The Correct CFM for Electric Furnaces

Zones

• Climate in your area helps determine your heating needs. Northern states need 45 to 50 Btus per square foot, while Southern states only need 25 to 30 Btus, according to the website AC4Life. Convert CFMs to tons for estimated air conditioning needs. If you need 2,000 cubic feet of airflow per minute, you’ll need a 4-ton or 5-ton air conditioner. A 1.5-ton or 2-ton unit produces 800 CFMs, and a 3-ton to 4-ton unit produces 1,600 CFMs. If you have kilowatt figures, estimate that a kilowatt produces about 3,400 CFMs of heat. Estimates aren't accurate enough for final calculations for your heating and cooling unit.

Annual Fuel Utilization Efficiency

• The Federal Trade Commission requires new furnaces to display an annual fuel utilization efficiency (AFUE) rating, or the ratio of heat output to the fuel or energy consumed. High-efficiency electric systems will have 90 percent to 97 percent AFUE, according to Energy Savers. A high AFUE rating indicates that you feel the effects of most of the energy used. The remaining percentage escapes through the duct work or in the attic.

Sizing

• Sizing electric heating units has changed in recent years. New homes are tighter construction with more complex designs. Air Conditioning Contractors of America provides Manual J for calculating the size of heating and air conditioning units. Computer programs can do the calculations as well. It’s important to have your furnace sized specifically for your home.

  Estimate the Btus needed to see how your estimate compares with the contractor’s calculations. Multiply the number of heated square feet in your home by the zone requirements. For example, if you have a 1,500-square-foot house in Zone 3, multiply by 35 Btus to arrive at 52,500 Btus to heat your home. Electric heating elements calculate the electricity in kilowatts, and 15 kilowatts is a common size. The 15-kilowatt unit provides 51,000 Btus in heating output. Convert CFMs to Btus with a complex formula or computer program that considers altitude and density along with the rise in temperature during the heating cycle.

Considerations

• The climate zone affects the size of the heating and air conditioning unit, but the square footage of your home and the orientation also matter. A home with many north-facing single-pane windows affects the heating. Windows may account for heat loss or heat gain, depending on orientation and type. Insulation levels and air infiltration matter, along with the number and age of the occupants. Lighting and appliances that give off heat can also affect the heating. Manual J for residential load calculation considers these factors. Your electric company or an electrical contractor can do these calculations, usually for free. Buying a software program for your computer allows you to work with variables to see how they affect CFMs, Btus and unit costs.