Posted by: lcowie | November 16, 2010

Heating and Cooling Degree Days

Your air is going to kick in when the day’s average temperature outside is above 76 degrees. Similarly, your heat is going to turn on when the day’s average temperature outside is below 65 degrees. These concepts are called Cooling Degree Days and Heating Degree Days. You can use this information to determine the cooling or heating loads in an area and compare them to a different geographic location.

Backing up a bit, a cooling degree day is the number of degrees that a day’s average temperature is above 76 degrees, the temperature above which buildings need cooling. So, if today’s high and low temperatures are 100 and 70, you will want to get the average ((100+70)/2=85). Then, you will find the difference of 76 degrees from 85 degrees. You get 9 cooling degree days for one single day.

A heating degree day is the number of degrees that a day’s average temperature is below 65 degrees, the temperature below which buildings need to be heated. If today’s high and low is 30 and 20, you will find the average ((30+20)/2=25), then find the difference from 65, so 65-25=40 heating degree days for one single day.

You can continue this process for each day in a month and determine the average number of heating or cooling degree days in the month. This information is useful for comparing heating or cooling loads in different geographic areas. The only problem with this process is the U.S. map that you use to compare the loads. The map is split into a variety of sections, but the sections are mostly vertical, meaning that Montana and Arizona would fall in the same zone. Montana and Arizona have very different climates! So if you do these calculations, you have to remember that the heating degree days in Arizona are going to be different than the heating degree days in Montana, even though they are in the same Mountain zone.

This is a pretty complicated concept to explain, but it is pretty interesting to compare states on the map. For heating degree days, the northern states generally have 5000-7000 HDD, and the southern states have 2000-3000 HDD. For cooling degree days, the northern states generally have 400-1000 CDD, and the southern degree days generally have 1000-3000 CDD. You see how the numbers flip, depending on HDD and CDD? It correlates, mostly, with the climates of each region.

The best thing to remember about BPI calculations and concepts is that you have to “think differently.” None of this material is hard to understand or work with. You just have to think differently. I kept telling myself that I could complete the BPI training and that I just needed to rearrange my thinking. I had never thought about temperature in such a way. You just have to go with the flow and innocently absorb what you are told. If you take in the information with an objective understanding, you will get it.

Leave a comment if you have any thoughts regarding HDD and CDD. It’s somewhat difficult to explain!

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