Written by Karie Lapham Fay; Updated June 19, 2017
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The biggest problem with sizing your furnace and air conditioner is that so few people know how to do it accurately. Even many contractors rely on a simplified rule of thumb (which is, incidentally, illegal) that translates square footage into a one-size-fits-all answer. The result is usually oversized units that cost more, use more energy and fail to maintain the temperature you desire inside your home. Professional calculation factors in many of your home's unique characteristics to obtain the correct HVAC sizing. Online calculators can often get you close. So will calculating some of these measurements yourself. It may not be as precise as an engineer's, but it will ensure you can spot any gross inaccuracies.
Find your home's total square footage if you will be heating and cooling the entire area. Multiply the length by the width to obtain the figure if you don't have the exact amount from the house plans or sales contract. Include home additions and multiple stories in the square footage. Exclude areas of your home that will not be heated nor cooled.
Calculate the British thermal units required to heat and cool your house, based on square footage alone. Multiply the square feet by 25. The result is the base BTUs needed.
Adjust the base BTUs if you have unusually high ceilings (more thab 8 feet). Multiply the BTUs by 25 percent. Add the result to the base BTUs.
Factor in the people who are usually in the house as each will affect the heat generated inside the home. Multiply the normal number of occupants by 400 and add that result to the base BTU calculation. Alternatively, for high levels of activity, boost the BTUs per person to 600, if desired.
Divide the total BTUs by 12,000 to calculate the equivalent air conditioning tonnage. Alternatively, use the BTUs to size the furnace required. With heating, the BTUs multiplied by the efficiency rating equals the output BTUs, which is the number your estimates require. For example, if you need 30,000 BTUs and the furnace is 85-percent efficient, you need a furnace with 36,000 BTUs to match an output of 30,000 BTUs.
Tip
With a variety of formulas available online, it's easy to get confused with which one to follow. Don't allow salesmen and contractors talk you into larger units. The formula above is a good guideline, but is still imprecise. The best calculation uses the 'Manual J,' a complicated guide of different factors to consider. Ask a third-party engineer - who will not profit from the sale of any given system - to use the 'Manual J' to calculate your values.
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Karie Fay earned a Bachelor of Science in psychology with a minor in law from the University of Arkansas at Monticello. After growing up in construction and with more than 30 years in the field, she believes a girl can swing a hammer with the best of them. She enjoys 'green' or innovative solutions and unusual construction.
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