HEATLOSSPROHow HeatLoss Pro calculates heat loss
New projects follow EN 12831-1:2017, the European standard for the design heat load, with the UK, German or French rules where they apply. Here is each step, where its figures come from, and what the calculation does not do.
In short: each room's loss through its walls, windows, roof and floor, with an allowance for thermal bridges; floors on the ground by EN ISO 13370; air leakage from how airtight the house is, or the room's minimum fresh air; and heat passed between rooms kept at different temperatures. The heat pump is sized on the whole house, from its own datasheet. UK projects use the method MIS 3005-D sets out, German projects DIN/TS 12831-1.
Which rules a project uses
A new project takes the rules of the language you start it in. You can change the rules, and each figure below, in the House card.
| Rules | Thermal bridges | Airtightness q50 | Behind a party wall |
|---|---|---|---|
| UK: the method MIS 3005-D sets out, with CIBSE data | +0.15 W/m²K | 12 m³/h·m² | 10 °C |
| Germany: DIN/TS 12831-1 | +0.10 W/m²K | 6 m³/h·m² | 14 °C |
| France: EN 12831-1 defaults, base temperature by département | +0.10 W/m²K | 12 m³/h·m² | 10 °C |
| EN 12831-1 defaults (Dutch and others) | +0.10 W/m²K | 12 m³/h·m² | 10 °C |
The airtightness is a default for a house that has not been tested; enter the result of a blower door test when you have one. The Dutch and French national annexes (ISSO 51, the Belgian and French annexes) are not built in: those projects use the defaults of EN 12831-1 itself. Under the French rules, choosing the département sets the design outdoor temperature to its base temperature up to 200 m, from NF P 52-612/CN; above 200 m or by the sea, type the corrected value.
Step by step
1. The design temperatures
Outside, the design outdoor temperature of the location. Under the UK rules HeatLoss Pro offers the nearest CIBSE Guide A weather station at 99.6%, 0.6 °C colder for every full 100 m above it, and the yearly mean temperature of the region. Inside, each room's temperature follows its room type: under the UK rules 21 °C in living rooms, 18 °C in bedrooms and 22 °C in bathrooms (21 °C everywhere but bathrooms from 2007 on); under the German rules 20 °C and 24 °C in bathrooms.
2. Walls, windows and roofs
Areas come from the drawing. A window leaves the wall above and below it at the wall's own U-value. Under the UK rules, the walls of a storey with another above it are 0.25 m taller, for the floor between them.
3. Floors on the ground
A floor on the ground loses heat to the ground, which is at about the yearly mean outdoor temperature. HeatLoss Pro takes the floor's equivalent U-value from EN ISO 13370, from the room's own characteristic dimension B′ = A / (0.5 × P), with P the length of its outside walls, and multiplies the difference by 1.45 for the yearly swing (1.15 more with shallow groundwater). A suspended floor over a ventilated void uses the EN ISO 13370 formula for one, to the outdoor design temperature.
4. Party walls and neighbours
A wall shared with the house next door loses heat to the neighbour's temperature in the table above, not to outside.
5. Heat between rooms
A bathroom at 22 °C next to a bedroom at 18 °C loses heat into it through the wall between them. Each room's load counts what it loses to cooler rooms and what it gets from warmer ones, so its radiator is big enough even when the room next to it is cooler. For the house as a whole these cancel out.
6. Air
fqv is the share of the leakage that comes through on a windy day: 0.05 for normal shielding, 0.07 for an exposed house and 0.03 for a sheltered one. A room with no outside wall has neither. With mechanical ventilation with heat recovery, the unit's air counts only for the share it does not recover.
7. The house and the heat pump
The house counts the leakage once and half of the rooms' minimum fresh air, as EN 12831-1 does, so its heat load is less than the rooms added up. Radiators and underfloor heating are sized on the room loads; the heat pump on the house's load, with its output at the design temperature taken from its own datasheet figure at that temperature, never a generic derating.
