wrong, and the answer is frequently surprising.
Ventilation loss is often the largest single item
Ventilation loss is 0.33 watt-hours per cubic metre per kelvin, times air changes per hour, times volume, times temperature difference. In a leaky older building at over one air change an hour, it can exceed the loss through any single fabric element.
It is also the part that responds to draught-proofing rather than insulation, and draught-proofing is far cheaper. Which is why knowing the split between fabric and ventilation loss changes what to spend money on.
A blower door test is the only reliable way to know the air change rate rather than guess it. Without one the figure is an assumption, and it is the assumption the whole calculation is most sensitive to.
Design conditions and margins
Design outside temperature is a cold percentile for the location rather than the record minimum. Sizing for the coldest hour ever recorded produces a system oversized for every other hour of its life.
Solar gain is excluded from a heating calculation because the design condition is a cold night. It is included in a cooling calculation for the opposite reason — which is why the two are not simply the same sum with the sign reversed.
The margin added covers recovery from a setback rather than uncertainty. Ten to twenty percent is common; beyond that the equipment short-cycles, which lowers seasonal efficiency and shortens its life. Oversizing is not a safe error.
Where to go next
The Heat Loss question rarely arrives on its own. These are the ones that usually come with it:
- HVAC BTU Calculator — Oversized is worse than slightly undersized.
- R-Value Calculator — R adds in series, U does not.
- Insulation Calculator — The first hundred millimetres does most of the work.
- Speed Distance Time Calculator — Any one of the three from the other two, in four units.
Frequently asked questions
How do I calculate heat loss?
For each element, area times U-value times the temperature difference. Add ventilation loss, which is 0.33 times air changes per hour times volume times the temperature difference.
Why not use watts per square metre?
Because two houses of identical size can differ threefold depending on insulation and air-tightness. A rule of thumb cannot know how the building is built, which is the whole question.
What are air changes per hour?
How many times the internal air volume is replaced each hour by infiltration. Around 0.3 for a very airtight build and above 1.0 for an older leaky one — it is often the largest single loss.
What design outside temperature should I use?
The published design figure for your location, which is a cold percentile rather than the record minimum. Sizing for the record low would oversize the system for every other day of the year.
How much margin should I add?
Enough to recover from a setback, commonly 10 to 20%. Beyond that the equipment short-cycles, which lowers seasonal efficiency and shortens its life.
Does this size individual radiators?
No — it gives the whole-house load. Room-by-room sizing needs the same calculation per room, including heat transfer between rooms at different temperatures.
How do I find my building's U-values?
From the construction if known, or from published default values for the era and construction type. An energy assessment gives better figures, and a thermal camera reveals where the assumptions are wrong.
What is a degree day?
A measure of how cold a period was relative to a base temperature, used to normalise energy use between years. It is the standard way to compare a heating bill against a previous winter.
How do I measure air-tightness?
A blower door test pressurises the building and measures the flow needed to hold the pressure. It is the only reliable way to know air change rate rather than guess it.
Does room-by-room sizing differ from whole-house?
Yes — each room needs its own calculation for radiator or emitter sizing, including heat transferred between rooms held at different temperatures. The room totals do not simply sum to the house figure.
What temperature should I design for?
Around 21 °C for living spaces and lower for bedrooms and circulation, though this varies by standard and by preference. Designing for a higher temperature than you will use oversizes the system.
How does solar gain fit in?
It reduces heating need on sunny days and does nothing at the design condition, which is typically a cold night. That is why heating is sized without it and cooling is sized with it.
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