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How Large Should My Furnace Be?

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How Large Should My Furnace Be?

Posted On: November 13, 2024

How Large Should My Furnace Be?

The size of a furnace is typically measured in BTUs (British Thermal Units), and the right size depends on various factors such as the size of your home, your climate zone, insulation quality, and the number of windows and doors. Here's a breakdown of what to consider when determining the appropriate furnace size:


1. Square Footage of Your Home

  • Generally, you can calculate the BTUs needed based on your home’s square footage. As a rule of thumb, most homes need between 30 to 60 BTUs per square foot.
  • For example, if your home is 2,000 square feet, a rough estimate for furnace size would be
  • 2,000 sq ft×40 BTUs per sq ft=80,000 BTUs2,000sq ft×40BTUs per sq ft=80,000BTUs

2. Climate Zone

  • Warmer climates require fewer BTUs per square foot, while colder climates need more.
  • Here’s a rough guide by climate:
    • Warm climates (like Florida): 30–35 BTUs per square foot.
    • Moderate climates (like the Midwest): 40–45 BTUs per square foot.
    • Cold climates (like New England): 50–60 BTUs per square foot.

3. Insulation and Windows

  • If your home has good insulation, you might need fewer BTUs. Poor insulation or old, drafty windows can increase the BTU requirements.
  • High-quality windows, doors, and insulation can reduce the furnace size by 10–20%.

4. Ceiling Height

  • Most BTU estimates assume an 8-foot ceiling. If your ceilings are higher, you may need a larger furnace to heat the increased volume of air.

5. Furnace Efficiency

  • High-efficiency furnaces produce more heat per BTU, so you can often go with a smaller size if you’re choosing a furnace with an efficiency rating of 90% or higher.

Example Calculation:

Let’s say you have a 2,000-square-foot home in a moderate climate with good insulation and standard 8-foot ceilings. You might aim for around:


2,000 sq ft×40 BTUs per sq ft=80,000 BTUs2,000sq ft×40BTUs per sq ft=80,000BTUs


If you choose a high-efficiency furnace (like 90% AFUE), you could reduce the size to around 72,000 BTUs.


When in Doubt, Consult a Professional

A qualified HVAC contractor can do a more precise calculation called a Manual J load calculation, taking into account all relevant factors, and recommend the right furnace size. This prevents you from oversizing or undersizing, which can lead to inefficiencies and comfort issues.



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See What Our Customers Are Saying:

by Anna Chin, Ajax on Twintech Heating and Cooling

On one of the coldest days of winter, our furnace stopped working. Our regular furnace company came and told us we had a CO2 leak and shut us off. Their quote for a new furnace was way too high and they couldn't even install it on the same day. TwinTech was recommended to us by a friend. We called and spoke to Dave who was very nice and knowledgeable. Grant showed up a short time later and a new furnace was installed the same day with no troubles at all. Best of all, we received a higher-end furnace than what was offered by our previous company and at a cheaper price. I would recommend TwinTech with no hesitation at all.

by Hiba, Pickering on Twintech Heating and Cooling

I installed a new furnace and Air conditioner with them. The installation is of high quality and according to standards. The guys are professional and reasonably priced.. I highly recommend them.

by Robert, Pickering on Twintech Heating and Cooling

Had new furnace installed and ran into some difficulties but the contractor responded in a timely and efficient manner - after further service calls were made, a reasonable resolution was found.

by Brian Burns, Whitby on Twintech Heating and Cooling

The work was done quickly efficiently neatly and professionally. Pleased with the service, information, product, and the company.

by Geoff, Port Perry on Twintech Heating and Cooling

Great sales member named Dave came in told us some amazing items and how they would be priced installed an all. We couldn’t believe it! The next day the ac was installed, that day one employee named Greg told us that our furnace was working way to hard and was causing us to use way too much gas. So two days later they came in an installed a new furnace!

by MB, Ajax on Twintech Heating and Cooling

Installation was done quickly and professionally. The new furnace is quiet and provides even heat.

by Richard from Oshawa on Twintech Heating and Cooling

They removed and discarded old furnace. Installed new furnace. Installed air intake pipe, that had never been installed by builder, no charge. Replaced loose hose on floor with hard pipe and rerouted it to drain, no charge. Almost 33% cheaper then quote from Enercare, which was the company we dealt with and had insurance with for 8 years. No yearly fee for cleaning/insurance, saves 300 a year. Courteous, professional, fast, and friendly. My wife and I have been recommending them to everyone we know and meet, and will continue to do so. Thanks for everything, Twintech!!!

by Darcy from Pickering on Twintech Heating and Cooling

This company was great to deal with. Our furnace went down and they installed a new furnace the next day with a lot of professionalism!

by gillian hooton from Uxbridge on Twintech Heating and Cooling

Finally! An honest company with highly knowledgeable and informative staff. Had a furnace service and found multiply errors in our system from an amateur installer. Very fair price to correct problems and will definitely use again.

by dflynn in Ajax on Twintech Heating and Cooling

Had to replace the heating & a/c unit in a condo apartment dwelling. this company recommended by the property manager and president of the condo building. found their workmanship and attention to details very good and very thorough. were able to resolve small problems and situations and happy to feel the heat in my unit. it's pricey, but happy was done by a very professional service. thank you.

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See What Our Customers Are Saying:

by Aaron on Twintech Heating and Cooling

We are very happy to recommend this company to everyone. Our furnace broke down on Sunday morning. We phoned them and before noon Grant the owner showed up. He worked on the furnace for about a hour not wanting to give up to fix the problem. The furnace was 17 years old. I was the one talking about replacing it. End result was the next day it was all done. On the Sunday Bob came over with heaters to warm up the house. We decided to replace the water heater at the same time. Thanks Grant for the excellent work. Thumbs up.

by Linda on Twintech Heating and Cooling

Right on time. Explained how to work the HRV in a different way so the house would not be so dry. Changed my whole house water filter for me as well. Friendly, helpful, caring service!

by Robert Rogers on Twintech Heating and Cooling

Very pleased with the professional service that TwinTech Heating and Cooling provided in replacing my furnace. I got several quotes from different service providers. Their price was excellent and they explained the features very well. We are very glad we went with TwinTech and highly recommend their services.

by Robert Mizuno on Twintech Heating and Cooling

We were new homeowners in need of a second opinion. TwinTech Heating & Cooling were Amazing and professional. Very knowledgeable and great service all around. Recommend to anyone looking for honest and reliable services. Treated with respect and care. Thank you Greg, Mitch and Bob!

by Ruth on Twintech Heating and Cooling

TwinTech is an excellent and honest company! I called them for a second opinion after being told by another company that I needed to replace my furnace. Turns out this was not the case at all! I did not need a new furnace. TwinTech did not push to make an unnecessary sale and saved me from a huge expense. Bob and Mitch were extremely professional and above all honest! I definitely recommend TwinTech!

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