Furnace size calculator
Enter the floor area, climate zone and how the house is built, or a heat loss from a Manual J if you have one. You get the heating load, the furnace output and input you need at your efficiency, and which standard furnace sizes are neither too small nor oversized.
Inputs
Result
| Input, BTU/h | Output | Of the load |
|---|
Heating BTU per square foot by climate zone
The rule-of-thumb output per square foot for average construction, and what it gives for common house sizes.
| Climate zone | BTU/h per ft² | 1,500 ft² | 2,000 ft² | 2,500 ft² |
|---|
How it is worked out
A 2,000 ft² house of average construction in climate zone 5 at 50 BTU/h per ft² needs about 100,000 BTU/h of heat. A 95% furnace rated 100,000 BTU/h input puts out 95,000, a little short; a 120,000 BTU/h input model puts out 114,000, or 114% of the load, inside the Manual S range of 100% to 140%. At 80% AFUE the 140,000 BTU/h furnace is the one that fits: 112,000 out.
Input and output
Furnaces are sold by input, the fuel they burn, but what heats the house is output. Output is input times the AFUE. An 80,000 BTU/h furnace at 80% puts out 64,000; at 96%, about 77,000. Comparing furnaces by input alone overstates the 80% one by a fifth.
Why not just go bigger?
ACCA Manual S, the sizing standard that codes reference, allows a furnace's output to be up to 140% of the heating load. A larger one short-cycles: it heats the house in quick bursts, wears its parts faster, makes temperatures swing, and often needs bigger ducts than the house has. If the house has had new windows, insulation or air sealing since the old furnace went in, the load is lower than it was.
Sizing the cooling side? Use the AC tonnage calculator, or the mini split size calculator room by room. For the gas line, the gas pipe sizing calculator.
Questions
What size furnace do I need for 2,000 square feet?
By the rule of thumb, about 60,000 BTU/h of heat in climate zone 1, 90,000 in zone 4 and 120,000 in zone 7, for average construction. At 95% AFUE that takes at least about 63,000, 95,000 and 126,000 BTU/h of furnace input. A Manual J on a newer house is often well below this.
How many BTU per square foot to heat a house?
The usual rule of thumb is 30 BTU/h per square foot in the warmest US climate zone, rising by 5 per zone to 60 in zone 7. Well-built newer houses need less, older leaky ones more.
What is the difference between furnace input and output?
Input is the fuel burned, output is the heat delivered. Output = input x AFUE: a 100,000 BTU/h furnace at 80% delivers 80,000, at 95% it delivers 95,000.
Can a furnace be too big?
Yes. ACCA Manual S limits furnace output to 140% of the heating load. A larger furnace short-cycles, wears faster and makes the house less comfortable.
What size furnace do I need for 1,500 square feet?
By the rule of thumb, about 45,000 BTU/h of heat in zone 1 to 90,000 in zone 7 for average construction; around 67,500 in zone 4, which an 80,000 BTU/h input furnace at 95% AFUE covers: 76,000 out, 113% of the load.
Should I replace my furnace with the same size?
Not without checking. Many existing furnaces were oversized, and new windows, insulation or air sealing lower the load. A heat loss calculation, or this estimate as a sanity check, tells you whether a smaller furnace will do.
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