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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 Peter, Pickering on Twintech Heating and Cooling

On coldest night of the year our furnace quit. At 8:40 in the morning Greg arrived and gave us a diagnosis and a recommendation. By 2:30 we had a new furnace and were warm again. Great company. Great customer service. Good reasonable price.

Peter, Pickering

by New Furnace Installed on Twintech Heating and Cooling

Twin tech team was very efficient and prompt. They provided a quote within hours of request and got the job done on time. No disruption to home function. We would highly recommend this team. Job well done.

by John from Toronto on Twintech Heating and Cooling

Our furnace bit the dust, and Twintech came out to troubleshoot and diagnose the problem within a couple of hours. Mitch was super helpful, patient, and answered all of our questions. When we decided to get the furnace replaced, the crew was over first thing the next morning and did a fantastic job. Not only did they complete the job in a few hours, but they cleaned up and respected our house as they went, and answered all of our follow-up questions. I've used Twintech for our HVAC needs in the past and will continue to do so. I'd recommend them to anyone, and it feels good to support a local, family-owned business.

by Ryan k on Twintech Heating and Cooling

Dave and the Twintech team were fabulous from day 1 and helped us through the process to identifying a new furnace and AC unit. This was our first time through this process and Dave did a thorough job explaining all the components and was very patience answering all our questions. Dave provided great insights and fair pricing.

The Twintech team did an awesome installation. Very clean and efficient. I would highly recommended their service.

by Louise in Scarborough on Twintech Heating and Cooling

We had a new Amana furnace installed by Twintech and we were very pleased with the installation. It is nice to have a warm house again.

by Private User on Twintech Heating and Cooling

Excellent job. Excellent service. Competitive price. Its rare to find a company with the knowledge and professional results we have experienced with Twintech. The clean and neat installation with attention to detail and drop sheers to respect our property is a clear indication of Twintechs high level of integrity. The 1-1/2 gas line upgrade they did for our pool 3 years ago was awesome. Can't say enough. Thank-you Twintech.

by Private User on Twintech Heating and Cooling

Twitch did a great job with the installation of our new furnace. I really appreciated their honesty and I could trust what they were telling me. Another company tried to suggest significantly more work at a greater cost claiming it was necessary to meet current building code. Twintech is very fair. All staff I meet were friendly, polite and trustworthy. I would recommend Twitch for a furnace.

by Private User on Twintech Heating and Cooling

This is a professional team that's easy to deal with. I would strongly recommend them

by hj.taggart in OSHAWA on Twintech Heating and Cooling

Very professional firm from start to finish!
They removed the old oil tank and furnace. They installed air conditioning and a new propane furnace.
Professional installation, on time and no mess.
Very pleased with their work and would highly recommend the company. Our house is three stories so they spent the time to do all the calculations to make sure we would have great performance on each floor.
Thank you again!

by susieschoeppler in Oshawa on Twintech Heating and Cooling

Upon a routine yearly maintenance, it was discovered there was a leak in the heat exchanger and a I needed a new furnace. I know nothing about these things and was completely stressed out. The company I had the maintenance plan with quoted me a huge number to rent a furnace, I just wanted to cry. Then I spoke to Dave at Twintech. He was so knowledgeable and calming, provided me a fair quote and within a few days I had an amazing crew show up to my house and install my new energy efficient furnace! I have already recommended these wonderful people at Twintech to a good friend of mine. Thank you Twintech!!

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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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