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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 Roy in Pickering on Twintech Heating and Cooling

I was referred to Twintech by a neighbour and was not disappointed. Not only was the customer service excellent their workmanship was second to none. Greg and Grant work extremely clean covering the pathway to the furnace room, labeling the essential parts, left the area spotless and were efficient. They cleaned up the mess left by the previous installers (wiring, improperly installed AC coil, and gaps in venting etc). They also ran through the operation of the new furnace and gave me some maintenance tips. Greg and Grant are professional and all around good guys. I will definitely recommend their service to anyone without hesitation.

by Peter Buck, Pickering on Twintech Heating and Cooling

Very satisfied with service from Twintech , beginning with all the information provided by Dave , which helped make the decision much easier . Installation was very smooth & done on time as promised & cleanup was well done . Would definitely recommend this company & use them again for any future needs . Well done.

by Angela from Ajax on Twintech Heating and Cooling

Twintech Heating & Cooling Inc. came in and replaced my 39 year old furnace with a beautiful Napoleon variable high efficiency furnace! Their work was efficient and professional and they were a pleasure to deal with, from sales to installation! I would highly recommend them.

by Scott and Sandra in Ajax on Twintech Heating and Cooling

After receiving numerous quotes to replace my old furnace and AC unit (which was 20 years old) I went with Twintech. Dave, in sales, answered questions professionally and promptly and worked diligently in order to secure my business. Installation was scheduled the week after and Greg and Peter showed up, as scheduled, and did a fine job installing both units. The site was left tidy and neat. I would highly recommend Twintech for your furnace and AC work.

by Linda from Whitby on Twintech Heating and Cooling

I chose TwinTech because they installed my new furnace a few years ago and did a great job. When they installed my new A/C, they replaced all parts -- the copper pipes, the housing on top of the furnace and the wiring. (Apparently, the wiring for my old A/C was insufficient.) Their pricing was competitive. The sales representative came to see me within a few hours of my call. My new A/C was installed a few days later and I couldn't be happier with Twintech's professionalism and work. I would recommend TwinTech to any homeowner.

by george polychronis from Whitby on Twintech Heating and Cooling

Had a great experience with this company. Great price and great customer service. You wont be disappointed with twintech. Discussed the job with Greg while he was installing who was extremely informative and obviously enjoys the work he does.

by Nuzio in Markham on Twintech Heating and Cooling

I needed my AC unit relocated due to upcoming landscape work. I reached out to the Twintech team through Homestars. Dave connected with me immediately to discuss the project and various options. We decided the best option was to mount the unit on the wall. Greg was in touch with me the same day as well and we scheduled the work for later that same week. This was a small job so I did not think I would get immediate attention. The Twintech team exceeded my expectations in every respect. They were very knowledgeable and friendly. The team arrived exactly on time on the scheduled day and worked efficiently to get the job done quickly. I would definitely use Twintech again for any Heating and AC projects.

by Leslie B, Pickering on Twintech Heating and Cooling

I have used Greg Dainty for over 15 years in 3 different homes, for heating, cooling, pool heaters, appliances and bbq lines. Greg has always been so professional, friendly and the workmanship nothing short of fantastic. Greg has always discussed my options and fixing versus replacing, giving his honest opinion and what works best for me cost effective wise and long term. I have valued his opinion so much and have not trusted my gas needs with anyone since using him. I have also referred him to many of my friends and family and have always been so thankful with how well and quickly he looks after us all. I hold Greg in the highest regard and would recommend him to anyone with complete confidence!

by Marcel M, Whitby ON on Twintech Heating and Cooling

Really nice honest guys. It was a pleasure to do business with a company that does exactly what they say they’re going to do! My basement was cleaner after they installed my new furnace and AC. Thanks again Greg and Grant and for doing such a great job.

by George In Pickering on Twintech Heating and Cooling

5 star service!! I had twin tech install a high efficiency furnace and hot water tank at my home. I am taking time to write this review as a thanks for what I viewed as next level service that went above and beyond. Prior to installation they explained my specific options without pushy sales tactics and up front costs. Greg and Grant replaced a large portion of the duct work (built in their own shop) that had rusted over the years which resulted in a great looking job. It was a pleasure to work with people are passionate and take pride in their work. Following the installation they explained all aspects and maintenance of the new equipment. Products have worked flawlessly. Thanks again Greg and Grant! Highly recommend!!

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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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