Understanding your result
The headline is the furnace input to buy, picked from the tiers that are sold (40,000, 60,000, 80,000, 100,000, 120,000 and 140,000 BTU per hour), with the house’s heating load in the sub-line. The load is an output figure, the heat the house loses on the design night; the input is the gas the furnace burns to supply it, and the two differ by the AFUE you entered. The stats repeat the load, the output of the chosen furnace and the blower airflow.
The oversize margin in the table is the furnace’s output divided by the load, minus one. Up to about 40% is tolerated by ACCA Manual S because furnace sizes jump in 20,000 BTU steps; above that the notes suggest a two-stage or modulating furnace, which runs at a fraction of its rating most of the time. A margin near zero is fine for a condensing furnace, which runs long and steady; it is not a reason to go up a tier.
The heat loss per degree row, the UA, is the load divided by the difference between 70 °F indoors and the zone’s design temperature. It is the figure the degree-day fuel calculator uses to turn a load into a season’s gas, and the heating cost by fuel calculator starts from the same load. If the house is a candidate for a heat pump instead, the heat pump size calculator uses the same loads and shows the balance point.
How we calculate this
The load per square foot is the heating figure for the climate zone on the BTU per square foot table, from 20 BTU/hr per sq ft in zone 1 to 60 in zone 7, for an average-insulated house with 8 ft ceilings at the zone’s typical 99% design temperature. The factors for insulation, ceiling height and windows are the same ones the cooling calculators use. The 1.08 in the airflow formula is the sensible heat factor for air at standard conditions (BTU/hr per CFM per °F), the constant every technician uses to check a temperature rise.
AFUE is the annual fuel utilisation efficiency on the yellow label: 80% for a standard induced-draft furnace, 90 to 98.5% for a condensing one, 100% for an electric furnace. The efficiency ratings table lists the federal minimums and what each tier means for the venting.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Heating load per sq ft | 20 to 60 BTU/hr per sq ft by zone (45 for zone 5) | Rule-of-thumb climate-zone sizing charts; see the BTU per square foot table |
| Design temperatures | 50, 35, 25, 15, 0, −10, −20 °F for zones 1 to 7 | Typical 99% heating design temperatures (ASHRAE climatic data), rounded by zone |
| Insulation, ceiling, windows | 1.2 / 1.0 / 0.85; +10% per ft above 8 ft; 0.95 / 1.0 / 1.1 | Common published adjustments; Manual J uses the actual construction |
| Stock input tiers | 40, 60, 80, 100, 120, 140 thousand BTU/hr | Residential gas furnace sizes sold in the US |
| AFUE | 95% | A common condensing furnace; the federal minimum for non-weatherized gas furnaces is 80%, rising to 95% for most new installations under the DOE 2023 rule effective 2028 |
| Sensible heat factor | 1.08 BTU/hr per CFM per °F | Standard air at sea level |
Assumptions last reviewed October 7, 2026.
The calculator does not model infiltration measured by a blower door, the heat gained from sun and occupants (which Manual J counts against the heating load), basement walls below grade or the altitude correction that de-rates furnaces above 2,000 ft. The guide to furnace input, output and AFUE explains how to read a nameplate, and the guide to BTU per square foot by climate zone says where the per-square-foot figures come from and how far to trust them.
Two worked examples
A 2,000 sq ft house in a cool climate
Zone 5 (Chicago, Denver, Boston), average insulation, 8 ft ceilings, average windows, a 95% AFUE furnace, 50 °F rise.
- Load: 2,000 × 45 = 90,000 BTU/hr output; all factors 1.0
- Input needed: 90,000 ÷ 0.95 = 94,737 BTU/hr
- Furnace: 100,000 BTU/hr input, delivering 95,000 BTU/hr; margin 6%
- Heat loss per degree: 90,000 ÷ 70 = 1,286 BTU/hr per °F
- Blower airflow: 95,000 ÷ (1.08 × 50) = 1,759 CFM
A 6% margin is as close as stock sizes get. The 1,759 CFM of heating airflow is more than the 1,200 CFM a 3-ton air conditioner on the same ducts needs, so the ducts, not the cooling, decide whether this furnace can breathe; a 60 °F rise would bring it down to 1,466 CFM.
An older house in a warm climate with a standard furnace
1,400 sq ft in zone 3 (Atlanta, Dallas), poor insulation, many windows, 8 ft ceilings, an 80% AFUE furnace.
