How do I calculate what size boiler I need?

Estimate your home's heat loss: multiply total square footage by an insulation factor (35 BTU/hr per sq ft for well-insulated, 50 for average, 65 for poor insulation) and by the temperature difference between your indoor target and outdoor design temperature. For North Dakota, the outdoor design temperature is -20 degrees Fahrenheit and the indoor target is typically 70 degrees F, giving a 90-degree difference. Multiply the result by 1.25 as a safety factor to get the recommended boiler output in BTU per hour. Then select the next standard boiler size above that number.

What is AFUE and why does it matter?

AFUE stands for Annual Fuel Utilization Efficiency. It measures what percentage of the fuel burned by a boiler is converted into usable heat for your home over a full heating season. An 80% AFUE boiler loses 20 cents of every heating dollar up the flue. A 95% AFUE condensing boiler loses only 5 cents. The difference adds up significantly in cold climates like North Dakota where the heating season runs 6 to 7 months per year. Federal minimum for non-weatherized boilers is 82% AFUE. High-efficiency condensing boilers run 90% to 98% AFUE.

What is the outdoor design temperature for North Dakota?

The ASHRAE 99% design dry-bulb temperature for most North Dakota locations is -20 to -25 degrees Fahrenheit. This is the temperature that will be equaled or exceeded 99% of winter hours. Bismarck is -23 F, Fargo is -24 F, Minot is -25 F. A boiler sized to the 99% design temperature will handle all but the most extreme cold snaps. Some contractors use -20 F as a round number for statewide ND estimates, which is what this calculator defaults to.

How much does it cost to run a boiler per month?

Monthly boiler operating cost depends on your heat loss, AFUE, fuel type, and fuel price. For a 2,000 sq ft North Dakota home with average insulation, heat loss is approximately 270,000 BTU/hr at design temperature. Over a full heating season averaging 35 degrees F outdoor temperature (vs. -20 F design), average load is roughly 30 to 40% of peak. With natural gas at $1.20 per therm and a 90% AFUE boiler, monthly heating cost in a ND winter typically runs $150 to $300 per month depending on how cold the month is. A 95% AFUE unit saves roughly 5 to 8% versus 90%.

Should I replace an 80% AFUE boiler with a 95% unit?

In a cold climate like North Dakota, the answer is almost always yes when the 80% unit is near end of life. The fuel savings of 15 percentage points (from 80% to 95%) on a 6-month heating season are substantial. Payback on the efficiency premium typically runs 4 to 8 years in a Zone 6 climate. The bigger issue: 80% AFUE boilers use a Category I venting system (B-vent), while 95% condensing boilers use Category IV (PVC flue through the wall). Switching requires replumbing the venting, which adds $500 to $1,500 in labor and materials but is almost always worth it in ND.

What insulation factor should I use for my house?

Use 35 BTU/hr per sq ft for well-insulated homes: R-19 or better walls, R-38 or better attic, modern double or triple-pane windows, and tight air sealing. Use 50 BTU/hr per sq ft for average insulation: R-13 walls, R-25 to R-30 attic, some air leakage. Use 65 BTU/hr per sq ft for poorly insulated homes: minimal or missing wall insulation, single-pane windows, drafty, pre-1970 construction that has not been upgraded. When in doubt, use 50 for a first estimate and refine after a blower door test.

Boiler Sizing Calculator 2026: Heat Loss, BTU Output, AFUE Fuel Cost

AFUE	Efficiency Tier	Annual Fuel Cost	vs. 80% AFUE
80%	Standard (minimum)	--	Baseline
90%	High efficiency	--	--
95%	Condensing Best	--	--

Shopping for a high-efficiency boiler in North Dakota?

Condensing boilers (90%+ AFUE) require PVC venting through the wall, not a B-vent chimney. Factor in the venting cost when comparing bids. Major manufacturers: Weil-McLain, Burnham, Navien, Rinnai. Lead times in ND can run 4 to 8 weeks in peak season. Order before demo starts.

The Formula This Calculator Uses

The heat loss estimate uses the simplified degree-day method, which is standard for residential budgeting and equipment selection. It is not a full ASHRAE load calculation, but it is accurate enough for boiler sizing decisions when the inputs are honest.

Heat loss (BTU/hr): square footage x insulation factor x temperature difference
Temperature difference: indoor target (70 F default) minus outdoor design temperature (-20 F for ND = 90 degree difference)
Recommended boiler output: heat loss x 1.25 safety factor
AFUE fuel cost: estimated using annual heating degree days, heat loss at average conditions, fuel type BTU content, and AFUE efficiency

Insulation Factors Explained

Factor	Description	Typical Home
35 BTU/hr/sqft	Well insulated	Post-2000 construction, R-19+ walls, R-38+ attic, tight air sealing
50 BTU/hr/sqft	Average	1970s-1990s construction, R-13 walls, some upgrades done
65 BTU/hr/sqft	Poor	Pre-1970, minimal insulation, single-pane windows, drafty

How the AFUE Fuel Cost Table Works

AFUE (Annual Fuel Utilization Efficiency) is the percentage of fuel energy that actually heats your home. An 80% AFUE boiler wastes 20% up the flue. A 95% condensing boiler wastes only 5%.

The annual cost calculation uses your heating degree days (HDD) to estimate total seasonal heat demand, then converts that to fuel units (therms for gas, gallons for propane or oil) and divides by AFUE to get actual fuel consumed. The result is your estimated annual fuel bill for heating only.

Natural gas BTU content: 100,000 BTU per therm. Propane: 91,500 BTU per gallon. No. 2 fuel oil: 138,500 BTU per gallon.

What heating contractors say about boiler sizing in cold climates

"The 1.25 safety factor is the right call for ND. You want the boiler to have headroom on the coldest days without over-cycling the rest of the season." -- r/HVAC

"Going from 80% to 95% AFUE in Bismarck typically saves $200 to $400 per year on gas. With the price premium on a condensing unit, payback is 5 to 7 years." -- r/HomeImprovement

"Old-timers always oversized by 50%. Modern equipment is so efficient that the right-sized unit is almost always smaller than the old one." -- r/Plumbing

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Not engineering advice. This calculator provides a simplified heat loss estimate for budgeting and preliminary equipment selection. It does not account for infiltration, duct losses, solar gain, occupant count, or detailed building envelope analysis. For permitted installations and accurate equipment selection, hire a licensed mechanical engineer or HVAC contractor. FigureNerd is not responsible for equipment selection, installation, or fuel cost outcomes.

Sources: ASHRAE Handbook of Fundamentals, Chapter 18 (heating and cooling load calculations) | ENERGY STAR AFUE definitions and minimum standards | ACCA Manual J 8th Edition (residential load calculation standard) | ASHRAE 99% design temperatures for ND cities (Bismarck -23 F, Fargo -24 F, Minot -25 F).

Boiler Sizing Calculator

Enter Your Home Details

Annual Fuel Cost at Three AFUE Efficiency Tiers

The Formula This Calculator Uses

Insulation Factors Explained

How the AFUE Fuel Cost Table Works

What heating contractors say about boiler sizing in cold climates

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