Heating Calculations for Room Sizes

Btu

• A Btu, or British Thermal Unit. is defined as the energy needed to raise the temperature of one pound of water by one degree. For example, one kilowatt hour of electricity puts out 3,412 Btus, fuel oil has 140,000 Btus per gallon, natural gas has 1.02 million Btus per thousand cubic feet and wood has 8,000 Btus per pound. Heaters and heating systems are normally sized according to Btus. A small heater might put out 5,000 Btus while a natural gas furnace for an entire home might have 90,000 Btus.

Rooms

• One way to calculate the amount of heating a room needs is to first multiply the length and width of the space in feet to produce the square footage. For example, a 12-by-12 room will have 144 square feet. Then, multiply the square footage by 10 for the number of watts needed by the heater. In this example, a 144-square-foot room need 1,440 watts. Finally, because each watt equals 3.41 Btus, multiply that number by the number of watts. For this case, a 1,440-watt heater provides 4,910 Btus, so that is the minimum heating size required by the room.

House

• Michaels Engineering, an environmental consulting firm, states that a quick way to estimate the Btus needed by an entire house is to multiply the square footage by 40 to 45. So, a 2,000-square-foot home needs 80,000 to 90,000 Btus per hour for heating. This method may overestimate the amount needed, however. Alpine Home Air Products suggests asking a neighbor whose house is similar in size and design to yours what furnace he has, its specifications and how he uses it. Take into account the number of hours his unit is on and the number of people living at home. This enables you to more accurately compare how closely your heating needs match his.

Contractors

• According to Consumer Reports, the most accurate way to calculate the heating required for a home is to hire a contractor. He can then use the industry standard calculation defined by Manual J of the Air Conditioning Contractors of America. This document accounts for climate; home design and location; the house’s orientation to the sun; insulation; number of windows and doors; and the number of inhabitants. He can also use Manual D to size the heating ducts using similar factors.