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PROMOTING EXCELLENCE IN NYC MULTI-FAMILY BUILDING OPERATION AND MAINTENANCE

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Porters, Handymen, and Doorman, or PHD's Blog
 
Degree Days
 

 

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What Are Degree-Days? And How to Use Them

If, on Day X, the average outdoor temperature is, say, 40 degrees Fahrenheit (40F), the difference between 65 and 40 is 25, or 25 Degree-Days (25 DD).

If, on Day Y, the average outdoor temperature is, say, 15F, then 65 - 15 = 50 DD. If your heating systems is properly controlled, you will have used twice as much fuel on Day Y as on Day X because 50 DD is twice 25 DD.

You can use this information to check on the operation of your building's heating system.

 The reason that the Degree-Day method works at all is that the fuel used to heat a building is roughly proportional to the difference between the indoor temperature and the outdoor temperature.

The colder it is outdoors, the faster the indoor heat flows through the walls and roof of the building to outside. The method assumes that your  indoor temperature is about 70F.

The difference between 70 and 65 is because of the heat that is generated inside the apartments, so that if you set your thermostat to maintain 70F in the apartment, it would have to go down to 65F outdoors before the temperature in the apartment went down to 65F and caused it to signal the boiler to go on.

On Days X or Y for instance, the normal fuel consumption for each building is different, depending on the size of the building, how well it is insulated, etc. Therefore, the building superintendent has to perform another calculation.

By dividing the gallons of oil or the therms of gas consumed by the DD for a particular day, a new figure, fuel consumed per DD is arrived at. Except during extremely mild weather, that number should remain fairly constant.

If it goes up, that is a signal to the superintendent that something is wrong with the heating system and it is his responsibility to try to find out what the problem is. (It could be that the burner went out of adjustment, for instance.)

The normal practice is to perform monthly calculations, dividing the month's consumption of fuel by the total of the DDs for the month.

The easiest and most accurate way to get the monthly DDs is to call the U.S Weather Service at 631-737-1000 and get a recording of lots of data, including DD total for the previous month.

This information is logged onto a sheet that looks something like this. (If you call Apartment House Institute, you will be mailed a blank one for your use.) 

 The log sheet looks like this:

HEATING DEGREE DAYS - BLDG. 1

Month Gal/Oil DD Gal/DD Notes
Jan '98 1500 1000 1.5
Feb 1000 700 1.4 good
Mar 800 400 2.0 too high!
Apr

Problem: If you are using the same fuel supply for both heating and DHW, you have to estimate how much of the fuel was used for heating, which is the number you enter into the log. Use your fuel consumption for July or August, and subtract.

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