Boiler sizing calculator

What size boiler do I need by square footage?

A correctly sized boiler matches the heating load at your local winter design temperature, not the square footage alone. For a typical home with average insulation, expect 25 to 35 BTU of output per square foot in a mixed climate and 45 to 60 BTU per sq ft in a cold climate. Quick reference for Zone 5 (Boston, Chicago) with average insulation, sized to output BTU at the 99% design temperature:

• 1,000 sq ft: 50,000 to 70,000 BTU input, 45,000 to 65,000 BTU output
• 1,500 sq ft: 70,000 to 90,000 BTU input
• 2,000 sq ft: 90,000 to 110,000 BTU input
• 2,500 sq ft: 110,000 to 140,000 BTU input
• 3,000 sq ft: 140,000 to 175,000 BTU input

Boilers are sold by input BTU rating, which is what the data plate shows and what a contractor quotes. Output equals input times AFUE. A 100,000 BTU input boiler at 95% AFUE delivers 95,000 BTU output. The same 100,000 BTU input boiler at 82% AFUE only delivers 82,000 BTU output. Same shopping size, very different real heating capacity.

Combi boiler sizing: heating load vs domestic hot water

A combination (combi) boiler handles both space heating and domestic hot water from one unit, which means it has to be sized to the larger of the two loads. Heating load is what comes out of the calculator above. DHW load is roughly 40,000 to 50,000 BTU per bathroom for simultaneous shower + sink demand. For a typical 2-bathroom home in a mild climate, the DHW load often drives the size up.

• 1 bathroom, 1,500 sqft mild climate: 45,000 BTU DHW > 30,000 BTU heating, size to DHW
• 2 bathrooms, 2,000 sqft mild climate: 90,000 BTU DHW > 60,000 BTU heating, size to DHW
• 2 bathrooms, 2,500 sqft cold climate: 90,000 BTU DHW vs 115,000 BTU heating, size to heating
• 3 bathrooms, 3,000 sqft cold climate: 135,000 BTU DHW vs 165,000 BTU heating, size to heating

The largest combi boilers top out around 199,000 BTU input. Above that load, a separate heating-only boiler plus a dedicated indirect water heater is usually the better install. The calculator above flags this automatically by sizing to the larger load.

How AFUE changes the boiler size you actually need

AFUE (Annual Fuel Utilization Efficiency) is the percentage of fuel energy your boiler converts to usable heat. The rest leaves through the flue or jacket losses. Higher AFUE means more heat per dollar of gas, but also a smaller required input rating for the same heating load. If your home needs 80,000 BTU/hr output to stay warm on a design day, the input you shop for varies by AFUE tier:

• 82% AFUE atmospheric: 80,000 ÷ 0.82 = 97,560 BTU input (round up to 110,000 standard)
• 87% AFUE mid-efficiency: 80,000 ÷ 0.87 = 91,950 BTU input (round up to 110,000)
• 92% AFUE high-efficiency: 80,000 ÷ 0.92 = 86,960 BTU input (round up to 90,000)
• 95% AFUE condensing modulating: 80,000 ÷ 0.95 = 84,210 BTU input (round up to 90,000)

The high-efficiency and condensing tiers often drop you one boiler size, which closes part of the upfront cost gap. The fuel savings from 82% to 95% AFUE run about 15 percent, so a home burning 900 therms a year saves around 135 therms ($180 at $1.35 per therm). Across a 20-year boiler life, that's $3,600 in fuel savings, which more than pays for the AFUE upgrade premium in most climates.

Natural gas vs oil vs propane boiler costs and fuel use

The output BTU you need stays the same regardless of fuel. What changes is the cost per BTU delivered and the annual fuel quantity. For a 60,000 BTU/hr output heating load through a typical Zone 5 winter (about 1.35 billion BTU annual heating load on 2,000 sqft):

• Natural gas (92% AFUE, $1.35/therm): 980 therms/year, $1,323 annual fuel cost
• Propane (92% AFUE, $2.80/gallon): 1,070 gallons/year, $2,996 annual cost
• Heating oil (87% AFUE, $4.10/gallon): 750 gallons/year, $3,075 annual cost
• Electric resistance (98% efficient, $0.18/kWh): 28,800 kWh/year, $5,184 annual cost

Natural gas is the cheapest fuel by a wide margin in most US markets. Oil and propane sit in a similar mid-cost range and are common in rural Northeast and Midwest installs where gas service does not reach. Electric resistance boilers are uncommon outside specific situations (no fossil fuel access, very mild climate). A cold-climate heat pump usually beats an electric boiler on operating cost by 60 to 70 percent in the same climate.

