The short answer
An 1,800 sq ft one-story house with average insulation needs a 60,000 BTU input furnace at 95% AFUE in Minneapolis, where the winter design temperature is -6°F. In Houston (37°F) the heating load is only 18,400 BTU/h, and the smallest common furnace, 40,000 BTU input, covers it.
The same house in every state
An 1,800 sq ft, one-story house with average insulation, at the winter design temperature of each state's largest city.
| Alabama Huntsville | 3A | 23 | 26,300 | 40,000 |
|---|---|---|---|---|
| Alaska Anchorage | 7 | -4 | 41,400 | 60,000 |
| Arizona Phoenix | 2B | 43 | 15,100 | 40,000 |
| Arkansas Little Rock | 3A | 25 | 25,100 | 40,000 |
| California Los Angeles | 3B | 47 | 12,900 | 40,000 |
| Colorado Denver | 5B | 7 | 35,200 | 40,000 |
| Connecticut Bridgeport | 5A | 16 | 30,200 | 40,000 |
| Delaware Wilmington | 4A | 18 | 29,100 | 40,000 |
| District of Columbia Washington | 4A | 23 | 26,300 | 40,000 |
| Florida Jacksonville | 2A | 42 | 15,600 | 40,000 |
| Georgia Atlanta | 3A | 27 | 24,000 | 40,000 |
| Hawaii Honolulu | 1A | 66 | 2,200 | 40,000 |
| Idaho Boise | 5B | 16 | 30,200 | 40,000 |
| Illinois Chicago | 5A | 6 | 35,800 | 40,000 |
| Indiana Indianapolis | 4A | 9 | 34,100 | 40,000 |
| Iowa Des Moines | 5A | 0 | 39,100 | 60,000 |
| Kansas Wichita | 4A | 13 | 31,900 | 40,000 |
| Kentucky Louisville | 4A | 16 | 30,200 | 40,000 |
| Louisiana New Orleans | 2A | 40 | 16,800 | 40,000 |
| Maine Portland | 6A | 7 | 35,200 | 40,000 |
| Maryland Baltimore | 4A | 19 | 28,500 | 40,000 |
| Massachusetts Boston | 5A | 14 | 31,300 | 40,000 |
| Michigan Detroit | 5A | 9 | 34,100 | 40,000 |
| Minnesota Minneapolis | 6A | -6 | 42,500 | 60,000 |
| Mississippi Jackson | 3A | 29 | 22,900 | 40,000 |
| Missouri Kansas City | 4A | 9 | 34,100 | 40,000 |
| Montana Billings | 6B | -4 | 41,400 | 60,000 |
| Nebraska Omaha | 5A | 4 | 36,900 | 40,000 |
| Nevada Las Vegas | 3B | 37 | 18,400 | 40,000 |
| New Hampshire Manchester | 5A | 9 | 34,100 | 40,000 |
| New Jersey Newark | 4A | 17 | 29,600 | 40,000 |
| New Mexico Albuquerque | 4B | 23 | 26,300 | 40,000 |
| New York New York | 4A | 18 | 29,100 | 40,000 |
| North Carolina Charlotte | 3A | 27 | 24,000 | 40,000 |
| North Dakota Fargo | 6A | -13 | 46,400 | 60,000 |
| Ohio Columbus | 4A | 11 | 33,000 | 40,000 |
| Oklahoma Oklahoma City | 3A | 19 | 28,500 | 40,000 |
| Oregon Portland | 4C | 30 | 22,400 | 40,000 |
| Pennsylvania Philadelphia | 4A | 18 | 29,100 | 40,000 |
| Rhode Island Providence | 5A | 11 | 33,000 | 40,000 |
| South Carolina Charleston | 3A | 32 | 21,200 | 40,000 |
| South Dakota Sioux Falls | 6A | -4 | 41,400 | 60,000 |
| Tennessee Nashville | 3A | 21 | 27,400 | 40,000 |
| Texas Houston | 2A | 37 | 18,400 | 40,000 |
| Utah Salt Lake City | 5B | 20 | 27,900 | 40,000 |
| Vermont Burlington | 6A | 0 | 39,100 | 60,000 |
| Virginia Virginia Beach | 3A | 27 | 24,000 | 40,000 |
| Washington Seattle | 4C | 30 | 22,400 | 40,000 |
| West Virginia Charleston | 4A | 16 | 30,200 | 40,000 |
| Wisconsin Milwaukee | 5A | 4 | 36,900 | 40,000 |
| Wyoming Cheyenne | 5B | 3 | 37,400 | 40,000 |
How it works
The formula
We add up the heat your house loses on a design-temperature night, the same idea as ACCA Manual J, then pick the smallest standard furnace whose output covers it.
walls, ceiling, windows = U × area × ΔT
floor edge = F × perimeter × ΔT
air leaks = 1.08 × CFM × ΔT
ducts = 15% of the above
output needed = total
input = output ÷ AFUE, rounded up to 40k, 60k, 80k, 100k, 120k
Assumptions
- Indoors is kept at 70°F, the Manual J heating condition. No credit is taken for sun, people or appliances, since the design night is cold and dark.
