Furnace size comes from heat loss at your county’s coldest design temperature, not a flat BTU-per-square-foot rule. Pick your county, describe the house, and get the output range ENERGY STAR’s design rules allow.
Location
Pick your county to fill the 99% heating design temperature from EPA’s ENERGY STAR reference guide, or type your own. Picking your county also sets its 2021 IECC climate zone (see the climate zone map).
Your home
Assumptions used (editable)
Furnace
Design heat loss
—BTU/H
Furnace output range
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Suggested input size
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Its output at this AFUE
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Heat loss per sq ft
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Where the heat goes
Part of the house
Heat loss at design
Share
Choosing the furnace size
ENERGY STAR’s HVAC design rules require the furnace’s output capacity to be 100–140% of the design heat loss when it only heats. When the same air handler also serves central air conditioning, 100–140% is still the recommendation, though up to 400% is allowed because the blower is often sized for cooling airflow. For two-stage and modulating furnaces, use the maximum output.
Input vs output: furnaces are sold by gas input (for example 60,000 Btu/h). Output is roughly input × AFUE, but AFUE is a seasonal rating, so read the actual output capacity on the furnace’s rating plate or spec sheet before deciding. The suggested input sizes are common nominal sizes, such as those in Lennox’s current model list.
Efficiency minimum: non-weatherized gas furnaces must be at least 80% AFUE today, rising to 95% AFUE for furnaces made on or after December 18, 2028 under 10 CFR 430.32(e). Gas utility groups petitioned DOE in 2026 to delay that date; DOE had not decided as of October 2026.
How the heat loss is calculated
Heat loss at the design temperature = (sum of each surface’s area × its U-factor + air leakage) × (70°F indoors − the outdoor design temperature). The 70°F indoor and the 99% outdoor design temperature are the conditions ENERGY STAR requires for HVAC design.
Design temperature: the county value from EPA’s ENERGY STAR Design Temperature Limit Reference Guide (2019 edition), which EPA compiled from ASHRAE and ACCA Manual J weather data. Where several weather stations fall within 40 miles of a county’s centre, EPA uses the coldest — so the value is on the safe side. A local jurisdiction’s own design temperature takes precedence.
Surfaces: the home is treated as a square box of the given floor area and stories, with windows as a share of wall area.
U-factors: the “new, 2021 IECC” option uses the code’s maximum U-factors for your climate zone. The other options are labelled assumptions, because no official values exist for existing homes; open “Assumptions used” to see or change them.
Air leakage: blower-door ACH50 converted to natural air changes with LBNL’s method (÷ about 19, adjusted for stories), then × 1.5 for design-day conditions (New York State Clean Heat guidance). The 2021 IECC limit is 3.0 ACH50 in zones 3–8 and 5.0 in zones 1–2.
Floor: floors over a crawlspace or unheated basement are taken to lose heat to outdoor temperature (conservative); slabs at half that rate (an assumption, since the ground is warmer than the air on a design day).
Not included: duct losses, internal gains and room-by-room effects. This is a whole-house planning estimate, not an ACCA Manual J calculation, which a contractor should run before buying.
FAQ
What size furnace do I need for a 2,000 sq ft house?
It depends on your winter design temperature and how well the house is insulated and sealed, not just its size. A typical 1990s–2000s 2,000 sq ft home near Chicago (design temperature −3°F) loses roughly 38,000 Btu/h, so a furnace with 38,000–54,000 Btu/h of output fits; a new home built to the 2021 code needs about 22,000 Btu/h.
Is a bigger furnace better?
No. ENERGY STAR caps a heating-only furnace at 140% of the design heat loss. An oversized furnace cycles on and off more, which is less comfortable and less efficient.
What is the difference between furnace input and output?
Input is the gas the furnace burns, and the number it is usually sold by. Output is the heat delivered to the house, roughly input × AFUE. Size the furnace on output, and check the actual output on the spec sheet.
What is a design temperature?
The outdoor temperature a heating system is sized for: the 99% value is exceeded (warmer) 99% of the hours in a typical year. The calculator fills it from EPA’s ENERGY STAR county reference guide.
What AFUE do furnaces need to have?
At least 80% AFUE for non-weatherized gas furnaces today, rising to 95% for furnaces made on or after December 18, 2028 (10 CFR 430.32). A delay petition was pending with DOE in 2026.