Boiler installation cost: gas, oil, and combi by AFUE tier

What you should expect to pay an HVAC or hydronic contractor for a boiler install, fully done including basic near-boiler piping, expansion tank, circulator, and permit. Based on current installed-quote data for mid-tier brands like Weil-McLain, Burnham, Buderus, Navien, and Bosch.

• 82% AFUE atmospheric gas boiler (heating only): $3,500 to $6,500 installed
• 87% AFUE mid-efficiency gas boiler: $4,500 to $7,500 installed
• 92% AFUE high-efficiency gas boiler: $5,500 to $8,500 installed
• 95% AFUE condensing modulating gas boiler: $6,500 to $10,500 installed
• Gas combi boiler (any AFUE tier): add $1,500 to $2,500 over heating-only price
• Oil boiler: $5,500 to $11,000 installed
• Electric boiler: $2,200 to $6,000 installed (cheapest install, highest run cost)

A full hydronic retrofit (new boiler + replacing old radiators with PEX in-floor) runs an additional $6,000 to $15,000 above the boiler-only number depending on home size and accessibility. If you are keeping the existing radiators and just swapping the boiler, the install cost stays in the ranges above.

Why oversizing a boiler ruins efficiency and shortens life

An oversized boiler cycles on and off too frequently to reach steady-state combustion. The burner fires, the boiler heats its water jacket to setpoint in 4 to 6 minutes, the burner cuts out, the circulator pump runs to deliver heat to the radiators, the jacket cools, and the cycle repeats. This short-cycling produces three problems: actual AFUE drops 5 to 15 points below the nameplate rating because the boiler never reaches steady-state efficiency, the ignition components see 3 to 5 times more starts per year and fail prematurely, and indoor comfort suffers from temperature swings in lightly-loaded zones.

A correctly sized boiler runs in long cycles of 15 to 30 minutes on a typical winter day. That matches the way hydronic radiators and PEX loops emit heat: slowly, evenly, over time. ACCA Manual S recommends sizing to the design-day load plus 10 percent for safety margin, which is the calculator's default above. Anything bigger than that wastes fuel and equipment life.

Combi boiler vs heating-only boiler with indirect tank

The combi-vs-conventional debate is the most common boiler buying decision. Combi units are cheaper to install and take up less space because there's no separate water heater. The trade-offs: simultaneous demand limits (running two showers + dishwasher pushes a combi beyond its flow capacity), shorter equipment life on the heat exchanger (10 to 15 years vs 15 to 25 for a heating-only boiler), and a small flow delay before hot water reaches the tap.

The heating-only-plus-indirect setup runs $1,500 to $2,500 more upfront but delivers unlimited simultaneous DHW capacity (the indirect tank holds 30 to 80 gallons of pre-heated water), and the boiler heat exchanger lives longer because it only sees space-heating cycles. For households of 4 or more with multiple bathrooms, the separate-tank approach usually wins on total cost of ownership. For 1- to 2-person households in tight basements, the combi wins.

Weil-McLain, Burnham, Buderus, Navien: which boiler brand to buy

All major residential boiler brands meet the same federal AFUE minimums and use similar cast-iron, stainless steel, or aluminum heat exchanger designs at the same price tier. The differences come down to warranty, regional dealer network, and combi feature set. Weil-McLain and Burnham are the legacy US brands with the longest cast-iron heritage and the deepest Northeast contractor network. Buderus (Bosch) and Viessmann lead on European-style modulating condensing technology. Navien dominates the combi market with the NCB and NFC series. Bosch Greenstar competes in the high-efficiency tier. Rinnai entered with the i-Series combi recently.

For an 82% or 87% AFUE atmospheric boiler, brand barely matters because the technology is mature and standardized. For a 95% AFUE modulating condensing combi, Buderus, Viessmann, and Navien are worth the premium for the modulation range and the longer heat exchanger warranty. The right install matters more than the brand. Bid the job out to two or three licensed hydronic contractors and pick the one who runs a Manual J in writing. Before you talk to them, run our heat loss calculator so you have an independent number to sanity-check whatever boiler size they propose.

When to consider a heat pump or hybrid system instead

Before signing a quote on a new gas boiler in a mixed or cool climate, run the heat pump vs gas furnace calculator with your local utility rates. In most of Zones 3 and 4 with electricity under $0.20 per kWh and gas above $1.20 per therm, a cold-climate air-to-water heat pump (Sanco2, Arctic Heat Pump, Chiltrix) feeding the existing hydronic loops beats a new gas boiler on 15-year total cost. The conversion is also rebate-eligible for heat-pump-specific HEAR and state programs that gas boilers don't qualify for. In Zones 5 to 7 with cheap gas, the gas boiler still wins, but the math is much closer than it was 5 years ago. A dual-fuel hybrid running the heat pump above 30°F outdoor and the boiler below that often beats either system alone.