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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 Anonymous from Pickering on Twintech Heating and Cooling

End to end service. From quote, overseeing installation, contacting and making arrangements for energy audit. Warranty program is comprehensive. Cleaned up mess of assistant walking through my place without proper indoor foot covering when I pointed out that I was not happy about footprints on my floors.

by Bob S in Pickering on Twintech Heating and Cooling

In April we hired Twintech Heating & Cooling to replace our 20 year old water tank & 35 year old air conditioner. We got 5 estimates for the job & decided to go with Twintech for the following reasons.

- The Sales Manager, Dave, was very personable & knowledgeable as all of our questions were answered. (We had a list of questions we asked each of the contractors when we got the estimates)
- We got 5 estimates for the job. All contractors verbally told us they had the required qualifications but Dave was the only one to physically show us their license & insurance documentation.
- Twintech carried the Napoleon air conditioner & we wanted to support Canadian industry as it is assembled in Barrie On.

Safety is important at Twintech as demonstrated when we discussed with Dave, the need to change our install date because of freezing rain. Access was required by navigating a set of outside steps & Dave identified that safety took precedence.
On the installation date, Greg, Peter & Grant showed up on time & methodically accomplished the removal & installation of both units. The guys did a great job cleaning up as well.

When the job was completed we mentioned to Greg that we were pleased with the work they had done & were going to have them come out & do our furnace inspection in Sept. Greg said we can save our money as he had inspected the heat exchanger on the furnace while doing the job & there was no need to have another inspection in Sept. We were very impressed with Greg’s initiative. This is top shelf service. Thank you Greg.

As the weather was too cool to complete a functional test of the air conditioner on the installation date, Dave made a follow-up call to us to see how things were going & Greg also came out to check the operation. No problems.

We are very happy we hired professionals like Dave, Greg, Grant & Peter & have no hesitation in recommending Twintech Heating & Cooling. They will now be our "go to" guys.

by ted diplock in Toronto on Twintech Heating and Cooling

These guys are the best. From start to finish they exceeded my expectations and delivered a great product. They accommodated the nuances of my 100 year old home which was really appreciated. The install was easy to schedule and done fantastically. I’m so excited to have an efficiently heated and cooled home.

by Private User from Kitchener on Twintech Heating and Cooling

Twintech were fast and efficient contractors. It only took 48 hours between an initial home visit and complete installation. The workmen were polite and hardworking. The site was left clean with the old furnace etc. removed. They helped arrange an environmental assessment in order that I could receive the maximum rebate. They even called me within a week to determine if I was satisfied with their work and to reassure me that I could call them anytime I experienced heating difficulties or had any questions. I recommend Twintech for your heating problems.

by dlcarroll in Vaughan on Twintech Heating and Cooling

Courteous and professional service from initial consultation through to the installation. We highly recommend Twintech for outstanding service in the industry. Competitive pricing, professional and accurate advice, attention to detail, respect for their client's home and cleaning up at the completion of installation.

by TraceyG from Toronto on Twintech Heating and Cooling

Last night was the coldest day of the year so far and I woke up to my house being 54 degrees... I called Twintech at 2:45am and Greg was on site by 3:30am. The furnace required a part and he returned to his shop and came back to my home around 4:15am, furnace up and running by 4:30am. THANK YOU TWINTECH! Your service is second to none.

by Cristelle from Ajax on Twintech Heating and Cooling

After much research & getting 4 quotes, we hired Twintech team for our new furnace installation, prices were fair, they carried great product brands & more importantly installation was expertly done by one of the owners (Greg & his assistant Peter) which gave me an added piece of mind. Dave was very organized & answered all my questions in great detail, especially re: the Enbridge & Govt. rebate program.

by David from Pickering on Twintech Heating and Cooling

My old air conditioner was 20 years old and developed a Freon leak and it was more cost effective to replace rather than repair. There are a large number of heating and air conditioning companies available in my area so I contacted several for quotes. Twintech heating and cooling responded quickly to the inquiry I left on their website and their sales person Dave came by the following day. He was knowledgeable about the field and was able to show copies of the various refrigeration and gas fitting licenses along with workmens compensation, TSSA and other certifications. He also suggested a particular manufacturer and model of AC unit appropriate to my house and gave me a quote. Since the price was reasonable, service had been good and the reviews on the internet were also good, I decided to purchase an AC unit from them. Installation was quickly scheduled. They showed up on time, did a good job installing the new unit including some sheet metal work to get the new heat exchanger to fit in the furnace duct, and cleaned up afterward. In addition, they noticed that my existing AC was installed too close to the furnace exhaust (based on building code) and were able to mount the new AC the appropriate distance from the exhaust. Overall, they did an excellent job and were a pleasure to work with.

by LP from Pickering on Twintech Heating and Cooling

All interactions from sales to installation were Courteous and professional. The installation was seamless and the attention to detail including clean up was outstanding. I highly recommend TwinTech Heating and Cooling.

by David in Pickering on Twintech Heating and Cooling

Twintech's Salesman Dave arrived promptly was friendly and courteous, listened and assured us the installation would be completed according to our requests. The crew arrived early morning on May 11th, 2018 and worked through till late afternoon non-stop. They were very friendly, very professional, their expertise was excellent, their workmanship outstanding and the installation was completed in a timely and efficient manner. We were completely satisfied and happy with the quality of the work.

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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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