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degree days

Predicting the amount of fuel needed to keep your home warm in the winter can be made more accurate by using degree days and the k factor. These two numerical measures provide an accurate baseline for assessing the amount of fuel used, the rate at which heating fuel is consumed, and the effects of outdoor temperatures on fuel usage.

Determining Degree Days

Degree days are used as a way to predict the heating or cooling requirements of a particular building. They can be calculated for both heating and cooling functions.

There can be more than one degree day in a 24-hour period. Degree days represent the number of one-degree decreases in temperature from an accepted average outdoor temperature that occur in a single 24-hour day. A winter day during which the temperature falls several degrees below the accepted average can represent several degree days, while a day when the temperature does not dip below that average will have zero degree days.

The calculation for determining degree days takes the high and low temperatures during a 24-hour period, adds them together, and divides the result by 2. The resulting number is subtracted from the accepted average temperature of 65 degrees to determine the degree days during a 24-hour period.

For example, assume the 24-hour period of Jan. 5 sees a high temperature of 30 degrees and a low of 20 degrees. The high and low will be added together to get the result of 50. The result of 50 is then divided by 2 to get 25. When 25 is subtracted from the accepted average temperature of 65 degrees, the result is 40 heating degree days.

Heating degree days can be determined from actual recorded temperatures or can be predicted using expected high and lows for a particular day.

Determining K Factor

The k factor is a calculation that shows how quickly a home is consuming the fuel it uses to produce heat. The k-factor calculation can be used in homes that use all types of combustible fuel, from natural gas to propane to heating oil.

A home’s k factor represents the number of gallons of fuel burned per degree day. Finding the k factor of your home requires longer-term records of fuel usage and the number of degree days between fuel deliveries. However, it can give you a useful baseline for assessing fuel usage and predicting when a new delivery of fuel will be needed.

The k factor is determined by taking the number of degree days between fuel deliveries and dividing it by the number of gallons of fuel delivered.

For example, assume a delivery of 100 gallons of fuel oil on Dec. 1 and another 100 gallons of oil on Jan. 1. In this example, 100 gallons of fuel were used during the period between deliveries. Assume that there were 800 heating degree days during that same period. The k factor would be determined by dividing 800 degree days by 100. In this case, that would result in a k factor of 8.0 for the period of time between fuel deliveries.

Using Degree Days and the K Factor

Degree days and the k factor can help you and your supplier determine when a new fuel delivery should be made. For example, if 100 gallons of fuel is the amount expected for each delivery, the number of gallons per delivery can be multiplied by the k factor to determine when a new delivery should be made.

Using the example above, 100 gallons of fuel per delivery multiplied by a k factor of 8 indicates that a new delivery of fuel oil should be made by the end of a period of 800 degree days. This type of calculation allows variables in seasonal temperatures to be taken into consideration to arrive at accurate delivery dates. If temperatures drop, more degree days will occur during a particular period, meaning that fuel delivery will have to be done sooner. If temperatures rise, fewer degree days will result, allowing a longer period of time before a new fuel delivery is necessary.

Griffith Energy Services, Inc. has served heating and cooling customers in the Mid-Atlantic region for more than 100 years, providing expert HVAC services to the communities of Baltimore, Hagerstown, Edgewater and Frederick, Maryland; Manassas and Berryville, Virginia; and Martinsburg and Charles Town, West Virginia. Contact us today for more information on degree days and the K factor and how these two figures can help you estimate your fuel needs for the winter.

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