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Furnace Sizing Calculator






Heating Square Footage Range by Climate Zone
ZONE 1  ZONE 2 ZONE 3 ZONE 4 ZONE 5
30 - 35 Btu's per square foot 35 - 40 Btu's per square foot 40 - 45 Btu's per square foot 45 - 50 Btu's per square foot 50 - 60 Btu's per square foot


Heating Guide

Use the lower of the two numbers if your home is well insulated and the higher number if it is older or poorly insulated. (Hint: Use the larger of the two numbers above if you're unsure of your home's insulation)

Simply multiply the appropriate factor above by your home's total heated square footage to arrive at your approximate required heating capacity. For example, if you live in the yellow zone, your home is well insulated, and you have 1900 heated square feet, the equation will look like this:

1900 square feet
X 40 heating factor (from the chart above)
76,000 Btu required to heat your home

Then, to calculate the output on a given gas furnace, multiply it's efficiency rating by it's listed input rating to determine the actual Btu output of heat. For example, if a furnace has a listed input rating of 90,000 Btu's and an efficiency rating of 80%, it will produce

90,000 Btu input
X .80 efficiency
72,000 Btu actual output

If the same 90,000 Btu furnace has an efficiency rating of 93% it will produce:
90,000 Btu input
X .93 efficiency
83,700 Btu actual output

For this example, using an 80% efficient furnace, the 1900 square foot home above would require a 90,000 Btu input furnace that produces 72,000 Btu's of heating, which is close enough to the 76,000 Btu's required using the climate heating factor.

Most furnaces are offered in 15,000-20,000 btu increments so you just need to get close in terms of sizing. If the furnace you selected is more than 10% below your heating requirement, we suggest you go up to the next size. A little under sizing or over sizing is fine, just don't over size by more than about 20% of your heating requirement, or short cycling can occur which wastes energy and reduces your comfort

If you're still unsure which system size is right for you, please e-mail us or call our toll free number at 1-866-862-8922. An experienced design technician will be happy to assist you. Your home's construction quality and insulation is unique and can greatly affect furnace sizing, so this information is intended to provide a general guideline but should not be the only consideration when selecting a furnace for your home.

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Insulation Variables

Variables such as your insulation, type and number of windows, number of stories, construction type, etc. will greatly affect the required Btu's per square for both heating and cooling. A general rule of thumb is that if your home is well insulated with newer style windows, you can select the smaller size system that falls within your total square footage.

If your home is two story it will place less of a load on the system in the downstairs area as the second floor acts as additional insulation. If your home is not well insulated, has older style windows, and/or a larger than average number of windows, you will want to select the larger system which falls within your square footage range. The less insulated and more windows within the environment, the more likely you will experience greater air and heat loss.
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Contact Us for More Info

If you're still unsure which system size is right for you, or if you're having difficulty downloading the Air Conditioner Size Calculator, please e-mail us or call our toll free number at 1-866-862-8922. An experienced design technician will be happy to assist you.

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