Using Fuel Consumption To Properly Size Heating Equipment

In the past, it was common for heating contractors to size heating equipment using either the “gorilla method” or the “gut feel method.” The gorilla method assumes that the person who installed the old equipment knew what he was doing. Therefore, new equipment is sized to match the old equipment. With the gut feel method, the contractor guesstimates what size equipment to use based on the size of the home.

The problem with both methods is that they are inaccurate. In today’s world of high energy prices and environmental concerns, there is no excuse for sizing equipment using an inaccurate method. We have found that the easiest and most accurate method to properly size new heating equipment is using fuel consumption data.

To use this method, it is necessary to have both delivery and degree day history for a heating season. Degree days are a measure of how cold it is, equal to the difference between the average temperature for the day and 65°. For example, if the average temperature on a winter day is 30°, that equals 35 degree days (65 -30 = 35).

Although Hart & Iliff has its own degree day meter, this information is readily available from the National Weather Service for virtually every major, and many minor, airports in the United States. By comparing fuel consumption with degree days over the course of a heating season, it is relatively easy to generate a usage factor or “burn rate.” Like miles per gallon in a car, this burn rate tells us the fuel consumption per degree day. The average burn rate for a home in the Sussex County area is approximately .16 gallons per degree day.

The other steps in the process are as follows:

1. Convert the design temperature into degree days. Design temperature is a very cold day that is used to properly size heating equipment. In Sussex County, the design temperature (which occurs less than 2.5% of the time) is 10°F. This corresponds to 55 degree days (65 -10 = 55).
2. Compute daily oil consumption. Daily oil consumption, at the design temperature, is computed by multiplying the burn rate times 55 degree days. This would be 8.8 gallons (.16 x 55 = 8.8) for the average Sussex County home.
3. Convert gallons per day to BTUs (a measure of energy) per day. Heating oil contains 140,000 BTUs per gallon. Gallons per day times 140,000 equals BTUs per day. In our example, this would be 8.8 x 140,000 = 1,232,000 BTUs per day.
4. Convert BTUs per day to BTUs per hour. BTUs per day is converted into BTUs per hour by dividing BTUs per day by 24 hours. In our example, this would be 1,232,000 ÷ 24 = 51,333 BTUs per hour.
5. Choose heating equipment. Since heating equipment is rated in BTUs per hour, it is easy to choose a boiler or furnace by comparing BTUs per hour (computed above) with the BTU per hour rating of new heating equipment.

We have used this fuel consumption method for sizing heating equipment for many years and have found it to be extremely accurate. While this article discussed how to use fuel consumption to size an oil-fired heating system, the same concepts can be used to properly size natural gas or propane heating equipment.