What Size Furnace Do I Need in Fairfield County, CT?
Whether you’re beginning to explore furnace replacement options or simply want to confirm that your contractor’s recommendation makes sense, this calculator provides a helpful estimate.
Calculate Furnace Size in BTUs
All calculations are based on typical conditions for Fairfield County, CT.
How to use the calculator
- Enter your home's square footage;
- Select your insulation level;
- Estimate your windows and air sealing; and
- Enter your ceiling height, that's it!
The calculator will take care of the rest!
What Impacts the Furnace Size You Need?
Several factors affect how many BTUs (British Thermal Units) your home needs to stay warm. The calculator uses a few key ones:
- Square Footage: Larger homes need more heat to maintain a comfortable temperature.
- Insulation: Well-insulated homes retain heat better, so they need fewer BTUs. Older or poorly insulated homes lose more heat, requiring a larger system.
- Windows and Air Sealing: Drafty or single-pane windows let warm air escape and cold air in, while newer, well-sealed windows help your furnace work more efficiently.
- Ceiling Height: Heat rises, so taller ceilings increase the volume of air that must be heated.
- Climate: In colder climates like in Fairfield County, homes typically need around 40 BTUs per square foot as a baseline to stay comfortable during winter.
Understanding Your Results
When you click “Calculate Furnace BTUs,” the calculator gives you four numbers:
- #1: Home Heating Load (BTU/hr): The estimated amount of heat your home needs to maintain comfort on a cold day.
- #2: Furnace size for standard gas (≈80% AFUE): Standard gas furnaces don’t convert 100% of fuel into heat. At about 80% efficiency, the furnace has to be a little bigger than your heating load to deliver the right amount of heat to the house.
- #3: Furnace size for high-efficiency gas (≈96% AFUE): High-efficiency furnaces waste less heat, so they don’t have to be as big. This number is usually closer to the actual heating load because more of the fuel is turned into usable heat.
- #4: Furnace size for electric (≈100% efficiency): Electric furnaces convert almost all of the energy into heat, so this number is typically the same as your heating load.
Find The Right Unit For Your Unique Home
Experts recommend anywhere from 40-60 BTUs per square foot of your home in a climate like Fairfield County, CT. But that’s a pretty wide range, and you might end up purchasing a furnace that’s too big or too small.
So to make sure you don’t waste any money, we start with the baseline of 40 BTUs. Then, we adjust that based on a few factors that impact how much heat output you need.
- Base load: 40 BTU per square foot (typical for southern New York and southwestern Connecticut).
- Insulation & window adjustments: Good insulation/windows reduce BTUs by about 10–15%. Poor insulation/windows increase BTUs by about 10–15%.
- Ceiling height: Homes with 9–10 ft ceilings require about 5–10% more heat.
- Furnace size for electric (≈100% efficiency): Electric furnaces convert almost all of the energy into heat, so this number is typically the same as your heating load.
How The Calculator Works
Want to learn exactly how the calculator works? Here are the 5 steps, with an example using a 2,000 sq. ft. home in Greenwich, CT with average insulation, average windows, and 8 ft ceilings:
1. Calculate the baseline BTUs
We first estimate the heat your home needs based on size and local climate: square feet × 40 BTU.
Example: 2,000 sq ft × 40 BTU = 80,000 BTU/hr.
2. Adjust for insulation
We adjust the base number based on how well the home is insulated.
- Average (typical CT home) has an adjustment factor of 1.00 (no adjustment).
- Good (updated, insulated) has an adjustment factor of 0.85.
- Poor (older, little insulation) has an adjustment factor of 1.15.
Example with average insulation: 80,000 × 1.00 = 80,000 BTU/hr.
3. Adjust for windows and air sealing
We then adjust again based on how tight the windows are.
- Updated or tight windows use 1.00 (no adjustment)
- Mixed or average windows use 1.08.
- Older or drafty windows use 1.15.
Example with mixed windows: 80,000 × 1.08 = 86,400 BTU/hr.
4. Adjust for ceiling height
Finally, we adjust for ceiling height.
- 8 ft ceilings use 1.00 (no adjustment).
- 9 ft ceilings use 1.05.
- 10 ft or higher uses 1.10.
Example with 8 ft: 86,400 × 1.00 = 86,400 BTU/hr (rounded to 86,500).
5. Convert to furnace sizes
Then we convert that to figure out the right size furnace, depending on whether you have a standard gas, high-efficiency gas, or electric furnace.
- a standard gas furnace at 80% efficiency,
- a high efficiency gas furnace at 96%, and
- an electric furnace at 100%.
Furnace Size Calculator for Fairfield County, CT Homeowners (Calculate BTUs)
Estimate BTUs needed in Fairfield County, CT
This is a planning estimate for Fairfield County, CT using about 40 BTU per sq ft as a starting point. Actual sizing should be confirmed on site by an experienced HVAC contractor who can look at ductwork, equipment location, fuel type, and your home’s level of insulation and air sealing.
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Want To Speak With Jack About Replacing Your Furnace?
Whether you’re just considering replacing your furnace, or it’s an emergency, you can reach out 24/7 to schedule your in-person quote. Call or text (203) 820-1776 or complete a short online form and Jack, our owner, will be in touch quickly!
