The short answer
An 1,800 sq ft one-story house with average insulation loses about 36,900 BTU/h on a design night in Minneapolis (-6°F), so it needs a 50,000 BTU input condensing boiler or a 75,000 BTU standard one. In Houston (37°F) the heat loss is only 16,000 BTU/h, and the smallest common boiler, 50,000 BTU input, is more than enough.
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 | 22,800 | 50,000 |
|---|---|---|---|---|
| Alaska Anchorage | 7 | -4 | 36,000 | 50,000 |
| Arizona Phoenix | 2B | 43 | 13,100 | 50,000 |
| Arkansas Little Rock | 3A | 25 | 21,900 | 50,000 |
| California Los Angeles | 3B | 47 | 11,200 | 50,000 |
| Colorado Denver | 5B | 7 | 30,600 | 50,000 |
| Connecticut Bridgeport | 5A | 16 | 26,200 | 50,000 |
| Delaware Wilmington | 4A | 18 | 25,300 | 50,000 |
| District of Columbia Washington | 4A | 23 | 22,800 | 50,000 |
| Florida Jacksonville | 2A | 42 | 13,600 | 50,000 |
| Georgia Atlanta | 3A | 27 | 20,900 | 50,000 |
| Hawaii Honolulu | 1A | 66 | 1,900 | 50,000 |
| Idaho Boise | 5B | 16 | 26,200 | 50,000 |
| Illinois Chicago | 5A | 6 | 31,100 | 50,000 |
| Indiana Indianapolis | 4A | 9 | 29,600 | 50,000 |
| Iowa Des Moines | 5A | 0 | 34,000 | 50,000 |
| Kansas Wichita | 4A | 13 | 27,700 | 50,000 |
| Kentucky Louisville | 4A | 16 | 26,200 | 50,000 |
| Louisiana New Orleans | 2A | 40 | 14,600 | 50,000 |
| Maine Portland | 6A | 7 | 30,600 | 50,000 |
| Maryland Baltimore | 4A | 19 | 24,800 | 50,000 |
| Massachusetts Boston | 5A | 14 | 27,200 | 50,000 |
| Michigan Detroit | 5A | 9 | 29,600 | 50,000 |
| Minnesota Minneapolis | 6A | -6 | 36,900 | 50,000 |
| Mississippi Jackson | 3A | 29 | 19,900 | 50,000 |
| Missouri Kansas City | 4A | 9 | 29,600 | 50,000 |
| Montana Billings | 6B | -4 | 36,000 | 50,000 |
| Nebraska Omaha | 5A | 4 | 32,100 | 50,000 |
| Nevada Las Vegas | 3B | 37 | 16,000 | 50,000 |
| New Hampshire Manchester | 5A | 9 | 29,600 | 50,000 |
| New Jersey Newark | 4A | 17 | 25,800 | 50,000 |
| New Mexico Albuquerque | 4B | 23 | 22,800 | 50,000 |
| New York New York | 4A | 18 | 25,300 | 50,000 |
| North Carolina Charlotte | 3A | 27 | 20,900 | 50,000 |
| North Dakota Fargo | 6A | -13 | 40,300 | 75,000 |
| Ohio Columbus | 4A | 11 | 28,700 | 50,000 |
| Oklahoma Oklahoma City | 3A | 19 | 24,800 | 50,000 |
| Oregon Portland | 4C | 30 | 19,400 | 50,000 |
| Pennsylvania Philadelphia | 4A | 18 | 25,300 | 50,000 |
| Rhode Island Providence | 5A | 11 | 28,700 | 50,000 |
| South Carolina Charleston | 3A | 32 | 18,500 | 50,000 |
| South Dakota Sioux Falls | 6A | -4 | 36,000 | 50,000 |
| Tennessee Nashville | 3A | 21 | 23,800 | 50,000 |
| Texas Houston | 2A | 37 | 16,000 | 50,000 |
| Utah Salt Lake City | 5B | 20 | 24,300 | 50,000 |
| Vermont Burlington | 6A | 0 | 34,000 | 50,000 |
| Virginia Virginia Beach | 3A | 27 | 20,900 | 50,000 |
| Washington Seattle | 4C | 30 | 19,400 | 50,000 |
| West Virginia Charleston | 4A | 16 | 26,200 | 50,000 |
| Wisconsin Milwaukee | 5A | 4 | 32,100 | 50,000 |
| Wyoming Cheyenne | 5B | 3 | 32,600 | 50,000 |
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 boiler whose output covers it. Unlike a furnace, a hot-water boiler has no duct losses: its pipes and radiators are inside the heated space.