- Load: 1,400 × 30 × 1.2 × 1.1 = 55,440 BTU/hr output
- Input needed: 55,440 ÷ 0.80 = 69,300 BTU/hr
- Furnace: 80,000 BTU/hr input, delivering 64,000 BTU/hr; margin 15%
- Heat loss per degree: 55,440 ÷ 45 = 1,232 BTU/hr per °F
- Blower airflow: 64,000 ÷ (1.08 × 50) = 1,185 CFM
The note points out that insulation and air sealing would cut the load to about 46,200 BTU/hr, which with a 95% furnace needs only 48,600 BTU/hr of input and drops the furnace to the 60,000 tier. In a warm climate the money is better spent on the envelope than on a bigger burner, and a heat pump would cover a load this small on its own.
Where to find your inputs
Heated floor area. The conditioned area from the plans or the listing, leaving out the garage, an unfinished basement and the attic. A finished basement that is heated counts at about half, because its walls are below grade.
Climate zone. By county on the DOE map; the dropdown names example cities and the BTU per square foot table lists more.
AFUE. On the yellow EnergyGuide label of the furnace you are considering, or in its model specifications. For an existing furnace, the nameplate inside the cabinet gives input and output; output ÷ input is the steady-state efficiency, close to the AFUE.
Temperature rise. On the furnace nameplate as a range, such as “RISE 35–65 °F”. Use the middle of the range for the airflow estimate.
Common mistakes
- Buying the same size as the old furnace. The old one was probably oversized; houses have also often been insulated since it was installed.
- Confusing input with output. A 100,000 BTU label is input. At 80% it delivers 80,000; at 96% it delivers 96,000.
- Rounding up twice. The tier above the input is already a round-up. Adding “a little extra for the cold snap” produces the 40% oversizes that short-cycle.
- Forgetting the ducts. A bigger furnace needs more airflow; if the ducts cannot carry it the high-limit switch trips and the heat exchanger suffers.
- Ignoring the air conditioner’s airflow. The blower serves both. Check the cooling CFM with the central AC size calculator.
- Skipping the Manual J. This is a rule of thumb. For a purchase that lasts twenty years, a room-by-room load calculation is worth insisting on.
Questions people ask
- How many BTU furnace do I need for 2,000 square feet?
- With the climate-zone loads this site uses, a 2,000 sq ft house with average insulation and 8 ft ceilings needs about 90,000 BTU per hour of output in a cool climate (zone 5), 76,000 in a mixed one (zone 4) and 50,000 to 60,000 in a warm one (zones 2 to 3). Divide the output by the AFUE to get the input size on the label. 90,000 BTU of output from a 95% furnace needs 94,700 BTU of input, so the stock 100,000 BTU furnace is the one to buy.
- What is the difference between furnace input and output?
- Input is the fuel energy the furnace burns per hour, which is the big number on the nameplate and in the model number. Output is the heat that reaches the ducts, which is the input multiplied by the AFUE. An 80,000 BTU furnace at 80% AFUE delivers 64,000 BTU per hour; the same input at 96% delivers 76,800. The house's load is an output figure, so a high-efficiency furnace can be a tier smaller than a standard one for the same house.
- Is it bad to oversize a furnace?
- Yes, though less dramatically than oversizing an air conditioner. An oversized furnace heats the house in short bursts, so rooms far from the thermostat never warm through, the temperature swings, the blower and igniter cycle many more times a year, and a single-stage unit runs at its least efficient for most of its life. Industry guidance (ACCA Manual S) allows a furnace up to 40% above the heating load, mainly because stock sizes are coarse; two-stage and modulating furnaces make the margin far less harmful.
- Which climate zone am I in?
- The US Department of Energy divides the country into zones 1 (hot, southern Florida) to 8 (Alaska) by county, and the International Energy Conservation Code uses the same map. Zone 4 covers a band from Washington DC through St. Louis to Seattle; zone 5 is Chicago, Denver and Boston; zone 6 is Minneapolis and northern New England; zone 7 is northern Minnesota and North Dakota. The BTU per square foot table on this site lists example cities and the zone map is on the DOE's website.
- Why is the blower airflow shown?
- Because a furnace needs enough air through its heat exchanger to keep the temperature rise inside the range on its nameplate, usually 35 to 65 °F, and that airflow has to pass through the ducts. The formula is output ÷ (1.08 × rise). A 95,000 BTU output furnace at a 50 °F rise needs about 1,760 CFM; if the ducts were sized for a 60,000 BTU furnace they will not carry it, the rise will climb, and the furnace will trip its high-limit switch. The duct size calculator checks the ducts.
- Should the furnace be sized for the air conditioner or the house?
- For the house's heating load, then matched to the air conditioner's airflow. The furnace blower must deliver the cooling airflow of about 400 CFM per ton as well as the heating airflow, and in warm climates the cooling airflow is the larger of the two, which can force a bigger blower than the heating load alone would need. A two-stage or variable-speed blower handles both; a contractor's Manual S selection checks the match.