Answers: heating, cooling degree days

By Jack Williams ©2015

Q: Several years ago, you wrote a short article for USA TODAY about heating degree days. I have depended on that article, and its links, to monitor the heating and cooling degree days in my area.  Now, I cannot access this article any longer through a USA TODAY search and I’d really like to have access to it.  I would appreciate some direction from you as to how or where I can find this article with its respective links now.

–Hank, Chesterfield, Mo.

A: The good news is that I found the article with a Google search–in two places–but the bad news is that the the link no longer works in the version in a back corner of the USA TODAY site. I also found it on the Cleveland National Weather Service office Web site, but that site doesn’t have the link.

I’m including the link you need in my discussion below with a little added information on how to use heating degree days and cooling degree days.

I won’t repeat here the explanation of how heating and cooling days are calculated since that’s described in the stories linked to above. As they say, these “days” are an easy-to-use way for heating and cooling engineers to relate the energy demands for heating and cooling to daily temperatures.

I think the major use a home owner has for them is to see how well improvements, such as better insulation, windows that don’t loose as much heat as the old windows, or a new furnace or air conditioner are paying off.

To do this, you compare the amount of energy used to the number of heating or cooling degree days before and after the improvements. The  companies you buy electricity, gas, or heating oil from should be able to supply the figures. In fact I found that the Web site of the  gas company we use lists the therms of gas used and the heating degree days for the past 12 months. The electric utility lists the number of kilowatt hours used each month, but doesn’t list cooling degree days.

You can find heating and cooling degree days by month on the Web site of the nearest National Weather Service by clicking on “Climate, Local” on the left side of the NWS office’s Web site home page. You want to select the “Observed weather” tab at the top of the page, and then select “Monthly Weather Summary (CLM)” under the “Products” listing. On the report for any month you will find heating and cooling degree days for that month and also for the “season” about half way down the page.

The quickest way to find the nearest local NWS office is to go to the NWS Organization page and look for the office nearest to where you live.

Instead of using the cost of the energy I recommend using measurements of the amounts of energy used such as kilowatt hours for electricity, gallons for fuel oil, and therms for natural gas. (A “therm” is the heat energy in gas equal to 100,000 British thermal units (BTUs), which is approximately the energy released by burning 100 cubic feet of natural gas.) My gas company lists the therms used but I can imagine cubic feet being used.

I recommend using these measurements rather than costs because you’d have to factor in changes in the price to compare your new furnace or air conditioner to the old one. For example, for heating I calculate the therms per HDD by dividing the number of therms by the HDDs. The figures I got for each month from last November through this April ranged from .14 to .20 terms per HDD.

I think this illustrates that for a good comparison you should average the figures for at least a few months. We have a gas water heater as well as furnace. I suspect the monthly differences are from using different amounts of hot water and also wanting the house warmer on some days than others.

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