What Size Furnace Is Perfect for Your Home?
In the colder months, your furnace can feel like the one thing standing between you and a chilly, uncomfortable house. So when it’s time to replace or install a new furnace, it’s natural to wonder what size is best for your home. Unfortunately, the moment you start Googling, you’re met with tons of technical jargon and confusing numbers instead of the simple answers you were looking for. If you want everything explained in beginner-friendly language, you’ve come to the right place!
Topics Covered:
- What are BTUs?
- What Does “Ton” Mean in a Furnace?
- Why Choosing the Right Furnace Size Matters
- Furnace Size by Square Footage
- Furnace Size by Climate Zone
- Other Factors That Determine Furnace Size
- How Do You Determine the Perfect Furnace Size for Your Home?
- Other Things to Consider When Purchasing a Furnace
- What is an AFUE? (Furnace Efficiency)
- Furnace Warranties
- Furnace Orientation
- Conclusion
Unlike your favorite coffee order, furnaces don’t come in “small, medium, and large.” A furnace’s “size” has nothing to do with its physical dimensions but instead refers to its heating power. That heating power is measured in BTUs.
What are BTUs?
BTU stands for British Thermal Unit, which measures the amount of heat a furnace can produce. The higher the BTU rating, the more heating power the furnace has. For example, a furnace rated at 80,000 BTUs can produce twice as much heat as one rated at 40,000 BTUs.
But BTUs aren’t the only way heating capacity is measured, you’ll often hear it described in tons too.
What Does “Ton” Mean in a Furnace?
Despite the name, a “ton” has nothing to do with weight. It’s just another way of expressing heating capacity. Think of it like miles vs. kilometers, both measure distance, just in different units.
Here’s the simple breakdown:
- 1 ton = 12,000 BTUs
- 2 ton = 24,000 BTUs
- 3 ton = 36,000 BTUs
- 4 ton = 48,000 BTUs
- 5 ton = 60,000 BTUs
Why Choosing the Right Furnace Size Matters
Getting the size right is crucial for comfort and efficiency.
- Too small: The furnace runs constantly, struggles to heat your home, raises your energy bills, and wears out early.
- Too big: It blasts your home with heat too quickly, shuts off, then repeats in short cycles. This uneven heating uses more energy and causes unnecessary wear.
Furnace Size by Square Footage
The bigger your home, the more space there is to heat. Here’s a basic rule of thumb:
Furnace Sizing Guide by Square Footage
- 600 – 1,000 sq ft: 18,000 BTU (1.5 tons)
- 1,000 – 1,300 sq ft: 24,000 BTU (2 tons)
- 1,300 – 1,600 sq ft: 30,000 BTU (2.5 tons)
- 1,600 – 1,900 sq ft: 36,000 BTU (3 tons)
- 1,900 – 2,200 sq ft: 42,000 BTU (3.5 tons)
- 2,200 – 2,600 sq ft: 48,000 BTU (4 tons)
- 2,600 – 3,200 sq ft: 60,000 BTU (5 tons)
Furnace Size by Climate Zone
Where you live matters too. Homes in colder climates require more heating power per square foot.
According to Energy.gov, the U.S. is divided into climate zones. San Antonio falls in Zone 2, which means your home will generally need 35–40 BTUs per square foot.
Climate Zone Heating Guide:
- Zone 1: 30–35 BTUs per sq ft
- Zone 2: 35–40 BTUs per sq ft
- Zone 3: 40–45 BTUs per sq ft
- Zone 4: 45–50 BTUs per sq ft
- Zone 5: 50–60 BTUs per sq ft
Other Factors That Determine Furnace Size
Square footage and climate aren’t the only considerations. Other factors include:
- Sun exposure: Homes with more sunlight naturally need less heating.
- Construction materials: Brick homes tend to retain heat better than drywall-heavy homes.
- Layout: Open floor plans are easier to heat evenly than homes with lots of small rooms.
- Windows: Larger windows can let heat escape in winter.
How Do You Determine the Perfect Furnace Size for Your Home?
You can estimate using square footage and climate zone, but for exact results, HVAC professionals use a Manual J load calculation.
A Manual J takes into account not just size and location, but also insulation, sun exposure, window placement, and more. According to the Air Conditioning Contractors of America (ACCA), this is the gold standard for accurate sizing.
Stay Cool offers free Manual J calculations with every furnace estimate, so you can be sure you’re getting the perfect fit.
Other Things to Consider When Purchasing a Furnace
What is an AFUE? (Furnace Efficiency)
AFUE, or Annual Fuel Utilization Efficiency, measures how efficiently a furnace converts fuel into heat. According to the U.S. Department of Energy, a furnace with a 95% AFUE converts 95% of its fuel into usable heat, wasting only 5%. The higher the AFUE, the lower your utility bills.
Furnace Warranties
A furnace is a big investment, so warranties are important. Most manufacturers offer 5–10 year coverage, but at Stay Cool, we provide extended warranty options to give San Antonio homeowners peace of mind.
Furnace Orientation
Depending on your home’s layout, furnaces can be installed upflow, downflow, or horizontal. Your HVAC professional will guide you based on available space.
Conclusion: There's No "One-Size-Fits-All"
Choosing the right furnace size is one of the most important steps in keeping your San Antonio home comfortable and energy efficient. While charts and climate zones provide a helpful starting point, factors like insulation, windows, and layout mean there’s no one-size-fits-all answer. That’s why the best way to get an exact recommendation is to have a professional perform a Manual J calculation.
If you’re ready for a free estimate and professional sizing consultation, contact Stay Cool today, we’ll help you find the furnace that’s the perfect fit for your home!