What the UK rules add to the report
- The design information MIS 3005-D 3.8.1 asks the installer to give before the installation: the hot water cylinder and its recovery time, pasteurising, metering and the SCoP.
- Daily hot water: 45 litres for each person, at least the number of bedrooms plus one.
- The performance estimate of MCS 031, from the degree days for the postcode area (or your own) and the SPF you enter from the Heat Emitter Guide.
- The noise check of MCS 020 a) for the outdoor unit, against the 37 dB(A) limit.
What the German rules add to the report
The figures the VdZ form for hydraulic balancing (method B) asks for: the heat load of each room and of the house, the flow temperature, each emitter's flow and the lockshield settings, all from the same drawing.
What the French rules add to the report
The sizing note (note de dimensionnement) the CEE fiches BAR-TH-171 and BAR-TH-172 ask for: the heat loss at the base temperature for the rooms the heating serves, whether the heat pump is a low or a medium and high temperature application, and each heat pump's output at the base temperature from its own datasheet, against that heat loss. It has lines for the installer's RGE qualification, the client, the date and the signature: the installer signs it and gives it to the client.
What it does not do
- It is not an MCS-approved tool, and its results are not a certified calculation.
- No extra allowance for rooms over 4 m high, where warm air gathers under the ceiling.
- No reheat allowance for heating that is switched off at night.
- Chimneys, flues and extract fans are not entered as separate air flows.
- The Dutch, Belgian and French national annexes are not built in.
What it was checked against
The method reproduces a published DIN/TS 12831-1 worked example within 1% (907 W against 912 W through the fabric, 207 W against 208 W for the air), and the worked examples of MCS 031 and MCS 020 exactly. Its formulas were also compared with Heaty, an open-source implementation of EN 12831-1: the same air formulas and defaults, and ground floors within 5–10% (Heaty fits a curve where HeatLoss Pro uses the EN ISO 13370 formula).
Projects saved earlier
A project saved before HeatLoss Pro used EN 12831-1 keeps the simplified method it was calculated with (U × A × ΔT, plus 0.33 × n × V × ΔT for the air), so its figures do not change. In the HeatLoss Pro designer, drawing is free; a calculation with the full report costs €1.
Frequently asked questions
Can I use HeatLoss Pro for MCS paperwork?
Not on its own. Under the UK rules HeatLoss Pro follows the method MIS 3005-D sets out and gives the design information it asks for, but it is not an MCS-approved calculation tool. For MCS paperwork, use an MCS-approved calculator; HeatLoss Pro is a quick way to design and check the house before you do.
Why is the sum of the rooms bigger than the heat load of the house?
Each room is sized as if its neighbours could be cooler, and each room gets its full minimum fresh air. The house as a whole counts the air leakage once and half the minimum fresh air, as EN 12831-1 does, and heat passed between rooms cancels out. Radiators and underfloor heating are sized on the room loads; the heat pump on the house's.
Does it follow DIN/TS 12831-1 for a German subsidy application?
With the German rules it uses the figures of DIN/TS 12831-1 (room temperatures, airtightness, thermal bridges, neighbour temperature) and gives the figures the VdZ form for hydraulic balancing asks for. Check what the funding programme asks for, and have the calculation signed off by whoever the programme requires.
Does it give the sizing note for a CEE file in France?
Under the French rules the report adds the note de dimensionnement the CEE fiches BAR-TH-171 and BAR-TH-172 ask for: the heat loss at the base temperature, the heat pump's application and its output there from its own datasheet. The installer checks it, signs it and gives it to the client; HeatLoss Pro does not check the rest of the CEE file.
Do old projects change?
No. A project saved before HeatLoss Pro used EN 12831-1 keeps the simplified method it was calculated with, so its figures stay the same. New projects use EN 12831-1.
Do the whole house in 5 minutes
Trace your floor plan and press Calculate for every room's heat loss, the heat pump size and the radiator check. Free to try, nothing to install, no sign-up.
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