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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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by Marcel M, Whitby ON on Twintech Heating and Cooling

Really nice honest guys. It was a pleasure to do business with a company that does exactly what they say they’re going to do! My basement was cleaner after they installed my new furnace and AC. Thanks again Greg and Grant and for doing such a great job.

by George In Pickering on Twintech Heating and Cooling

5 star service!! I had twin tech install a high efficiency furnace and hot water tank at my home. I am taking time to write this review as a thanks for what I viewed as next level service that went above and beyond. Prior to installation they explained my specific options without pushy sales tactics and up front costs. Greg and Grant replaced a large portion of the duct work (built in their own shop) that had rusted over the years which resulted in a great looking job. It was a pleasure to work with people are passionate and take pride in their work. Following the installation they explained all aspects and maintenance of the new equipment. Products have worked flawlessly. Thanks again Greg and Grant! Highly recommend!!

by Donna J, in Ajax on Twintech Heating and Cooling

Twin Tech was great; they were honest, reliable and very thorough. I would recommend their services and will definitely be using them in the future.

by Sheila C. Pickering on Twintech Heating and Cooling

Hi Dave, Thank You Dave for coming to my aid this past summer when my AC quit. Your men did a great job installing and cleaning the area. The unit is working very well and i would highly recommend your company TwinTech. Dave you delivered what you sold and i truly appreciate it.

by Darin O. Whitby on Twintech Heating and Cooling

My experience with TWINTECH has been excellent, start to finish. While doing market research I made a list of several local companies and called them for some high-level information. Dave was the first to call me back, and the only one to book a meeting to see my place in order to give an informed, accurate estimate. He reviewed the different AC makes and models with me, explained the technology, and made a recommendation appropriate to my home. The installation went smoothly. I had one post-installation issue – there was moderate noise/vibrations coming from the duct system. I called about this, booked an appointment, and the technician resolved the issue during the service call. I feel assured having a 10-year warranty on parts & labour. I would recommend TWINTECH to anyone considering an AC or furnace for their home. Regards,

by Bill P. Whitby on Twintech Heating and Cooling

Thank you for your attention to detail regarding my purchase of a new air conditioner. Twintech arrived at the appointed time and were totally professional on the installation process completing the job quickly. PS thanks for the assistance applying for the government rebate

by Sam C, Pickering on Twintech Heating and Cooling

Dave and the guys from Twintech did a great job of installing my AC. They came on time did a nice neat job and cleaned up after themselves! Do not waste your time call Twintech first.

by Mary W, Whitby on Twintech Heating and Cooling

My AC unit was not working at all and our furnace that was still working but not giving us any trouble was over 15 years old. So I had 3 heating companies come out to give us a quote on replacing both the AC and the furnace at the same time. Two companies right away recommended that I replace both units simply because they were old. Dave from twintech however suggested that we save some money and replace the AC only because it seemed to him that the furnace was still in great shape and was running fine. He told us that he would have his technicians inspect the furnace thoroughly when they had it apart to install the AC. My husband was at home when the furnace was apart and was very impressed when the tech took him downstairs to personally show him that the furnace was in excellent working condition. He also cleaned the furnace at no extra charge. So now I have an honest heating and AC company that I can highly recommend to other home owners. Great job Thank You!

by Scott B, Whitby on Twintech Heating and Cooling

I had made the decision to replace my furnace and AC earlier this year. After meeting with several HVAC contractors I decided to go with Dave and his company. Dave was on time, polite, friendly and very professional. He took the time to answer all my questions and to explain the scope of work that would be required to do the job. The technicians arrived on time did a professional installation and completed the job on schedule. They respected my home by putting down vinyl runners to protect my floors and did a very thorough clean up. I’m very pleased with my decision to go with Dave and his team. I would highly recommend them to anyone who is considering upgrading their furnace and air conditioner.

by Mel F, Oshawa on Twintech Heating and Cooling

I have no problem in recommending Dave and the TWINTECH team to any home owner who is looking for quality heating and AC products and expert service.

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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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