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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 Eric Y, Greenbank on Twintech Heating and Cooling

Dave was great in helping me get the proper units for my property, as well as getting them installed. The installers arrived when they said they would, and completed the project on time, and professionally. If I needed another furnace, I would use Dave again.

by Ed B., Whitby on Twintech Heating and Cooling

I have known Greg and Grant Dainty for many years and have used their company for my house and business. I recommended them also for my parents and sister and they all have been very satisfied with the quality of work for a reasonable price. I just recently had them install a new HVAC system in an older house and was very pleased with their work. Craig in Pickering. After meeting with various heating and air conditioning contractors I was referred to Twin Tech Heating and Cooling through a family member. Greg and Grant arrived at my home and provided me with an estimate and a plan as to how they were going to remove and replace my furnace and air conditioner. On the day of install Greg and Grant arrived at my home on time and were very professional and extremely courteous as to any questions or concerns that I had. The removal and install was truly professional and the attention to detail was exactly what I was looking for. I’ve seen many installs from other contractors and I couldn’t be any happier to have had such a clean looking install. The sheet metal work, ducting and venting all look fantastic. Upon completion Greg and Grant left me feeling extremely happy and very fortunate of a job well done. Thanks again Twin Tech!!

by Floreene in Whitby on Twintech Heating and Cooling

My old furnace had finally broken down for the last time when contacted twintech to have a new furnace installed…They were prompt, knowledgeable and very reasonably priced. They did such a great job i tell all my friends and family about this company.

by Damien McIsaac. Ajax on Twintech Heating and Cooling

To whom it may concern, I recently had TwinTech (Greg and Grant Dainty) replace my furnace and service both my gas fireplaces and air conditioning unit. They were professional, courteous and very fair in their pricing. I would gladly recommend TwinTech to anyone needing work done. I can be contacted at 905-626-XXXX.

by Blair N. - Toronto on Twintech Heating and Cooling

My Fiancé and I have relied on Twintech Heating and Cooling now for the better part of the last 10 years. In that time, Greg has installed a furnace, an air conditioner, and a heat-pump at our residence in Toronto. In each instance Greg and his team were a pleasure to deal with. They were courteous and professional throughout the whole process and the quality of their work is first rate. Our experiences have always been extremely positive and we have referred his services numerous times to our friends and family. Without hesitation, I would recommend Greg and TwinTech Heating and Cooling to anyone looking to service or replace their existing heat/AC equipment

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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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