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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 Private User in Whitby on Twintech Heating and Cooling

Very impressed they arrive to quote even though I had forgotten to call them back to confirm date & time. They showed the day before installation to deliver the unit. Returned the next day as promised at 9am. Project was completed in a professional manner. They even fixed a few things that they noticed were not installed correctly, or to code by previous heating & air contractors we had at our residence before. Would recommend this company to everyone!

by Private User from Bowmanville on Twintech Heating and Cooling

Greg removed old oil furnace and oil tank from basement relocated new Napoleon propane furnace to allow duct work to be rerouted to a much more suitable location. Moved air conditioner to the new furnace. Dave recommended two available grants to assist with cost. They kept to their original quoted price which was substantially lower than a competitors for the same furnace and work. They completed the work as stated when stated and left the area clean and tidy. We are very pleased with the outcome.

by astephenson in Oshawa on Twintech Heating and Cooling

Twintech installed a Goodman furnace and ac init. Very professional install, both units are working well.

by rcnichols electric in Whitby on Twintech Heating and Cooling

We had Grant and Greg Dainty quote to replace our 20 yr old furnace. I have known them for years and have had other jobs done by them ( AC and Pool Htr). The price was very reasonable and the job was completed in one day.

by Rick. F. in Pickering on Twintech Heating and Cooling

TwinTech was the right choice. Our consultation with Dave set the tone. Professional and knowledgeable, we even received a follow-up call within hours of our install completion, wow! TwinTech arrived as scheduled to install our A/C unit. Within minutes of arrival Greg noted and explained a few out of code faults on our furnace installed by another company, problems fixed, thanks TwinTech. Greg, Grant and "Subaru" Pete were meticulous start to finish. Highly recommended!

by Taresh, Pickering on Twintech Heating and Cooling

Twintech replaced our aging +20 yrs old Furnace & AC with the variable high efficiency Napoleon Furnace matched with the Goodman AC. Sales guy ,Dave was great, gave a no pressure estimate, explained about the Government Rebate program which they took care of completely including arranging the home energy audit. On the scheduled day Greg and his team arrived on time, did a great job and completed the same day , extremely professional and courteous, cleaned up everything , tested the system, Greg took his time to explain the setup. Very impressed with the Napoleon Furnace 9600 which did a great job in maintaining a comfortable temperature last winter and now with the hot weather the Goodman AC cooled the house in much less time than the old AC unit could ever. From start to finish Twintech were great and have no hesitation in recommending them 100%.

by Private User Pickering, ON on Twintech Heating and Cooling

The Twintech team did an excellent job installing our new Goodman air conditioner. They were polite and professional. Greg was knowledgeable and clearly experienced. Thank you Twintech for bringing our cooling system up to current regulations. Job well done!

by Y. M. from Pickering on Twintech Heating and Cooling

Dave from TwinTech was very knowledgeable, professional and thorough in the buying process. He helped us to identify the best furnace for our home and our budget. When we needed a new filter a year later and seemed to have a problem finding the right one, I called Dave up and he delivered a filter to me the next day! The cost was on par with other retailers, but it was this unexpected personal service that really was impressive. The furnace? Works like a charm.

by JLH from Pickering on Twintech Heating and Cooling

Imagine, it's 5 minutes after the midnight countdown New Year's eve and the furnace quits. We called TwinTech fully expecting to leave a message and actually spoke with Greg, one of the owners! We had an immediate repair early the next morning & a new furnace within the week. We have entrusted TwinTech with all our home HVAC service needs for over 6 years. That's why this week with an opportunity to purchase a new BBQ, we called them 1st to see about a gas line install. It's chicken on the grill tonight! They are efficient, reliable, honest, hard working, and -- attentive is an understatement.

by Curtis, Ajax on Twintech Heating and Cooling

Wonderful customer service, starting from the job quote (Dave), to the clean install, to the follow-up. Twintech is a small company which installed my Napoleon furnace, A/C, John Wood tank, and ran the line for my gas dryer. The installation was completed as promised in one day. The work area was kept very clean, and they even took care of my old rental tank. The professionalism and wonderful customer service is second to none. They completed all of my rebate paperwork, and now I'm getting $2000 back! You will not be disappointed if you hire this company for your project.

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See What Our Customers Are Saying:

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!

by Anna Chin on Twintech Heating and Cooling

Amazing experience with TwinTech Heating & Cooling! Their team was professional, knowledgeable, and efficient from start to finish. They installed my new furnace quickly and left everything spotless. The technicians took the time to explain everything and made sure I was comfortable with the system before they left. Great customer service and fair pricing. I highly recommend them for anyone needing HVAC services!

by Connor Agnew on Twintech Heating and Cooling

Great experience all around. Bob was sharp and easy to talk to, and Greg handled the installation perfectly. Would hire them again in a heartbeat.

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