Heat Loss & Heating BTU Calculator

About the Heat Loss & Heating BTU Calculator

Work out how much heat each room loses on a cold day — and so how big its radiator, heater or the whole heating system must be. For every wall, window, door, roof and floor the calculator multiplies its U-value by its area and by the temperature difference across it; it adds the heat carried away by air leaking in and out (0.33 × air changes per hour × volume × ΔT), and shows each room in watts and BTU/h and the house in total.

Room temperatures start from the CIBSE values that the UK’s MCS heat pump standard uses, cold-day design temperatures for 14 UK sites are built in (or type your own), the air leakage can be estimated the way SAP 10.2 does, and a ground floor’s U-value can be worked out by ISO 13370. Radiators are sized for the water temperature you run — a 75 °C boiler or a 45 °C heat pump. An estimate for choosing heating — not a professional design.

Without a pass, the free preview shows the design heat loss of the whole home in full, with the first rows of the room table; the complete result, Copy result and the CSV come with a Pro pass.

Common uses

• Size a radiator, a panel heater or an oil-filled heater for one cold room.
• Find the whole-house heat loss before choosing a boiler or a heat pump.
• Check whether existing radiators are big enough to run at heat-pump temperatures.
• See what new windows, loft insulation or draught-proofing would save in peak heating power.

The calculation

• Fabric loss of each element: Φ = U × A × (θ_room − θ_beyond), in watts. Walls are entered with their full area; the windows and doors in them are taken off.
• What is beyond: outside air at the design temperature; for a solid floor on the ground, the annual average outdoor temperature (MCS MIS 3005-D, after MGD 007); a suspended floor uses the outdoor design temperature; an unheated garage or a neighbour’s home has its own temperature, which you enter.
• Ventilation and infiltration: Φ = 0.33 × n × V × (θ_room − θ_outside), where n is air changes per hour and V the room volume in m³; 0.33 is air’s heat capacity, 1.2 kg/m³ × 1,005 J/kg·K ÷ 3,600 s.
• Thermal bridging (optional): an allowance y added to the U-value of every element facing outside, the ground or unheated space — SAP 10.2 uses 0.20 W/m²K when the junction details are unknown.
• BTU/h = watts × 3.412. The total is the heat the heating must deliver on the design day; a margin can be added for reheating after setbacks.

Temperatures

Inside (CIBSE, as MIS 3005-D Table 1 lists them): living, dining and bedsitting rooms 21 °C, bedrooms, halls and landings, kitchens and toilets 18 °C, bathrooms 22 °C. CIBSE’s Domestic Heating Design Guide uses 21 °C throughout newer, well-insulated homes, with 22 °C in bathrooms — tick “newer, well-insulated home”.

Outside: the design temperature is a cold-day value that is exceeded most of the time, not the record low. For the UK the calculator has CIBSE Guide A Table 2.5 for 14 sites at the 99.6% and 99% levels (London Heathrow −3.0 / −1.7 °C, Glasgow −5.6 / −3.5 °C), lowered by 0.6 °C for each whole 100 m your home stands above the site. Elsewhere use your national value — for example the ASHRAE 99.6% heating dry-bulb for North America, or the national annex to EN 12831 in Europe.

Air leakage (SAP 10.2)

Leaky homes lose much of their heat through air. The SAP 10.2 worksheet adds: 80 m³/h for each open chimney, 20 for an open flue or a blocked chimney, 10 for a chimney with a closed fire, 20 for a solid-fuel boiler flue, 35 for another heater’s flue, 10 for each intermittent extract fan or passive vent and 40 for a flueless gas fire, divided by the home’s volume; then 0.1 for each storey above the first, 0.35 for masonry or 0.25 for timber or steel frame, 0.2 for an unsealed suspended timber floor (0.1 sealed), 0.05 without a draught lobby, and 0.25 − 0.2 × the share of windows and doors draught-proofed. An air-pressure test result (m³/h per m² at 50 Pa) divided by 20 replaces the construction terms. Each sheltered side takes off 7.5%; SAP then scales the rate by the month’s average wind speed ÷ 4 m/s (its UK average for January is 5.1 m/s), so enter the wind of your coldest month — 4 m/s leaves the rate as it is. Below one air change an hour natural ventilation gives 0.5 + 0.5 × n². CIBSE’s guide sets 0.5 air changes an hour as the minimum for a room with outside walls.

Ground floors and radiators

Ground floor U-value (ISO 13370, a slab on the ground): B′ = floor area ÷ (half the exposed perimeter), d_t = wall thickness + λ_ground × (0.17 + R_floor + 0.04); U = 2λ ÷ (πB′ + d_t) × ln(πB′ ÷ d_t + 1), or λ ÷ (0.457B′ + d_t) for a well-insulated floor. Clay or silt has λ = 1.5, sand or gravel 2.0 and rock 3.5 W/m·K. An uninsulated 100 m² floor with 40 m of outside wall on sand is about 0.76 W/m²K.

Radiators are sold by their output at ΔT 50 — water averaging 70 °C (75 °C in, 65 °C out) in a 20 °C room (EN 442). At other temperatures the output scales with (ΔT ÷ 50)ⁿ, where n comes from the radiator’s test data — the calculator starts at 1.3; use your radiator’s own figure. A heat pump running at 45/40 °C in a 21 °C room has ΔT 21.5, so a radiator gives about a third of its catalogue output — which is why heat pump homes often need bigger radiators.

Sources

• MCS MIS 3005-D heat pump design standard — room temperatures, ground and suspended floors
• MCS heat load calculator: design conditions — CIBSE Guide A Table 2.5 sites, altitude correction and UK annual average temperatures
• SAP 10.2, the UK Standard Assessment Procedure (gov.uk) — ventilation heat loss, the infiltration worksheet, thermal bridging and Table 6e window U-values
• BEE Eco-Niwas Samhita 2024 — material conductivities behind the wall and roof U-values offered
• BS EN 12831-1 (design heat load), ISO 13370 (heat transfer via the ground), EN 442 (radiator output), CIBSE Guide A

Limitations

• An estimate for choosing heating — not a professional design. MCS-certified heat pump installations in the UK are designed from a full room-by-room BS EN 12831 calculation (MIS 3005-D); other countries have their own rules.
• The result is the peak heating power on the design day, not the energy used over a winter.
• U-values must be right for the result to be: ask for the values of your windows and insulation, or work a wall out with the R-value calculator. Ceilings to a cold loft are treated as if the loft were outside air.
• Air leakage is an estimate; real homes vary widely, and an air-pressure test is the only way to know.
• It does not include heat from people, appliances or the sun, which help in milder weather, or extra heat for rooms heated only some of the time.

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