Degree Days and Heating Degree Days

How to Calculate Degree Days and Heating Degree Days

Since the macro output contains the cumulated values of the degree days since application start, we recommend at start-up to initialize the degree days with the current values since the beginning of the current year.

The macro updates the degree days every hour and adds this value to the total cumulated value at the corresponding output.

Degree Days

For degree days, the macro calculates the difference between the room temperature setpoint and the hourly average outside air temperature. If the average outside air temperature of the current hour is less than the room temperature setpoint, the macro adds the following quantity to the output value "DegreeDays":

(RMT-01 – average outside air temperature of the current hour)/24

Heating Degree Days

For heating degree days, the macro calculates the difference between the heating limit and the hourly average outside air temperature. If the average outside air temperature of the current hour is less than the heating limit, the macro adds the following quantity to the output value "HtgDegreeDays":

(HDD-01 – average outside air temperature of the current hour)/24


Fig. 1.   Degree Day and Heating Degree Days


When to Use the Two Different Values?

Degree days are the correct value for an energy balance, where solar and internal heat gains have to be considered during the heating season and the heat demand will be reduced correspondingly.

Heating degree days are the correct reference value to compare the energy consumptions of different years and normalize them to clean them up from climate influences. Normalization means to divide the consumed energy by heating degree days. By this way one will get the heat demand depending on temperature difference. Comparing these numbers for several heating periods, one can recognize whether his energy efficiency has increased or not.