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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

Twintech promptly responded to request for quotation. They were thorough and helpful in explaining options. The quote was very competitive. Installation of new furnace was very professionally completed. Excellent work.

by Rita, Scarborough on Twintech Heating and Cooling

When our old oil furnace finally broke down we decided it was time to finally convert to natural gas and finally install central air conditioning at the same time. Knowing it was going to be a significant investment for us, we wanted to make the right choice. Twintech came highly recommended by a friend and we were not disappointed. From the first contact with the salesperson, until the final touches on the installation, we were so pleased with their service. Everyone from the company was so friendly, efficient and a pleasure to deal with. Best of all, the cost was very competitive. We would certainly recommend Twintech to anyone requiring HVAC work in their home.

by Bob S, Pickering on Twintech Heating and Cooling

Mitch from Twintech came out for our annual furnace inspection. We found him to be friendly, conscientious & he left the work area clean. We were also very impressed with the thoroughness of the job he did. This is the 2nd job we’ve had Twintech do for us & have been very happy with the results for each. We highly recommend this company. Thank you, Mitch, Dave & Greg.

by Brent L on Twintech Heating and Cooling

Work was done on schedule, professionally, according to quote and with minimal inconvenience to building users. Very happy with the end result. The only area for improvement from my experience - need more timely quoting process.

by Karen on Twintech Heating and Cooling

The team was great! Very knowledgeable, professional and extremely helpful. A few days after install the unit didn’t seem to be working, user error, but within minutes of contacting them..on a Saturday.. a tech was on the phone troubleshooting with us. I highly recommend Twintech for your cooling or heating needs. They stand by their products and a pleasure to deal with.

by Rob on Twintech Heating and Cooling

During the hottest week in the summer our old A/C decided to call it quits. I called Twintech Heating & Cooling after receiving a referral from a family member; came highly rated for being an honest, family-run business, who appreciates their customers. Dave was able to accommodate my hectic schedule and book a late evening appointment to walk through the specifics. Greg and Mitch arrived exactly on time a few days later, were very polite and friendly, and within a couple of hours had a new unit up and running. I also received a follow-up call to ensure we were happy with the Amana.

by Linda Robinson on Twintech Heating and Cooling

They were on time and very professional. I'm so glad I decided to go with Twintech, thanks!

by Famcam1953 on Twintech Heating and Cooling

Timely and appreciated - this was during the hottest days this year when our Air and Furnace way.
Efficient. Professional. No BS or overselling. What was promised, was delivered at a cost-effective price point.
Down to earth.

by DG Sellers on Twintech Heating and Cooling

Professionally done and took care to cover all entry floors to the house for the installers to remove the old furnace and install a new furnace.
I also made recommendations of the tankless water heater that we had installed and items to address. We did not know they also installed tankless water heaters.

by Bill, Pickering on Twintech Heating and Cooling

Price was not the cheapest nor the most expensive of my quotes. It's the quality of the installation that makes for a good AC system. The twins' knowledge & professionalism was evident in the small details:
Very clean work. Greg made sure that the refrigerant level & temperature were correct. They reinstall the insulation & vapor barrier around the wall. The outside electrical cords were secured to the wall. Proud to support this local company. Thank you.

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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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