How to Size a Boiler for Your Home

The formula, in one paragraph

Start with the floor area, the climate zone, the ceiling height and the insulation. Multiply them into the heat load, divide by the boiler efficiency to get the input, round up to the next standard size, and multiply the input by $55 per 1,000 BTU for the installed cost.

Step 1: Work out the heat load

The heat load is the heat your home loses on the coldest day. Multiply the floor area by the BTU per square foot for your climate zone: Zone 1 is warm at 30, Zone 2 moderate at 40, Zone 3 cold at 50, Zone 4 very cold at 60. Adjust for ceilings and insulation: ceiling height divided by 8, times the insulation factor (1.15 poor, 1.00 average, 0.90 good). At the example numbers that is 2,000 times 50, which is 100,000 BTU/h.

Step 2: Divide by the AFUE

AFUE is the share of the fuel’s heat that reaches your home, so the boiler must burn more than it delivers. Divide the load by the AFUE as a decimal: 100,000 divided by 0.85 is 117,647.06 BTU/h of input. A 95% AFUE boiler needs only 105,263.16 BTU/h of input for the same load, which is 12,383.90 BTU/h less — and that smaller input is what makes the high-efficiency boiler cheaper to buy, before a dollar of fuel savings.

Step 3: Round up to a standard size

Boilers come in seven standard sizes from 50,000 to 200,000 BTU. Pick the smallest size that is at least the input. At the example numbers the input is 117,647.06 BTU/h, so the size is 125,000 BTU — the 100,000 BTU size would fall short by 17,647.06 BTU/h. Always round up, never down: a boiler that is too small cannot keep the home warm on the coldest day, while a boiler that is one size too big only costs a little more to buy.

Step 4: Work out the installed cost

The installed cost is $55 per 1,000 BTU of input: 117,647.06 times 55 divided by 1,000 is $6,470.59. A 95% AFUE boiler needs only 105,263.16 BTU/h of input, so it costs $5,789.47 installed — $681.12 less. The price follows the input, not the size on the box: the 125,000 BTU boiler costs $6,470.59 because its input is 117,647.06 BTU/h, not because 125,000 is printed on it.

That box is the whole formula.

What changes the size most

The zone moves the size fastest. The same home needs a 75,000 BTU boiler in Zone 1 at $3,882.35, and a 150,000 BTU boiler in Zone 4 at $7,764.71. A 1,500 sq ft home in Zone 3 needs a 100,000 BTU boiler at $4,852.94. Nine-foot ceilings push the default home to a 150,000 BTU boiler at $7,279.41. Poor insulation pushes it to a 150,000 BTU boiler at $7,441.18, while good insulation drops it to a 125,000 BTU boiler at $5,823.53 — a $1,617.65 difference on the boiler alone.

Boiler vs furnace: the same math, different numbers

A boiler and a furnace use the same four steps, and the same heat load: a 2,000 sq ft home in Zone 3 needs 100,000 BTU/h of heat either way. The numbers part company at the efficiency and the price. A boiler runs at 85% AFUE by default, so it needs 117,647.06 BTU/h of input, while a furnace runs at 80% AFUE and needs 125,000 BTU/h of input for the same heat. Boilers also cost more per unit of input — $55 per 1,000 BTU against $45 for a furnace — so the same home costs $6,470.59 for a boiler and $5,625.00 for a furnace. The standard sizes differ too: boilers run 50,000 to 200,000 BTU in seven sizes, and the default home lands on a 125,000 BTU boiler against a 140,000 BTU furnace.

Work backwards from a boiler you already have

When the boiler is fixed, run the formula in reverse. A 100,000 BTU input boiler heats at most 1,700 sq ft in Zone 3 at 85% AFUE, and a 150,000 BTU boiler at 95% AFUE heats 2,850 sq ft. On a budget: $5,000 heats 1,545.45 sq ft and $7,500 heats 2,318.18 sq ft. Above 200,000 BTU of input, consult an HVAC pro: a 5,000 sq ft home in Zone 4 needs 352,941.18 BTU/h of input, which no single residential boiler covers.

Frequently asked questions