Shorts-Climate Zone and Heating Degree Days

I have a confession to make, but you will have to read this blog post before you’ll know what that is.

I woke up this morning to a very cold temperature.  Nowhere near the coldest I’ve ever experienced, which was right around -50°F.  (I remember that stretch of weather back in 1996, Minnesota set its all-time low temp about an hour away from where I live, that temp was -60°F.)  This got me thinking about the climate zone I live in and heating degree days.

I live in climate zone 7, but what does that mean?

The climate zones are based on the average heating degree temperature (HDD), (we will get into what an HDD is in a bit.)  The cool and cold climate zones across the US start in climate zone 5 (in my opinion) where they have less than 5,400 heating degree days.  Climate zone 6 has between 5,400 and 9,000 heating degree days and my area, climate zone 7 has 9,000 to 12,600 heating degree days.  There is also a climate zone 8, those lucky folks in Canada and a good portion of Alaska will experience more than 12,600 heating degree days.  The map is basically an indication of a winter’s intensity.

What is a heating degree day?  A heating degree day is the number of degrees that the average temperature is below a given temperature, in the US, that average is 65°F (18°C).  yesterday’s average temperature (February 23, 2022) in Grand Rapids, Minnesota was -9°F.  The formula for calculating heating degree days is 65°F-the average temp of a given day.  65°F minus -9°F equals 74 heating degree days.  If we add all those heating degree days over the course of a year, this tells us which climate zone we are located.  Over the past 12 months, Grand Rapids, Minnesota has had 10,498 heating degree days, firmly in climate zone 7.

How is an HDD useful?   I often use heating degree days to calculate the cost of an air leak when performing an energy audit.  This can be done with the formula: (26 x HDD x cost of heating fuel/BTU x CFM50(the CFM blower door number)) / (n-factor x heating efficiency) x .6.  This will give me information I can use to determine if an energy cost upgrade will have a good return on investment.  Not the only metric used to determine if making improvements to a home is a good idea, but it does add a monetary amount to the decision.  There are a few other formulas that use HDD, cost of ventilating a home is one, calculating the heat loss across a building shell is another.

Where can you calculate your heating degree days?  I use a website when I need to calculate the HDD for my location.  Heating & Cooling Degree Days – Free Worldwide Data Calculation.  Enter in your location along with the time period and you’ll get a nice spreadsheet showing the HDD info.

Now for my confession, I’m ready for winter to be over!  I enjoy the changes in the season.  I like seeing the snow and not being bitten by mosquitos for 9 months out of the year.  But this year has been too cold for too long with more snow than we’ve had in several years.  Spring can come anytime now.   I’m ready to see our heating degree days closer to 0.

2 Replies to “Shorts-Climate Zone and Heating Degree Days”

  1. Hi Randy,

    It has been a tough winter in northern MN. Really brutal winds in February in NW MN. Can you break down your formula for costing air leakage?
    (26 x HDD x cost of heating fuel/BTU x CFM50) I follow it except the 26.

    Heating degree days is a wonderful tool, can be used in many ways .

    Thank you,

    1. Hi Doug,

      According to my Residential Energy book I got several years ago when I took energy auditing training, “the number 26 combines the heating capacity of air (.018) with the factors 24 and 60 for relating CFM to HDD.” I had the same question a couple years ago and had to look it up. The last part of the formula, you have to multiply the result of the formula times .6. I believe that is an adjustment needed because of a difference comparing actual heat loss costs to the estimated amount using the formula.


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