- The house is a simple box with a square footprint, the same as the AC calculator. Average insulation is R-13 walls (U 0.075), an R-30 attic (U 0.033), double-pane windows (U 0.5) and 0.45 air changes an hour.
- Floor loss is counted along the slab or foundation edge (0.55 BTU/h per foot per °F for average). A heated basement with uninsulated walls loses more.
- Ducts run through unheated space and lose 15% of the heat. Sealed ducts inside the house lose almost none.
- Manual S allows furnace output from 100% to 140% of the load. In mild climates the smallest furnace made is often bigger than that; it still works, it just cycles more.
Worked example: 1,800 sq ft in Minneapolis
One story, 8 ft ceilings, average insulation. Winter design -6°F, so ΔT = 70 − (-6) = 76°F.
- Walls 0.075 × 1,088 ft² × 76 = 6,200; ceiling 0.033 × 1,800 × 76 = 4,500; windows 0.5 × 270 × 76 = 10,300.
- Floor edge 0.55 × 169.7 ft × 76 = 7,100; air 1.08 × 108 CFM × 76 = 8,900.
- Ducts +15% = 5,500. Total heating load 42,500 BTU/h.
- At 95% AFUE: 60,000 × 0.95 = 57,000 BTU/h covers it, so buy a 60,000 BTU furnace. At 80%: 60,000 × 0.80 = 48,000, so 60,000 BTU.
Too big or too small
- Too big: short cycles. The furnace heats in quick bursts and shuts off, which wears the igniter and heat exchanger and wastes fuel.
- Too big: hot and cold swings. Rooms overshoot, then cool off, and the far rooms never quite warm up because the blower doesn't run long enough.
- Too big: noise and drafts. A large furnace on small ducts pushes air fast and loud.
- Too small: it can't keep up on the coldest nights, running nonstop while the house slips below the setting. A few hours a year is normal; every cold snap is not.
- Between sizes? A two-stage or modulating furnace runs on low fire most of the time, so the larger size causes fewer problems.
Questions people ask
What size furnace do I need for an 1,800 sq ft house?
With average insulation, an 1,800 sq ft one-story house in Minneapolis (winter design -6°F) loses about 42,500 BTU per hour on a design night, so it needs a 60,000 BTU input furnace at 95% AFUE (57,000 BTU output). The same house in Houston (37°F) needs only about 18,400 BTU/h: even the smallest common furnace, 40,000 BTU input, is more than enough.
What is the difference between furnace input and output BTU?
Input is the gas the furnace burns per hour; output is the heat it delivers to the house. Output = input × AFUE. An 80,000 BTU input furnace at 95% AFUE delivers 76,000 BTU per hour; at 80% AFUE it delivers 64,000. Size on output.
How many BTU per square foot do I need to heat a house?
It depends mostly on your climate and insulation. With average insulation our calculator gives about 10 BTU/h per sq ft in Houston and 24 in Minneapolis. Rules of thumb of 30 to 60 BTU per sq ft come from older, leakier houses and usually oversize a furnace.
Is a bigger furnace better?
No. An oversized furnace heats in short bursts, which wears it out, makes rooms swing between too warm and too cool, and can be noisier. ACCA Manual S calls for a furnace whose output is 100% to 140% of the heating load. If you have central air, the blower size often matters more than the burner size.
Should I get an 80% or 95% AFUE furnace?
A 95% furnace burns about 16% less gas for the same heat and needs PVC venting through a wall instead of a chimney. In cold climates it usually pays back; in warm ones, where the furnace runs little, an 80% model can make sense if your venting suits it. Local codes may require 90%+ in new homes.
What is a winter design temperature?
The outdoor temperature your heating is sized for: only 1% of the hours in a year are colder. We compute it for your county from ten years of hourly NOAA weather data. On the few colder hours, a correctly sized furnace runs nonstop and the house may dip a degree or two.