How Do I Know What Size Furnace I Need…..Is Bigger Better?

Furnace sizes vary by different gas input values, the correct terminology is “British Thermal Unit; or “BTU”. One BTU is equivalent to the heat given off by burning one wooden match.

Heat is lost through the materials used to construct your home. Every structure is different. A heat loss calculation should be performed to determine the exact amount of heat your home will lose on an extreme day. Let’s discuss the primary components.


  • Windows: Most people want lots of natural light in their home, however, windows usually account for as much as 50% of the homes heat loss.
  • Doors: Better insulated then windows, but they still open and close leaving gaps where heat can be lost even if the door remains closed most of the time.
  • Walls: Taking out the square footage of all the doors and windows still leaves a lot of walls. Filling in the wall cavities with insulation does help, using a house wrap helps reduce air leaks and some exterior sidings used helps prevent the transfer of heat. Additionally, a 2”x6” wall can take on more insulation then a 2”x4” wall. Walls below grade take advantage of the heat held below the surface of the earth.
  • Ceiling/Roof: This also has the potential to be a major source of heat loss especially with a warm air furnace. It is very important to fully insulate the attic evenly so as to not leave places for the warm air to escape and prevent the cold air from dropping into the living space from the attic.
  • Floors: Weather you have a basement, crawl space or concrete slab on grade, the floor is another building component that contributes to a structures heat loss. Slabs tend to lose heat at a greater rate than the crawl space or basement floor because they are not below grade and are not afforded the consistency of the ground temperature.


By measuring each of these various building components and multiplying by the correct insulating factor, a structures heat loss can be determined and matched to a furnace with the appropriate heat output. If the furnace is undersized, it will not provide enough heat on the coldest of days; if it is oversized, it will cycle often, run less efficient and you will also observe noticeable temperature swings. Bigger is not better, properly sizing equipment for a given structure is best.


A properly sized furnace will provide you with enough heat for the extreme cold but will still have an adequate cycle time on milder days to provide an even temperature. Call RJ Kuhn in Oswego to find out more. We will be happy to discuss with you the new technology such as two stage furnaces that have a high heat output for the extreme days and a low heat output for the milder days or the variable heat output models that match the heat needed on any given day. Whichever type of furnace you choose, if you get it from RJ Kuhn, you can rest assured that it has been properly sized.