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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

It was a refreshing experience interacting with the Twintech team. From Bob, the sales consultant, who articulated in layman's terms all the options available and reasons for making any selection. This was done without the prevailing pressure tactics so commonplace today.

Before proceeding Mitch, the technician, at our request conducted a service check on our 30 yr old furnace. His report was positive indicating no urgency to replace it. Again no pressure tactics.

Three days after first meeting with Bob, the installation team of Greg and
Spencer completed the installation within the planned time frame.

A very professional, courteous and transparent team which delivered on their promise. I highly recommend Twintech.

by Private User on Twintech Heating and Cooling

Initial contact with sales resulted in an immediate response, which led to a timely installation. Both sales and installers were all very professional. I have no hesitation in recommending TWINTECH.

by Private User on Twintech Heating and Cooling

I had a great experience with TwinTech. The company offered a reasonable quote, and performed a quick and clean installation. The installer Greg was experienced, professional and courteous. He explained the services he performed and gave recommendations on maintain the new furnace.

The salesman Bob was honest
and professional and educated me on how to achieve optimal performance.

All communication from them was quick and they came the following day. I was pleased with the service and would recommend them.

by Lousan, Ajax on Twintech Heating and Cooling

From quote, to installation, to follow up, Twintech provided excellent service. I would highly recommended their services.

by John, Pickering on Twintech Heating and Cooling

I had a really great experience with these guys. The salesman Bob was clearly very knowledgeable about everything to do with heating and cooling a home, spent a lot of time with us going over the details of the job, never rushed us and came back after the install to make sure we were happy. The installation crew was excellent, they covered the floors all the way from outside down to the basement furnace room, kept the place clean, got the work done on time and we’re very friendly. I’ve got 15 years in the contracting/ construction industry and these guys are one of the most professional company’s I’ve dealt with. Great company!

by Debra Robertson on Twintech Heating and Cooling

Had great service and clean-up was good. Was happy with my salesman. They were very friendly and helpful. I needed a new furnace as our was over 20 years old. The furnace and air-conditioner work great and they are very quiet. I am very pleased with the service and product I received.

by Private User on Twintech Heating and Cooling

Met with Dave for the sales process and he was great about explaining and going over everything to us. The install happened very quickly and the team were super professional and friendly. We are very happy with this company and their services and we definitely recommend them to others. Thankful my sister recommended them to me!

by Private User on Twintech Heating and Cooling

Reviews for Twintech were good, and they did not disappoint. After an initial visit to discuss the suitable AC unit for replacement, an installation date was available in just a couple of days, and everything was installed without a hitch. Highly recommended

by Private User on Twintech Heating and Cooling

Very pleased with the installation of our new furnace and A/C unit. From quotation stage to the completion of the job Twintech was very professional.
Thank you for a job well done.

by Darren, Pickering on Twintech Heating and Cooling

My old water heater quit on me. I called TwinTech. In less than 4 days, I had my new water heater installed and the old one taken away. The whole process was quick, easy, pleasant, and well done. This is the third time I've dealt with TwinTech and I've been satisfied each time.

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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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