HEATLOSSPRODesign outdoor temperature
Every heat loss calculation starts from one number: how cold a day the heating must cope with. Here are typical values, and what choosing a different one does to the result.
In short: the design outdoor temperature is a cold day that is only rarely colder. Typical values are about −2 to −5 °C in the UK, −10 °C in the Netherlands, about −8 °C in Belgium and −10 to −16 °C in Germany. The heat loss scales with the indoor-to-outdoor difference, so each degree colder adds about 4% at 20 °C inside.
Typical design outdoor temperatures
| Country / region | Design temperature | Where it comes from |
|---|---|---|
| London and south-east England | about −2 °C | MCS, per postcode area (CIBSE data) |
| Midlands and Wales | about −3 °C | |
| Northern England | −3 to −4 °C | |
| Scotland | −4 to −5 °C | |
| Netherlands | −10 °C | ISSO 51, Dutch annex to EN 12831 |
| Belgium (Brussels) | −8 °C | Belgian annex to EN 12831, colder with altitude |
| Germany: Cologne | −10 °C | DIN/TS 12831-1, per location |
| Germany: Hamburg, Frankfurt | −12 °C | |
| Germany: Berlin | −14 °C | |
| Germany: Munich | −16 °C |
These are typical values to get started. For a formal design, use the figure the standard or scheme you work to gives for the exact location and altitude.
What a different design temperature does
Heat loss is proportional to the difference between inside and outside. If you know the heat loss at one design temperature, you can work it out at another.
Why it matters for a heat pump
A heat pump gives less heat as it gets colder outside, so its output must be checked at the design temperature, from its datasheet, not at the +7 °C rating on the front page of the brochure. Pick the design temperature first, calculate the heat loss at it, then choose the smallest heat pump whose datasheet output at that temperature covers the load.
In the HeatLoss Pro designer you set the design outdoor temperature once for the project, and every room's heat loss, the heat pump size and the radiator check follow from it. Drawing is free; a full calculation with a PDF report costs €1.
Frequently asked questions
What is the design outdoor temperature?
The outdoor temperature a heating system is sized for: a cold day that is only rarely colder, typically one exceeded on about 99% to 99.6% of the hours in a year. The heat loss is calculated at that temperature, and the heat pump must give that output at that temperature.
What design outdoor temperature should I use in the UK?
Roughly −2 °C in London and the south-east, −3 °C in the Midlands and Wales, −3 to −4 °C in northern England and −4 to −5 °C in Scotland, colder on high ground. MCS publishes the figure for each postcode area; use that for an MCS-compliant design.
What design outdoor temperature is used in the Netherlands?
−10 °C for the whole country, the value used with ISSO 51 and the Dutch annex to EN 12831. Belgium uses about −8 °C for Brussels, colder with altitude; Germany uses −10 to −16 °C depending on the location.
What happens if I choose a colder design temperature?
The calculated heat loss rises in proportion to the indoor-to-outdoor temperature difference. At 20 °C inside, going from −3 °C to −5 °C raises the heat loss by about 9%. Choosing it too cold oversizes the heat pump, which then cycles on mild days; too warm, and the backup heater runs on cold ones.
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