The temperature a heating system is designed for
A heating system does not have to be designed for the coldest day of the century, but for a statistically defined low outdoor temperature. This design outdoor temperature is the most important climate value in a heat load calculation. It determines the temperature difference between inside and outside, and therefore directly the heating power needed.
Since April 2020 Germany has used the technical specification DIN/TS 12831-1 for this. It assigns a value to every postcode and replaces the older tables listed by city. The design outdoor temperature is a two-day mean. Individual nights can be considerably colder, but the thermal mass of the building evens out short cold spells.
Values for larger cities
The values below come from the climate map of the German heat pump association, which returns the design outdoor temperature to DIN/TS 12831-1 per postcode. Each value applies to the postcode area given; outer districts and higher locations differ.
| City (postcode) | Design outdoor temperature |
|---|---|
| Düsseldorf (40213) | −6.9 °C |
| Cologne (50667) | −7.0 °C |
| Essen (45127) | −7.1 °C |
| Kiel (24103) | −7.9 °C |
| Hamburg (20095) | −8.2 °C |
| Bremen (28195) | −8.6 °C |
| Frankfurt am Main (60311) | −9.2 °C |
| Stuttgart (70173) | −9.3 °C |
| Freiburg im Breisgau (79098) | −9.7 °C |
| Hanover (30159) | −10.0 °C |
| Rostock (18055) | −10.7 °C |
| Berlin-Mitte (10115) | −11.1 °C |
| Munich (80331) | −11.1 °C |
| Nuremberg (90402) | −11.3 °C |
| Leipzig (04109) | −11.7 °C |
| Erfurt (99084) | −11.9 °C |
| Dresden (01067) | −12.3 °C |
| Chemnitz (09111) | −13.0 °C |
| Augsburg (86150) | −13.5 °C |
| Hof (95028) | −14.6 °C |
| Oberstdorf (87561) | −16.7 °C |
Retrieved on 24 September 2026. For your exact address, look up the value by postcode or have your heating contractor confirm it.
What the difference means
The range is wider than many expect. A house at 20 °C indoors has a design difference of 27 kelvin in Cologne but almost 37 kelvin in Oberstdorf. With an otherwise identical building, it needs about a third more heating power there. Altitude and urban climate also count within a region: city centres are often warmer in winter than the surrounding countryside.
Common misunderstandings
- The design outdoor temperature is not the record low. Measured minimums are lower almost everywhere. A correctly sized heating system still copes on those days, because the building stores heat and use rarely demands maximum output in every room at the same time.
- Older values are often colder. Many planning documents still use the earlier city tables. The new values are based on a more recent climate series and are somewhat milder in many places. Calculating with old values tends to oversize the heating system.
- A few degrees of extra safety are not safety. Calculating five kelvin colder just to be careful raises the heat load by 15 to 20 percent. For a heat pump, that means a more expensive unit that cycles more often.
The indoor temperature counts too
The temperature difference has two ends. The standard assumes 20 °C for living rooms and bedrooms and 24 °C for bathrooms. Calculating the heat load for 22 °C throughout the house raises the difference in Berlin from 31.1 to 33.1 kelvin, and the heat load by just over 6 percent. That is why the calculator lets you set the room temperature separately.
Design outdoor temperature in the calculator
In the second part of the U-value calculator you choose one of the cities from the table or enter your own value. The calculator uses it for the heat load and for the temperature profile through your building element. That way you can see how warm the inner surface of your wall stays on the design day. How the temperature difference translates into heating power is explained in Estimating heat load.