Don’t Confuse Design Temperature With Degree Days

People new to building science and HVAC are confronted with a lot of terms for things that are related but different.  Load and capacity.  Energy recovery ventilator (ERV) and heat recovery ventilator (HRV).  Hot water heater and cold water heater.  Another pair of terms—design temperature and degree days—describes how the local climate impacts the heating and cooling needs of a house.  They’re both useful but they give us different information.

Design temperatures

The design temperature is just what it says:  the temperature used in the design of the heating and cooling systems.  There’s one for heating and another for cooling, and they’re based on how low the temperature gets in winter and how high it gets in summer.

Winter design temperature  –  Defined as the temperature that a location stays above a certain percentage of the hours in a year.  The 99% design temperature is the one usually used.  That means a place stays above the 99% design temperature 99% of the hours in a year.  The total number of hours in a year is 365 x 24 = 8,760.  Ninety-nine percent of that is 8,672 hours, which means that if your 99% design temperature is 0° F as it is in Fort Collins, Colorado, your temperature will be below 0° F for only 88 hours per year, on average.

Summer design temperature – Defined as the temperature that a location stays above a certain percentage of the hours in a year.  Ha!  That can’t be right, you’re thinking.  It’s the exact same definition as for heating.  Well, it is right because the percentage isn’t specified yet.  For summer, it’s the 1% design temperature.  Only about 88 hours per year—on average—will your location go above your 1% design temperature.

You may live in Houston and object to the idea that your 1% design temperature could ever be as low as 95° F, but it is.  Some years are hotter than others, which why we use long-term averages.  Yes, it’s easy to remember that heat wave when it got to 106° F or that time it hit 99° F every day for a month, but the best temperature to use for designing your cooling system is the 1% design temperature.  (Of course, with climate change, average summer temperatures are increasing, but I’ll look at that issue in another article.)

Referring to the winter and summer design temperatures by numbers can be confusing, but there’s an easy way to remember which is which.  The number always refers to the amount of time a location stays above the design temperature.  So the 99% design temperature has to be winter because the actual temperature will be above that temperature 99% of the time.  Likewise, the 1% design temperature must be for summer because the temperature will go above that only 1% of the time, on average.

Degree days

Degree days is another way of combining time and temperature, but it has different implications for heating and cooling than does design temperature.

Heating degree days, HDD  –  This is the number that tells you how long a location stays below a special temperature called the base temperature.  I find it easiest to start with degree hours.  For example, the most commonly used base temperature for heating is 65° F. (Yeah, yeah, I wish we used Celsius, too.)  So if the temperature at your house is 50° F for one hour, you just accumulated 15 degree hours.  If the temperature is 56° F for the next hour, you’ve got another 9 degree hours and 24 degree hours total.  To find the number of degree days, you divide by 24, so you’ve got one degree day in this example.  You can do that for every hour of the year to find the total and then divide by 24.  Or you can use the average temperature for each day to get degree days directly.

Cooling degree days, CDD – Same principle as for heating degree days but usually with a different base temperature.  It gives you an idea of how hot and for how long it is at a location.

Seattle vs Atlanta – an example

Here’s an example to help you see the difference between these two different climate metrics.  Seattle and Atlanta have similar 99% design temperatures.  It’s 26° F for Seattle and 23° F for Atlanta.  That tells us something important about the peak heating load.  If we build the same exact house in those two cities—same floor area, same windows, same orientation…same everything—they’ll need a heating system about the same size.

What’s different, however, is that the people who live in the Seattle house will pay more for their annual heating than the people in Atlanta.  That’s because Seattle, with 4,800 HDD, has a lot more heating degree days than does Atlanta, with 2,800 HDD.

And that’s your takeaway.  Design temperatures are related to the size of the heating or cooling system you have to install, and degree days correlate with the annual heating and cooling bills.  It’s similar to the difference between power and energy, which I wrote about in my last article.

Allison Bailes of Atlanta, Georgia, is a speaker, writer, building science consultant, and founder of Energy Vanguard.  He is also the author of the Energy Vanguard Blog and writing a book.  You can follow him on Twitter at @EnergyVanguard.

Related Articles

We Are the 99% — Design Temperatures & Oversized HVAC Systems

The Fundamentals of Heating and Cooling Degree Days, Part 1

Do You Know Your Building Science Climate Zone?

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