HEATLOSSPROHow to calculate heat loss
The formula, a worked example for one room, and a free calculator. Then the quick way: draw the whole house and get every room's heat loss in about 5 minutes.
In short: a room's heat loss is the sum of its fabric loss and its ventilation loss. For each wall, window, floor and roof, multiply the U-value by the area and by the difference between the indoor and the design outdoor temperature (U × A × ΔT). Add the ventilation loss, 0.33 × air changes per hour × room volume × ΔT. Add up the rooms to get the whole house, and size the heat pump on that total.
The heat loss formula
1. Fabric loss: walls, windows, floor and roof
The U-value says how many watts pass through one square metre for each degree of difference between inside and outside. The lower it is, the better the insulation. Calculate each surface separately: subtract the windows from the wall they sit in, and use the glazing U-value for the windows themselves.
Floors on the ground lose less than the full ΔT suggests, because the ground stays warmer than the winter air. A common simplification multiplies a ground floor's loss by about 0.75. Walls to a heated neighbour, or floors and ceilings between heated rooms, lose nothing.
2. Ventilation loss: air leaking or ventilated out
A typical value for n is about 0.5 air changes per hour for a reasonably airtight house. Use 1.0 or more for draughty older houses, and lower values with heat recovery ventilation.
3. Add it up
This is the room-by-room method behind EN 12831, the European standard for heat load calculations. The standard adds some refinements (thermal bridges, heat flows between rooms at different temperatures), but the core is these two formulas.
Worked example: a living room
A living room of 5 × 4 m and 2.5 m high, with two outside walls (5 m and 4 m long) of uninsulated cavity brick, one 3 m² window with modern double glazing, an uninsulated concrete floor on the ground, and a heated bedroom above. Indoors 21 °C, design outdoor temperature −3 °C, so ΔT = 24 K.
| Surface | Area | U-value | Calculation | Heat loss |
|---|---|---|---|---|
| Outside walls (22.5 m² less the window) | 19.5 m² | 1.40 | 19.5 × 1.40 × 24 | 655 W |
| Window | 3.0 m² | 1.30 | 3.0 × 1.30 × 24 | 94 W |
| Floor on the ground (× 0.75) | 20.0 m² | 1.10 | 20 × 1.10 × 24 × 0.75 | 396 W |
| Ceiling to a heated room | 20.0 m² | — | no temperature difference | 0 W |
| Ventilation (50 m³, 0.5 per hour) | — | — | 0.33 × 0.5 × 50 × 24 | 198 W |
| Room heat loss | 1,343 W | |||
That is about 1.34 kW (4,580 BTU/h), or 67 W per m² of floor. The radiator or underfloor loop in this room has to deliver 1.34 kW at the heat pump's flow temperature. Repeat for every room and add them up for the heat pump.
Free heat loss calculator for one room
Change the figures to match your room. The U-values are the same typical values the HeatLoss Pro designer uses.
The 5-minute way: draw it instead
Calculating by hand means measuring and typing in every wall, window, floor and ceiling of every room. For a whole house that is easily two hours in a spreadsheet, and one typo throws the result off.
In the HeatLoss Pro designer you paste or drop a floor plan, trace the rooms over it, and pick wall, window, floor and roof types. Areas, volumes and adjoining rooms are worked out from the drawing, and each room's heat loss appears as you draw. A typical house takes about 5 minutes.
- Paste the floor plan (an image or PDF) and set its scale from one known length.
- Trace the rooms; corners snap together, and shared walls between heated rooms lose nothing.
- Set the constructions: wall, window, floor and roof types, for one room or many at once.
- Read the results: heat loss per room and for the house, the heat pump size at your design temperature, and a radiator and underfloor heating check. Check it in 3D across several floors.
Drawing and the live estimates are free and need no sign-up. A full calculation with a PDF report for your client costs €1. See how it compares with spreadsheets and other tools.
From heat loss to heat pump size
The house's total heat loss at the design outdoor temperature is the output the heat pump must deliver on the coldest day. Heat pumps lose output as it gets colder, so compare the load with the output on the datasheet at that outdoor temperature, not with the rated output at +7 °C. Choose the smallest model that covers the load: an oversized heat pump cycles on and off and runs less efficiently.
Each room's figure sizes its radiator or underfloor heating loop. Heat pumps run at low flow temperatures (35–50 °C), where a radiator gives much less than its catalogue rating at ΔT 50, so check every room, not only the total.
Frequently asked questions
What is the formula for heat loss?
For each wall, window, floor and roof: heat loss (W) = U-value × area (m²) × temperature difference (K). Add the ventilation loss: 0.33 × air changes per hour × room volume (m³) × temperature difference. The sum is the room's heat loss; the sum of all rooms is the house's.
How long does a room-by-room heat loss calculation take?
In a spreadsheet, a typical house takes two hours or more, because every surface of every room is measured and typed in. In HeatLoss Pro you trace a floor plan and set wall, window, floor and roof types, and each room's heat loss appears as you draw. A typical house takes about 5 minutes.
What indoor and outdoor temperatures should I use?
Indoors, the usual design values are 21 °C for living rooms, 18 °C for bedrooms and 22 °C for bathrooms. Outdoors, use the design temperature for your location: roughly −2 to −5 °C in much of the UK and around −10 °C in the Netherlands, Belgium and Germany.
What is a typical heat loss per m²?
As a rough guide at design conditions: 30–50 W/m² for a well-insulated modern house, 50–80 W/m² for a house from the 1970s to 1990s, and 80–120 W/m² or more for an older uninsulated house. These figures are only a starting point; the rooms of one house can differ by a factor of two.
Can I size a heat pump from a W/m² rule of thumb?
Not reliably. A rule of thumb gives no figure per room, so radiators and underfloor heating cannot be checked, and it often oversizes the heat pump, which then cycles on and off and runs less efficiently. Size it from a room-by-room calculation at the design outdoor temperature, using the output the heat pump's datasheet gives at that temperature.
How many BTU do I need to heat a room?
Work out the room's heat loss in watts and multiply by 3.412 to get BTU per hour. A typical 5 × 4 m living room in an older house loses about 1,300 W, or about 4,600 BTU/h, on a cold day. Rules of thumb that use only the room's volume ignore insulation, windows and outside walls, so they can be out by a factor of two either way.
Is HeatLoss Pro free?
Drawing, tracing floor plans and the live heat loss estimates are free, with no sign-up. A full room-by-room calculation with its PDF report costs €1, or there is a monthly subscription for unlimited calculations.
Do the whole house in 5 minutes
Trace your floor plan and see every room's heat loss, the heat pump size and the radiator check as you draw. Free to try, nothing to install, no sign-up.
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