How Do You Size an Electric Furnace?

Step 1: Find Your Heating Requirement

Start with your climate zone and your home's total heated square footage. Each zone has a BTU per square foot factor. Use the lower number in your range if your home is well insulated and the higher number if it is older or poorly insulated. If you are unsure of your insulation, use the larger number.

Multiply the factor by your heated square footage. For example, a 1,500 square foot home in Zone 2 that is not well insulated would use the 35 factor: 1,500 times 35 equals 52,500 BTU required to heat the home.

Step 2: Match a kW Heating Element

Each kilowatt of an electric heat strip produces about 3,400 BTU, so the element options translate directly into heating output.

Electric furnaces come in roughly 7,000 to 10,000 BTU increments, so you only need to get close. If the furnace you pick falls more than 10 percent below your requirement, step up to the next size. A little under or over sizing is fine, but do not oversize by more than about 20 percent or short cycling can occur, which wastes energy and reduces comfort. If you need more than 68,000 BTU, consider two electric furnaces on separate duct systems or a gas furnace. Remember that a larger element consumes more power.

Step 3: Select the Right CFM

The CFM, or cubic feet per minute, rating refers to the volume of air the unit moves and matters mainly for air conditioning. If you plan to add cooling later, base your CFM choice on a professional estimate of your future needs. For heating only, choose the least expensive or middle CFM option within your heating range.

Your home's construction quality and insulation are unique and can greatly affect sizing, so treat these factors as a general guideline. When you buy from AC Direct, free technical support can help you confirm your selection before you order.