Hello all,

To my surprise and happyness, I discovered that I now can enter the north-south orientation of windows in the heat-demand-calculator.

However, I also noticed that now, when I change the room-temperature in the heat-demand-calculator (while all other data stay the same), the gradtage changes.

To my knowledge gradtage is the year sum of all delta's between reference temperature (18degree) and avg.day temperature.

Can anybody explain the dependency to room-temperature?

If you choose a higher room-temperature there are more days needing heating- consequently there are more Gradtage. Same vice versa.

Andreas Teich

I expected the Gradtag to be an international standard. But according wikipedia it is not:

wikipedia.nl:
Een graaddag is relatief ten opzichte van een referentie temperatuur, meestal die waarbij geen verwarming meer nodig is (typisch 18 graden Celsius).

wikipedia.de:
Allgemein berechnet man für jeden Tag einen Temperaturwert in °C (beziehungsweise K)[1] oder angloamerikanisch °F, und summiert über die Tage. Darüber hinaus gibt es oft bestimmte Korrekturfaktoren.

wikipedia.org (en):
Degree_Day
If the air temperature outside is below 15.5 °C, then heating is required to maintain a temperature of about 18 °C. If the outside temperature is 1 °C below the average temperature it is accounted as 1 degree-day. The sum of the degree days over periods such as a month or an entire heating season is used in calculating the amount of heating required for a building. Degree Days are also used to estimate air conditioning usage during the warm season.

The Dutch use one fixed reference at 18degree, in Germany there are multiple references (depending on the usage). The English use the same standard as the Dutch.

@Andreas Teich
I do not agree with your answer. Degree Day is temperature difference to a reference. Room temperature can be chosen as a reference, but Wikipedia seems to disagree.

The amount of DegreeDay depends on the outside temperature. When you chose a room temperature, you need more/less energy to keep that room temperature with more/less DegreeDay. The room temperature does not change the amount of DegreeDay in my opinion.

Following your definition of DegreeDay would make it a unit which can not be compared. You can only compare the amount of DegreeDay in Aachen to that in Bonn, when you know the room temperatures by which they are given. That would make it an unpractical unit in my opinion.

Dear VAlKo,

if you read a little bit further in Wikipedia or on other webpages, they do say that the Gradtage depend on the room temperature and that the numbers are hardly comparable.

E.g. in the german Wikipedia:

Die Berechnung der Wärmesummen erfolgt – je nach Betrachtungskontext – in unterschiedlicher Weise. Das führt dazu, dass sich Ergebnisse so stark unterscheiden, dass sie untereinander kaum vergleichbar sind.

[...]

In der Klimatechnik der Gebäude gilt für die Heizgradtage (G, auch Gradtagzahl Gt, durch das Zeitintervall geteilt) eine Rechnung zwischen einem Innenraumheizziel von 20 °C und Tagesmitteltemperatur außen, und zählt nur die Tage oberhalb einer Heizgrenze bei 15 °C (G15 etwa deutsche VDI 2067/DIN 4108 T6, VDI 3807) oder 12 °C (G12 für Österreich, Schweiz, Liechtenstein), oder eine ortsübliche Heizperiode. Analog werden die Kühlgradstunden berechnet.

Or on the webpage of the DWD:

https://www.dwd.de/DE/service/lexikon/Functions/glossar.html?lv2=100932&lv3=101064

Best regards,
Ralf Plag

It's not really unpractical because 'Gradzahltage' take into account the different climate in different areas compared with the inner room temperature.
Consequently the need for heating is less in Freiburg compared to a building situated at the Zugspitze.
Besides- the chosen room temperature may also be different in different countries.

Considering the number of 'Gradzahltage' and the U-values of a building, heating system etc you can compare heating costs of buildings in different aereas.

Andreas Teich

@Ralf Plag
I stand corrected. Thanks for pointing me to this.

@Andreas Teich
Lets agree to disagree.

In my opinion it is unpractical, since GradTage in one city cannot be compared to another city, because the room temperature is not known. For example: the GradTage in a city near The Alps measured with room temperature of 5 degree may be less then the GradTage in a city near the coast measured with room temperature of 25 degree. Only when the room temperature is the same (when you have a fixed reference), the GradTage can be compared. Therefore i think the GradTage referenced to room temperature is unpractical.

But, as I said, lets agree to disagree.

The supposed room temperature is always the same, usually 20 or 21 degrees.

Only the outside temperature is different, according to the relevant climate datas.

Using these you get the 'Gradzahltage', enabling you to compare the demand for heating of buildings in different aereas.

Andreas Teich
Gebäude-Energieberater, Planungsbüro, Bau- und Selbsthilfeberatung, Begutachtungen