Furnace Size Calculator
Estimate the furnace size a home needs. Enter the heated area and a climate heating factor, then add an efficiency rating to see both output and input BTU.
How to size a furnace
- Enter the heated floor area in square feet.
- Set the climate heating factor in BTU per square foot.
- Optionally enter the furnace efficiency as a percent.
- Read the required output BTU, the input BTU and the tonnage.
Examples
1,500 sq ft, moderate climate
1,500 sq ft, 45 BTU per sq ft
67,500 BTU output, about 5.6 tons
1,500 sq ft at 95% efficiency
1,500 sq ft, 45 BTU per sq ft, 95%
67,500 BTU output, 71,052.63 BTU input
Frequently asked questions
How many BTU per square foot does heating need?
It varies by climate. A common range is about 30 BTU per square foot in mild zones up to 60 in very cold zones, with 45 as a moderate middle. These are guidelines.
What climate zone factor should I pick?
Roughly 30 to 35 for hot southern zones, 40 to 45 for moderate zones, 45 to 50 for cool zones and 50 to 60 for cold northern zones. Adjust for insulation and air sealing.
What is the difference between output and input BTU?
Output is the heat delivered to the home. Input is the fuel the furnace burns, which is higher because no furnace is 100 percent efficient. Input equals output divided by efficiency.
How does AFUE efficiency affect the size?
AFUE is the percent of fuel turned into usable heat. A 95 percent furnace wastes only 5 percent, so its input BTU is just above its output. A lower AFUE needs more input.
Is a rule-of-thumb size enough?
It is a useful starting point, not a final answer. A Manual J load calculation that accounts for windows, insulation and air leakage gives a more accurate furnace size.
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