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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 on Twintech Heating and Cooling

I had them replace my furnace last year. When it came time for the air conditioner this year, I did not hesitate to call Twintech again. I will also have them perform regular maintenance on them.

by Robert, Ajax on Twintech Heating and Cooling

We required a new HVAC replacement after running the old
system for 18 years.
Dave understood our requirements and gave us a quote that
covered all the necessary items.
Installation was fast and very tidy. Everything works as
it should. Our thanks to Greg, Mitch and others for their
excellent work. Recommend this family owned business to
anyone else, considering HVAC or other work.

by John Armstrong, Pickering on Twintech Heating and Cooling

I needed a new furnace and A/C to replace my 21 yr. old units, and went to the internet to view potential companies in May, I received 3 quotes,
and chose Twintech based on their salesman Dave who was very detailed and covered off everything on what Twintech was all about,
an extremely dedicated company with so many years of experience, and after reading the on line reviews I was sold on Twintech, who did not disappoint as the installation team were top notch with Greg leading,
I would highly recommend this company without any doubts, in my books they are first class all the way. Our new Amana furnace and A/C are working perfectly !

by Private User on Twintech Heating and Cooling

Flawless transaction. They did exactly what they promised, showed-up on time, cleaned-up, and were highly professional. Highly recommended.

by Bill Edmondson on Twintech Heating and Cooling

Salesperson and 2 man installation team were all professional and
knew their stuff. All three were friendly and helpful, answering all
my questions. The quality of their work demonstrated their experience
and expertise in their field.

by Private User on Twintech Heating and Cooling

A dear late cousin of mine recommended this company and I have been happy with the service.

by Private User on Twintech Heating and Cooling

We are very pleased with the work done by Twintech. They were very professional from start to finish. Dave came to our home and gave us a very competitive quote for new high efficiency furnace and air conditioner. The installers were very courteous and professional. We would highly recommend them.

by W1n5t0n1987! on Twintech Heating and Cooling

Great experience getting an old furnace replaced.
Sales Guy (Dave) was great - honest, responsive, and friendly.
The work itself was done quickly and we are very happy with the outcome, thank you! We recommend TwinTech!

by Private User on Twintech Heating and Cooling

This review is late coming so my apologies to the amazing Twin Tech team! I contacted Dave at Twin Tech for a quote this past Feb (as I was collecting a few) and his recommendation was excellent. From the first meeting with Dave to the team installing the new furnace, not once did i feel uncomfortable . The team were extremely professional yet personable and clean and tidy. Decent companies still exist!
The new furnace just purrs and its just amazing the difference it has made!
(PS my sibling ended up getting a furnace from them as well)

by Private User on Twintech Heating and Cooling

The service was Twintech was excellent.
Dave our salesperson was easy to talk to and was professional. The unit recommended meets our needs and budget.
Greg and his team arrived on time for the installation and even got rid of the garbage the contractors left in our furnace.
Would definitely recommend Twintech.

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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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