walls, ceiling, windows = U × area × ΔT
floor edge = F × perimeter × ΔT
air leaks = 1.08 × CFM × ΔT
net rating = input × heating-capacity ratio ÷ 1.15
boiler = smallest of 50k, 75k, 100k, 125k, 150k, 175k, 200k with net rating ≥ heat loss
Assumptions
- Indoors is kept at 70°F; no credit for sun, people or appliances on the design night.
- 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.
- Ratings: a condensing boiler's heating capacity is about 93% of its input (Weil-McLain ECO-Tec 150: 139,000 of 150,000 BTU/h, 95% AFUE); for a standard boiler at the DOE minimum of 84% AFUE we take capacity as 84% of input. The net rating is capacity ÷ 1.15, the industry allowance for piping and pickup. Check the actual net rating on the boiler you buy. A boiler that also heats your hot water (an indirect tank) usually needs no extra size.
- Steam boilers are sized differently, to the radiators' rating (EDR), not to the heat loss.
Worked example: 1,800 sq ft in Minneapolis
One story, 8 ft ceilings, average insulation. Winter design -6°F, so ΔT = 76°F.
- Walls 6,200 + ceiling 4,500 + windows 10,300 + floor edge 7,100 + air 8,900 = 36,900 BTU/h
- Condensing: 50,000 × 0.927 ÷ 1.15 = 40,300 BTU/h net covers it, so a 50,000 BTU boiler
- Standard: 75,000 × 0.84 ÷ 1.15 = 54,800 BTU/h net, so a 75,000 BTU boiler
Too big or too small
- Too big: short cycling. The boiler fires, heats the water fast and shuts off, again and again, which wastes fuel and wears the burner and controls.
- Too big: condensing boilers lose efficiency if they can't turn down far enough for mild days. A modulating boiler with a wide turndown range handles both cold nights and mild days.
- Too small: it can't keep up on the coldest nights. A few hours a year is normal at the design temperature.
- Check the radiators. The boiler can only deliver what the radiators can give off at the water temperature you run.
Questions people ask
What size boiler do I need?
One whose output covers your house's heat loss on a design-temperature night. An 1,800 sq ft house with average insulation in Minneapolis (winter design -6°F) loses about 36,900 BTU/h, so a 50,000 BTU input condensing boiler (net rating about 40,300 BTU/h) or a 75,000 BTU standard one. A 3,000 sq ft two-story house there needs about 53,700 BTU/h.
How many BTU per square foot for a boiler?
It depends on climate and insulation, not a fixed rule: about 9 BTU/h per sq ft for the example house in Houston and 21 in Minneapolis. Rules of thumb of 30 to 60 BTU per sq ft come from older, leakier houses and tend to oversize.
Should I size a new boiler to match the old one?
Not without checking. Many old boilers were sized by rule of thumb or for a house before insulation and new windows, and are far bigger than the heat loss. An oversized boiler short-cycles, wastes fuel and wears out sooner. Have a contractor do a heat-loss calculation, and check that the radiators can deliver the heat at the water temperature a condensing boiler needs.
What is the difference between input, heating capacity and net rating?
Input is the fuel the boiler burns per hour. Heating capacity (the DOE rating) is the heat it makes: Weil-McLain's ECO-Tec 150 burns 150,000 BTU/h and makes 139,000. The net (AHRI) rating takes off about 15% for piping and warming up a cold system, 121,000 for that boiler; size on the net rating. AFUE is a seasonal efficiency, not the boiler's output.
Is a condensing boiler worth it?
A condensing boiler at about 95% AFUE burns roughly 12% less fuel than a standard 84% one for the same heat, but it only condenses, and reaches that efficiency, with cooler return water, which suits radiant floors and large radiators. With small baseboard sized for very hot water, the gain is smaller.