How Many BTU Furnace Do I Need? Sizing Guide
Furnace BTU Chart by House Size
Based on 80% AFUE furnace in ASHRAE Climate Zone 4. For 96% AFUE, multiply output column by 1.2. For Zone 5-6, multiply by 1.4–1.6.
| House Size | Input BTU (80% AFUE) | Input BTU (96% AFUE) | Output BTU |
|---|---|---|---|
| 800–1,000 sq ft | 40,000–50,000 | 35,000–45,000 | 32,000–43,000 |
| 1,000–1,200 sq ft | 50,000–60,000 | 45,000–50,000 | 40,000–48,000 |
| 1,200–1,500 sq ft | 60,000–75,000 | 50,000–65,000 | 48,000–62,000 |
| 1,500–1,800 sq ft | 75,000–90,000 | 65,000–80,000 | 60,000–77,000 |
| 1,800–2,200 sq ft | 90,000–110,000 | 80,000–95,000 | 72,000–91,000 |
| 2,200–2,800 sq ft | 110,000–135,000 | 95,000–115,000 | 88,000–110,000 |
| 2,800–3,500 sq ft | 135,000–150,000 | 115,000–130,000 | 108,000–125,000 |
Understanding AFUE Efficiency
AFUE (Annual Fuel Utilization Efficiency) measures how much fuel energy becomes useful heat. The rest is lost through exhaust gases.
- 80% AFUE: 80 cents of every dollar spent on gas becomes heat. Standard efficiency. Minimum allowed in southern US states
- 90% AFUE: 90 cents becomes heat. Mid-efficiency. Required in northern US states (DOE regional standards)
- 95–98% AFUE: Condensing furnace. Extracts heat from exhaust gases. Highest efficiency. Payback in 5–7 years in cold climates
Key formula: Output BTU = Input BTU × AFUE
Example: 80,000 BTU input × 0.96 AFUE = 76,800 BTU output
How to Calculate Furnace Size
The accurate method uses ASHRAE's heating load calculation:
Q = Qwalls + Qwindows + Qinfiltration + Qventilation
- Qwalls = Wall area × U-value × ΔT (design temp difference)
- Qwindows = Window area × U-value × ΔT
- Qinfiltration = 0.018 × ACH × Volume × ΔT
- Qventilation = 0.018 × CFM × ΔT
Where ΔT = indoor design temp (typically 70°F) − outdoor design temp (ASHRAE 99% value for your location).
For a step-by-step example, see our heating load calculation guide.
Furnace Sizing for Different Climates
| Climate Zone | Outdoor Design Temp | BTU/sq ft (80% AFUE) | Example Cities |
|---|---|---|---|
| Zone 2–3 | 25–35°F | 25–35 | LA, Houston, Atlanta |
| Zone 4 | 10–20°F | 35–45 | NYC, Denver, Seattle |
| Zone 5 | 0–10°F | 45–55 | Chicago, Boston, Detroit |
| Zone 6 | -10–0°F | 55–70 | Minneapolis, Buffalo |
| Zone 7 | -20–-10°F | 70–85 | Duluth, Anchorage |
Common Mistakes
- Ignoring AFUE when comparing prices: A cheap 80% AFUE furnace costs more to operate than a pricier 96% AFUE unit. Calculate total cost of ownership (purchase + 15 years of fuel)
- Oversizing by 50%+: Extremely common. Contractors oversize to avoid callbacks. Result: short-cycling, uneven heat, higher bills, shorter equipment life
- Using cooling tonnage for heating: Heating and cooling loads are different. A house in Chicago needs 3 tons of cooling but 80,000 BTU of heating
- Not considering altitude: Gas furnaces lose ~4% capacity per 1,000 ft above sea level. At 5,000 ft (Denver), a 100,000 BTU furnace delivers only 80,000 BTU
Frequently Asked Questions
How many BTU furnace for a 1,500 sq ft house?
In moderate climate (Zone 4): 60,000–80,000 BTU input (80% AFUE = 48,000–64,000 BTU output). In cold climate (Zone 5-6): 80,000–100,000 BTU input. Always size based on heat loss calculation.
What AFUE rating should I choose?
Minimum AFUE for new gas furnaces is 80% (DOE 2021). 90%+ AFUE is required in Northern states. High-efficiency condensing furnaces (96–98% AFUE) pay back in 5–7 years in cold climates.
What is the difference between input and output BTU?
Input BTU is fuel energy consumed. Output BTU is heat delivered. Output = Input × AFUE. Example: 80,000 BTU input × 0.96 = 76,800 BTU output. Always compare furnaces using output BTU.
Is it better to oversize or undersize a furnace?
Neither. Oversized furnaces short-cycle, creating temperature swings. Undersized furnaces can't maintain temperature on the coldest days. Size within 10–15% of calculated heat loss.