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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 Karen Morgan on Twintech Heating and Cooling

Once again Twin Tech came through and fixed my furnace. A young man by the name of Mitch came by tonight and he was so nice explained everything he was quick he was clean he turned off the lights even when he left the furnace room it’s just above and beyond Twin Tech always delivers and I just found out about them last year and I will use no one else but thanks again Mitch you are fantastic

by Richard Liu on Twintech Heating and Cooling

I had a problem with the Furnace (Armstrong ultra V) and Greg diagnosed it on the phone. Even told me what the issue was and I was able to fix it. This level of service from a Business that is almost unknown today.

If you require service, reach out to Greg. I will definitely use them in the future!

by Tony Forde on Twintech Heating and Cooling

We have been shopping for a heat pump for our home. We had 3 reps come in and make recommendations and provide a quote. Of the 3, Bob was so clearly so much more qualified, thorough and informed than the other two that the decision was a no brainer. As soon as we get our audit results Twintech will be installing our heat pump.

by Private User, Pickering on Twintech Heating and Cooling

knowledgeable sales person, did not try to up-sell unneeded features,
and was easily contacted to ensure customer satisfaction.
Installation was expertly done by friendly guys, everything explained for use of the products. Old products were removed and returned to rental company as part of the service. Follow up call was received to check all was well. This is my second time using this company as I was fully satisfied with my previous air conditioning unit installation

by Private User, Courtice on Twintech Heating and Cooling

Twintech recently installed a new furnace for me. Bob did a great job assessing what I had and needed. All details well explained including the importance of proper sizing of furnace and the value of multi stage heating and the variable speed blower. As a nurse, indoor air quality is a high priority so the variable speed motor running continuous is so good. 24/7 air cleaning. Greg did a great job on install, attention to detail so appreciated. Already notice such an improved comfort and heat. Thank you Twintech! You come as advertised. I will recommend to anyone in need of heating or cooling!

by Walter, Whitby on Twintech Heating and Cooling

We received various quotes from 4 different companies and right away, the prompt and thorough communication from Bob from TwinTech stood out. He introduced himself, sent sample sites and information, and always confirmed any agreed to dates and times.

Bob was friendly and extremely knowledgeable about the topic of HVAC and it was clear he was passionate about his profession. He approached the job as if he was installing in his own home.

Prior to the new furnace & A/C installation, our old units were loud and inefficient - having several rooms feeling too cold in the winter and too hot in the summer. After installing the recommended units, all rooms stay at a consistent temperature- even the finished basement.

I highly recommend the TwinTech team - just an outstanding job at a reasonable price point.

by Private User on Twintech Heating and Cooling

The experience with Twintech was great from the start. Bob the sales manager was friendly and professional and did not use any pressure tactics. He was very knowledgeable.
The installation was completed by the twin owners in about 6-7 hours. Clean quiet easy going didn't mind when I poked my head in to see progress. Highly recommend

by Private User on Twintech Heating and Cooling

Just choose Twintech!

I do not usually write reviews and should have done this a few days sooner. But I was and still am enjoying the increased comfort in my home from the new furnace installed by Twintech.

From my initial call and conversation with Bob, I already had a good feeling that I would like this company.

I was provided information, pictures of great quality work previously completed.

When Bob came for the inspection of the furnace he was amazing. He gave so much great advice and told me all the areas that his team would resolve. I was very impressed as some of what he had suggested I had only seen in Youtube videos. The honesty and options for payment made this a no brainer. I decided I would like Twintech to replace my furnace.

The work was scheduled promptly week and took place within half a day. Gregg came to install the new furnace. Again amazing they carried out all the work promised. What was left was the best I have seen and speaks to how they take pride in the work they do and deliver. Gregg was really polite and professional and the furnace in my basement is a work of art.

by Private User on Twintech Heating and Cooling

They were friendly, fast, clean, & answered any questions I had. They are also offering an interest free 12 month payment plan - that really helped me out since this was an unexpected expense.

by Jamie, Windsor on Twintech Heating and Cooling

I had Twintech heating install a new furnace and ac unit. Bob, the sales rep, the installation team of Greg and Mitch were a 10 across the board. They were never pushy, gave me time to think about it. Once we decided , they were there in 24hrs. Great company to deal with. And they were nearly 5k less than a competitors quote for a top of the line furnace/ac unit verse mid level.
I highly recommend Twintech heating to anyone interested in anything HVAC.

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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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