When it's cold out, we want to stay warm. A well-insulated house is a start. But it's also important to have the right-sized furnace. Not too big, and not too small. Most of the time, we want to be able to set the thermostat, and forget about what's needed to get our house to that comfortable level. We also don't want to pay for more heating fuel than is necessary, or to have a furnace working beyond its capacity. To get your home to that ideal temperature, it's good to have the furnace that suits your square footage and the climate you live in.

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How to Calculate Furnace Size

BTUs

Energy output is measured in British Thermal Units, or BTUs. Put simply, a BTU is the enrgy needed to heat one pound of water by 1 degree Fahrenheit. So the higher the BTU rating of your furnace, the greater the heating capacity. BTUs are also used to calculate how large an air conditioner needs to be, by calculating how much heat it can remove from the surrounding air. So now you'll want to figure out how many BTUs, on average, you want your furnace to generate.

Calculate Your Heating Factor

Climate is key to figuring out the capacity of your heater. A.C. Direct has a handy map that shows you how to calculate your heating factor, based on information from the U.S. Department of Energy. If you live in Chicago or New York, for example, you're in Zone 4, which means you'll have a heating factor of 45 to 50, and you'll need 45 to 50 BTUs per square foot. If you live in Los Angeles or Atlanta, you're in Zone 2. You'll have a heating factor of 35 to 40, and you'll need BTUs per square foot. Use the lower of the two numbers if your house is new or well insulated, and the higher of the two if your home is older or poorly insulated. If you're not sure, go with the higher number to be safe. But at least try to come close, so you don't buy a furnace that has much more capacity than you need.

Say your house has 1,900 square feet, and you live in a well insulated house in Zone 3, which includes Washington, D.C. Multiply 1,900 (your square footage) by your climate zone of 40, and your estimate is 76,000 BTUs to comfortably heat your home.

But you need to consider more than just climate and insulation. BTUs needed can also vary according to the efficiency of your windows, the number of your windows, the placement of your house in sun or shade and whether your home is a single story or two-story home. Two-story homes require slightly fewer BTUs, because the upper story acts as an insulator. A large number of windows can tilt you toward the upper end of your heating factor, especially if those windows are older. You should factor in all that when you calculate your heating factor.

Furnace Efficiency

When you're looking at furnaces, you'll notice they come with two important numbers. One is a listed input rating, measured in BTUs, and the other is an efficiency rating percentage, which show how efficient the furnace is at converting air to heat. You don't want to make the mistake of just looking at input rating, or BTUs, because that's an incomplete picture of how efficient the furnace is. You'll know your actual BTU output when you take into consideration how efficient the furnace is.

So, back to our 1,900 square-foot home. If you're input rating is 90,000 BTUs, and your efficiency rating is 80 percent, you'll have a 72,000 actual BTU output. So this furnace would work very well for an older house in the Los Angeles or Atlanta area, and be more than enough for a well insulated, new home. If you're in Washington, D.C., in the 1,900-square-foot home, and the furnace you're considering has an efficiency of 80 percent, you'll want your input rating to be 100,000 BTUs. You can calculate this with any size home. Just substitute your own total square footage, and multiply it by your regional heating factor.

Many typical inexpensive furnaces are in the 80 percent efficiency range, while high energy models are typically at least 93 percent efficient. Gas or oil furnaces typically don't reach 100 percent efficiency, while electric furnaces often do offer 100 percent efficiency. Gas is the most common heating fuel. A gas furnace in the 1970s had an efficiency rating of about 65 percent. Today, regulations require that new furnaces must offer at least a 78 percent efficiency rating, and some more expensive gas furnaces approach 97 percent efficiency.

Manual J Load Calculator

Many contractors use what's called the Manual J Load Calculator to add in all this information and calculate the ideal furnace size. You can download this form from the Air Conditioning Contractors of America, and go through it yourself or with a contractor. It's complex, but it'll give you a very clear picture of how big your furnace should be.