The temperatures where we live can be represented in many ways
Temperatures will vary from year to year. So the question is how to summarize them in such a way that they can be used for calculations on your needs.
This is done in many ways. Two of the common ones are describing the lowest average outside temperature (lowest DUT) seen during the last years, and the average temperature for different locations.
Lowest average outside temperature in Malmoe over the years
The lowest daily average temperatures vary a lot. They are important to know in order to know how likely it is to need to handle a certain temperature. But, in my opinion, they vary too much to dimension the heat pump after. It is enough to use them to dimension the needed total max heating power, thus to dimension possible additional heating capabilities and also to dimension the radiators as well as deciding to add additional radiators.
Below are temperatures taken from Copenhagen, but I use them for Malmoe. As can be seen in the diagram, Malmös lowest temperature over the last years will vary heavily due to free temperature spikes. Being sure to deliver heat enough for that is one thing, but dimensioneing the whole heat pump for those spikes is creating an inefficient heating solution (Malmös lowest temperatur for dimensioning for example has been -17 for many years now, and suddenly it will increase to minus 12, which is a huge difference).
Yearly average outside temperature in Malmoe
These are good to know as well. But maybe the most important notice is that there is a huge difference between the lowest temperatures and the averages. Using only these two figures for dimensioning a heating system will be a very rough work. I thus prefer doing calculations based on daily average temperature data.
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