Heating Degree Days, Heating Days, Heating Limit..

What is behind these terms?

Heating Days, Heating Limit and Heating Degree Days: Behind these terms, there are characteristic values for evaluating the energy consumption – precisly the heating energy consumption – of buildings. The very similar sounding metrics posses similar calculation basis with different parameters.

You may wonder:

  • What is the difference between a Heating Day and Heating Degree Days?
  • How are Heating Days defined?
  • How do I calculate Degree Days?

The following information will give you an overview of the definitions and the calculation methods to answer your questions above.

Classification of Terms

There are various methods for assessing consumption parameters of buildings, i.e. energy and water consumption. Heating Degree Days are used to describe, evaluate and compare the heat consumption of buildings. The metric reflects the climate conditions of a location, which influence the heating demand. In this way, heating energy consumption of a buildings can be compared over various heating periods and take the weather influence into account. DIN EN ISO 50001 refers to this as Normalization of Consumption.


Find out more about Heating Degree Days in 2 minutes video and how to apply it – illustrated with a simple use case.

Heating Limit

The Heating Limit is a threshold value of the outside air temperature below which heating is required to maintain the desired room temperature. It depends highly depends on the building type.

In context of Degree Days the Heating Limit is also called Base Temperature or Balance Point.

If you do not find any information on the Heating Limit Temperature, a fixed heating limit of 15°C can be assumed according to German publications.

Heating Day

Simply explained a Heating Day is a day when you need to heat a building or on other words when the outside temperature is below the Heating Limit. According to the German standard VDI 4710 Sheet 1, Heating Days are “days on which the Daily Mean Temperature is below the Heating Limit Temperature”.

Many standards and calculation methods only accumulate Heating Degree Days on Heating Days calculated by the daily mean temperature. Other methods simply take the average of minimum and maximim air temperature (our Degree Day Calculator handle both approaches and the more precise “Integration Method”)

Heating Days are counted over a defined period – typically a heating period, calendar year or calendar month – and a Heating Limit Temperature is determined.

Heating Degree Days

Heating Degree Days (HDD) are the product of a day and the difference between the Heating Limit Temperature and the outside air temperature. The unit is Kelvin days (K • d). HDD are only determined on Heating Days, respectivly if the outside air drops below the Heating Limit, otherwise the HDD = 0.

By this, Heating Degree Days is a suitable key figure to carry out a climate adjustment for measured consumption.

“Room Heating Degree Days“ (German Method)

“Room Heating Degree Days“ (DD) are a method for Heating Degree Days, mainly used in Germany, that includes the aspect of the desired room temperature (German: “Gradtagzahl”). A “Room Heating Degree Day“ is the product of a day and the difference between the desired Room Temperature and the Daily Mean Temperature applicable on that day. The units are Kelvin days (K•d). It is determined only on heating days, respectivly if the outside air drops below the Heating Limit, otherwise the DD = 0.
According to the German standard VDI 3807, in Germany 20°C is used as the Mean Room Temperature and 15°C as the Heating Limit Temperature for weather adjustment.
“Room Heating Degree Days“ are the correct input variables for an energy balance calculation that also takes solar and internal gains into account.

It is usually given as the sum for a heating season, calendar year or calendar month. The calculation method is specified in the VDI guidelines VDI 2067 and VDI 3807.

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