How to Size a Furnace in a Residential Home

A correct-size furnace is indispensable for efficient home heating. A furnace that is too small for the space it's expected to heat will run too much and will not heat the house effectively on the coldest days, while a unit that is too large will cycle on and off --- called "short cycling" --- resulting in a loss of efficiency. A properly sized system, according to HomeTips.com, "will run almost continuously on very cold days."

• 1

  Understand BTUs. A furnace's heating capacity is measured in BTUs (British Thermal Units). Input BTUs measure the amount of fuel burned during operation, and output BTUs measure the actual heating capacity of the unit.

• 2

  Calculate the output BTU by performing a simple calculation: multiply the input BTU by the efficiency percentage, as Alpine Home Air Products recommends. The result is the amount of heat the unit puts out.

• 3

  Purchase the same-size unit as the one you're replacing. If you're upgrading to a high-efficiency unit, HomeTips.com advises that you can't simply replace an old furnace with a high-efficiency unit without matching the output capacities first. Make sure you do your output calculation before making a purchase and double-check with the salesperson or a licensed HVAC contractor.

• 4

  Consult a licensed HVAC contractor, especially for new construction that doesn't have a furnace yet. A contractor will evaluate your entire home and calculate home energy loss from the size of the house, window areas, insulation levels and related features, according to HomeTips.com.