Heat Loss Calculator
Fabric plus ventilation, with the assumption stated.
Work out Heat Loss. Fabric plus ventilation, with the assumption stated. Shows the working, not just the answer.
Design heat loss
5.56 kW
55.6 W/m² at a 24° difference
Heat loss splits into fabric — through walls, roof and floor — and ventilation, the energy carried out by air leaving the building. Air has a volumetric heat capacity of about 0.33 Wh per cubic metre per degree, which is where the ventilation term comes from. The annual figure uses degree days rather than a flat multiplier, because heating demand is proportional to how far and how long the outside temperature sits below the base temperature — not to the coldest day of the year. A well-insulated modern house runs 30 to 50 W per square metre at design conditions; a solid-wall Victorian terrace can be three times that. That ratio is the entire argument for insulating before changing the heat source.
How the Heat Loss Calculator works
Whole-house heat loss at design conditions, split between fabric and ventilation, with an annual estimate from degree days. A rule-of-thumb figure for sanity-checking a quote — not a substitute for a room-by-room calculation.
Also known as: what size boiler do i need · heat loss of my house · kw needed to heat a room · is my quote oversized
Frequently asked questions
How do I calculate heat loss?
Fabric loss through walls, roof and floor, plus ventilation loss from air leaving the building. Both scale with the temperature difference between inside and out at design conditions.
What size boiler or heat pump do I need?
Something matching the design heat loss, not comfortably above it. Oversizing is expensive twice — once to buy, and again for its whole life, because an oversized unit short-cycles and runs at poor efficiency.
What are degree days?
A measure of how far and how long outside temperature sits below a base temperature over a year. Annual heating demand is roughly proportional to them, which is why they give a better annual estimate than scaling the design-day figure.
What is a good heat loss per square metre?
A well-insulated modern house runs 30 to 50 W/m² at design conditions. A solid-wall Victorian terrace can be three times that, which is the entire argument for insulating before changing the heat source.
How accurate is this?
Good enough to spot a quote that is wildly wrong, and not good enough to size equipment. A proper calculation works room by room from actual U-values, orientation, glazing and measured air tightness.
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