What Size Furnace Do I Need for My Home?

Most homeowners know that installing a furnace isn't a DIY kind of job, but you might be surprised to know that merely sizing a furnace requires some special knowledge. That's because choosing the right furnace isn't as simple as looking up a home's square footage on a chart; several factors come into play, including the local climate, the layout of the home and the condition of insulation and weather stripping.

How Furnaces Work

Not all homes are heated by furnaces. Many homes in warmer climates use heat pumps, while others in remote areas may use wood or oil burning heaters to stay warm off the grid. But for most of North America, natural gas or electric furnaces are the norm.

When a natural gas furnace turns on, the pilot light shares its flame with a series of burners inside a combustion chamber. These burners produce heat that flows into a heat exchanger, and an air handler blows air around the heat exchanger to warm it while distributing it throughout the home. An electric furnace works similarly, except instead of a pilot light and gas-powered burners, heat is produced through a series of electric heating elements like those on an electric stove.

What Size Furnace Do I Need?

Just as with air conditioners, choosing the right size furnace is extremely important. If your furnace is too small, it will take too long to heat your home and may never be able to reach your thermostat setting. That nonstop work is also inefficient, which means you'll pay more for poor performance.

On the other hand, if your furnace is too large, it may overheat some rooms. It will also cycle on and off too frequently, which is both inefficient and hard on system components, leading to more frequent repair calls.

You may find a furnace size calculator or chart designed to help you find your perfect furnace size on your own, and those tools can be helpful in approximating heating capacity. But before you place your faith in a furnace BTU calculator, consider that a new furnace is a high-involvement purchase that you'll be living with for a long time. For a purchase this important, it's worth the time to schedule a professional sizing consultation with a licensed HVAC professional.

HVAC technicians use industry standards established by the Air Conditioning Contractors of America (ACCA) to guide their furnace sizing calculations. This process requires a home inspection and takes into account factors including:

  • Total heated square footage
  • Total heated air volume
  • Local climate zone
  • Condition and efficiency of ductwork
  • Condition of weather stripping and other home sealing elements
  • Type, amount and condition of insulation
  • Insulation value and configuration of windows and doors
  • Sunlight exposure

How to Find the Right Size Furnace

Regardless of whether a furnace is powered by electricity or natural gas, its heat output is measured in British Thermal Units, or BTUs. When an HVAC technician calculates the heating demands of your home, the complicated part is determining the right number of BTUs for your unique space. Once you have that figure, picking out the right size furnace is relatively easy.

There are two very important numbers when choosing a furnace: the input BTU and the efficiency rating. The input BTU is the amount of heat in BTUs that the furnace generates. The efficiency rating is a percentage that tells you how much of that heat will actually go toward heating your home.

For example, imagine your HVAC technician calculates that it will take 80,000 BTU to properly heat your home. A furnace with an input BTU of 100,000 and an efficiency of 80 percent would be a perfect fit, because 80 percent of 100,000 BTU is 80,000 BTU. But if you chose a furnace with a higher efficiency, like 93 percent, you could downsize to a smaller input BTU, closer to 85,000.

You may not find a furnace that hits your target BTU count on the nose, but you should be able to find one within a few thousand BTU of that number. Whether you should opt for the slightly bigger or slightly smaller furnace can come down to a number of factors, including the prices of the units and the energy efficiency of your home, so you should consult with your HVAC technician before making a final decision.

Higher input BTUs and higher efficiency ratings both lead to higher upfront costs when purchasing a new furnace. But higher efficiency ratings can pay off in the long run in the form of lower energy bills, since your heating and cooling alone represent almost half of your home's energy use. If you have the budget for the upfront costs of a more efficient model, it's usually a smart investment.

What About My Old Furnace?

If you're replacing an existing furnace and you're happy with your current heating performance, you may decide to simply stick with your current BTU output. But if you go this route, you should first verify the output BTU of your current furnace by checking the input BTU and efficiency rating (usually stamped on a metal plate on the furnace) and crunching the numbers. Once you have that output BTU, shop around for more efficient furnaces that have a similar output BTU, because one of the best parts of upgrading your furnace is the huge gain in energy efficiency.

If you're not completely happy with the performance of your current furnace, it could be that it's not properly sized to your home. Work with a licensed HVAC technician to calculate your home's needs the right way and make your next furnace a perfect fit.

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