Back to UtilityDirect Main Menu

Understanding Heating and Cooling Degree Days

Weather is a key factor in determining the efficiency of your buildings. This page is designed to give you an understanding of how Heating and Cooling Degree Days are calculated and what they tell us. This will help you correlate utility usage with trends in weather in your area. Identifying abnormalities can assist you in pin-pointing potential usage issues and inefficiencies with your buildings. This can help you implement strategies to save money.

What are Heating Degree Days and Cooling Degree Days?

Heating Degree Day

  • Definition of Heating Degree Day (HDD): The number of heating degrees in a day is defined as the difference between 65°F and the mean temperature (average of the daily high and daily low).
    • Example:

     

     

    • So, if the Average Daily Temperature is 30°F then the difference from 65°F is 35°F. Therefore, you have a 35 Heating Degree Day. In other words, you have had to heat the building by 35°F that day to reach 65°F.
      • Mean daily temperature (MDT) = (Daily High Temp + Daily Low Temp)/2
      • HDD = 65°F – MDT

Cooling Degree Day

  • Definition of Cooling Degree Day (CDD): The number of cooling degrees in a day is defined as the difference between the mean temperature (average of the daily high and daily low) and 65°F.
    • Example:

     

     

    • In this case, the Average Daily Temperature is 80°F then the difference from 80°F to 65°F is 15°F. Therefore, you have a 15 Cooling Degree Day. In other words, you have had to cool the building by 15°F that day to reach 65°F.
      • Mean daily temperature (MDT) = (Daily High Temp + Daily Low Temp)/2
      • CDD = MDT - 65°F

Why is 65°F Used When Calculating Heating and Cooling Degree Days?

Heating Engineers needed a way to relate the temperature everyday to a building’s demand for fuel to heat them. This led to the development of Heating Degree Days and conversely, Cooling Degree Days. 65 is a neutral base temperature where it is generally not too hot and not too cold, and is adequate for human comfort.

 

The calculation shows how much is needed to get to the base temperature of 65°F. The main thing to remember about HDD and CDD is that it is an approximation and gives a general idea as to how much energy is required to heat and cool your buildings.

Why is this Important?

Calculating your Heating and Cooling Degree Days gives you wealth of information. As an example, if you have an abnormally high Natural Gas Usage and Cost in May, then you'll want to ask questions to find out why this occurred. Was there a cold snap? Were the set points for the HVAC system set too high? Did someone leave a door or window open that affected the cycles the HVAC?

 

The first place to look is at the Weather Trend Report in UtilityDirect. It will show the monthly HDD and CDD totals for all 12 months of one or more calendar years. Months like January will usually just have HDD, while July will just have CDD. More mild months like April and October will probably show some of both and this is normal. Of course, this is based on Zip Code, so the numbers will be in line with where you are in the country.

 

If the spike in use and cost is experienced in May and there are little or no Heating Degree Days, then there may be another reason this occurred, be it human, mechanical, or building envelope related. Since weather is not a factor more investigation should be warranted to find out the reason for the abnormality. Finding the root of the issue could save you a lot of time, money, and resources in the future.

 

Click here to view the National Oceanic and Atmospheric Administration’s explanation of Degree Days.

 

Did you find this helpful? 

 

Back to Top