Energy Audit-Heating Degree Days (HDD)

On occasion I need to look up the heating degree days (HDD) for an area over a period of time to complete an energy audit.  Weather data can be useful for estimating heating costs.  So, what is a heating degree day?

A heating degree day is a unit of measurement comparing an average outdoor temperature to a 65°F indoor temperature over a 24 hour period.  The 65°F temperature is used because this is considered a heat balancing point, or a temperature where heating will not be required inside a home or building.  Of course HDD is used if you live in a heating climate like me.  If you live in a cooling climate, cooling degree days (CDD) is more important.  CDD uses a balance point temperature of  78°F.

HDD are figured by averaging the daily temperature (24 hour period) and subtracting that value from 65°F.  Lets say you have daily high temperature of 20°F and a low temperature of -20°F, the average daily temperature is 0°F.  Subtract that from the 65°F and you have 65 heating degree days.

A very cold climate, such as one of the colder cities in the US, International Falls, Minnesota has a HDD of 10,000 over the course of a year.  In comparison, Washington DC has around 4,200 HDD in a year.  Some areas of Alaska see more than 12,000 HDD.

I find HDD data from the Weather Underground web site,  A very handy tool I use when needing weather related information.  A good example is the weather for my area at the end of January of 2019.  We were under a polar vortex with average daily temperatures of less than -20°F.  These single days were producing 80-90 HDD.  Check out zip code 55744 over the date of January 31, 2019, a very cold day in Northern Minnesota.

So, how can this be helpful with an energy audit?  Lets say your monthly heating bill is higher than normal.  The reason may be outdoor temperature related.  Higher than average heating degree days over the course of a month will require more indoor heat to maintain comfort, increasing heating costs.  I often compare data from previous years to the current period to confirm heating costs to temperatures comparisons.  Weather data can also be used to compare summertime cooling needs and the associated air conditioning costs.

HDD and CDD, another tool used in energy auditing.

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