Enter your design heating load and the furnace AFUE. You get the input and output BTU per hour, with a range, or you can check a quote.
What “furnace size” means here
A furnace is sized from the heat the house loses on a design winter day, called the design heating load, and not from square footage alone. The load comes from a Manual J calculation, and Manual S is the step that turns the load into equipment. The U.S. Department of Energy’s sizing guideline says Manual S recommends that the furnace stay within 140 percent of the peak heating load estimate. This calculator takes your load and the furnace AFUE and returns the input and output Btu per hour with that range.
We could not find an official, published Btu-per-square-foot figure by climate zone, insulation level and efficiency, so the tool does not guess one. If you do not have a load yet, ask your contractor for the Manual J report, which lists it.
How it is calculated
AFUE is the annual ratio of the energy a furnace delivers to the home’s ducts compared with the energy in the fuel, so a furnace with 96 percent AFUE delivers 96 percent of its input as heat. The tool therefore treats the load as the output the furnace must deliver, and divides by AFUE to get the input rating.
Output upper limit = 1.4 × design heating load
Input needed = output ÷ AFUE (as a decimal)
Input range = load ÷ AFUE up to 1.4 × load ÷ AFUE
Load per sq ft = design load ÷ heated floor area
The headline input is rounded up to the next 1,000 Btu per hour so the furnace is never below the load, and the upper limit is rounded down. The second tab does the reverse for a quote: it multiplies the furnace’s input rating by its AFUE to get the output, then shows what share of your load that is.
Worked examples
A home with a 40,000 Btu per hour design heating load and a 96 percent AFUE furnace
Output needed: 40,000 Btu per hour. Upper limit: 1.4 × 40,000 = 56,000.
Input needed: 40,000 ÷ 0.96 = 41,667, rounded up to 42,000 Btu per hour. Upper limit: 56,000 ÷ 0.96 = 58,333, rounded down to 58,000.
Input range: 42,000 to 58,000 Btu per hour (12.3 to 17.0 kW).
The Chicago house example from the sizing guideline
The guideline’s Manual J result is a heating load of 41,660 Btu per hour, so the maximum target capacity is 41,660 × 1.4 = 58,324.
With the guideline’s 92.5 percent AFUE furnace the input range is 41,660 ÷ 0.925 = 45,038 up to 58,324 ÷ 0.925 = 63,053, shown by the tool as 46,000 to 63,000 Btu per hour.
Checking a quote: a 60,000 Btu per hour furnace at 96 percent AFUE for a 40,000 load
Output: 60,000 × 0.96 = 57,600 Btu per hour. That is 144 percent of the load, above the 140 percent limit, so the tool flags it as larger than the sizing method allows.
Low-load homes
The Pacific Northwest National Laboratory notes that a common definition of a low-load home is one with design heating and cooling loads of 10 Btu per hour per square foot or less. For a 1,200 to 2,400 sq ft home that is 12,000 to 24,000 Btu per hour. The same review explains that equipment built for larger loads can cycle on and off in short bursts, which lowers efficiency and comfort. The tool shows a note when your load per square foot is 10 or less.
Frequently asked questions
What size furnace do I need for a 2,000 sq ft house?
There is no honest single number from square footage alone, because the load depends on climate, insulation, windows and air leakage. Ask for a Manual J load calculation, then enter its result here. For reference, a low-load home of 2,000 sq ft has a design load of 20,000 Btu per hour or less.
What is the difference between input and output Btu?
Input is the fuel energy the furnace burns, and output is the heat it delivers to the ducts. Output equals input times AFUE, so an 80 percent AFUE furnace with a 60,000 input delivers 48,000 Btu per hour.
Is a bigger furnace better?
No. Manual S limits a furnace to about 140 percent of the design heating load, and a furnace that is much too large cycles on and off in short bursts, which lowers efficiency and comfort.
What does AFUE mean?
AFUE stands for Annual Fuel Utilization Efficiency. It is the annual ratio of the energy a furnace delivers to the home compared with the energy in the fuel, shown as a percentage on the label.
How do I find my design heating load?
It comes from a Manual J calculation, which a heating contractor or an energy professional can run for your house. Ask for the report and use the heating load figure in Btu per hour.
Related calculators
Sources
Every figure on this page comes from the sources below. They were opened and checked while building the tool.
- U.S. Department of Energy (2011): Building America Strategy Guideline: HVAC Equipment Sizing (Manual S limits, Chicago house example)
- Pacific Northwest National Laboratory (2013): Review of Residential Low-Load HVAC Systems, PNNL-23017