jsnover / Getty Images
Whenbuying a furnace, you want one that’s the right size.
Buying one too big or too smallwastes energyand can lead to costly maintenance projects.
Precise calculations are best left to qualified HVAC professionals.
jsnover / Getty Images
But if you’re wondering, “What size furnace do I need?”
A furnace that falls outside that range is costly, wastes energy, and breaks down faster.
Want more home reno project tips and inspiration?
Climate Factors
Your area’s climate zone is a significant factor when determining furnace size.
House Factors
The house’s square footage is the main factor when determining a furnace’s size.
But other factors like sunlight, ceiling height, and insulation should be considered, too.
Or, check with your county tax assessor.
Public records indicate a home’s square footage.
Ceiling Height
House square footage estimates are based on standard 8-foot-tall ceilings.
If your home has unusually tall ceilingsa cathedral orvaulted ceilingthis will increase the furnace size requirements.
Roof Color
Mostroof materialsare dark colored, like black, gray, or brown.
Subtract 10 percent if the house gets an unusual amount of sun.
If the amount of sun seems average, leave the number as it is.
Insulation
Standard fiberglass insulation in the walls and attic is assumed when calculating a furnace’s size.
Homes with heavier insulation require a smaller furnace than one without insulation or with poor insulation.
The standard for measuring a furnace’s heating capacity is BTUs (British Thermal Units).
Furnaces with higher BTU capacities put out more heat than those with lower BTU numbers.
This efficiency is called AFUE, or Annual Fuel Utilization Efficiency.
AFUE is a percentage, typically ranging from 56- to 98-1/2-percent.
AFUE is multiplied by heat output (BTUs).
Manual J pulls hundreds of factors into the equation for a precise estimate.
Technology Fact Sheet: Right-Size Heating and Cooling Equipment: U.S. Department of Energy.
Climate Zones: DOE Building America Program.
U.S. Energy Information Administration.
Volume 7.3 Guide to Determining Climate Regions by County, August 2015.
Pacific Northwest National Laboratory / U.S. Department of Energy.
Section C301 Climate Zones.
2012 International Energy Conservation Code (IECC).
U.S. Department of Energy.
