BOILER SIZE
About this calculator
This boiler size calculator returns the heating output needed for a residential hydronic system, whether the distribution is radiators, baseboard, or radiant floor. The rule of thumb is the same as forced-air furnace sizing — 25-60 BTU/ft² depending on climate zone — but boilers are quoted in input BTU at the gas burner and the typical efficiency profile (AFUE) ranges from 82% non-condensing to 95% modulating-condensing. For radiant floor systems specifically, design heat loss is typically lower than radiator systems because the entire floor is the emitter, but the boiler sizing follows the same envelope load. Boiler sizing should be based on a measured load calculation per ACCA Manual J — rule-of-thumb sizing routinely over-sizes by 30–50%.
How to use this calculator
Enter the conditioned area in ft², pick your climate zone (BTU/ft² multiplier from 30 mild to 60 very cold), choose distribution (radiators/baseboard at full load, or radiant floor at 10% reduction), and pick AFUE — 82% for cast-iron atmospheric vent, 87% mid-efficiency, 90-95% for modulating condensing.
The calculator returns output BTU/hr (heat delivered to the home), input BTU/hr (nameplate rating), and the next standard boiler size to buy. Hydronic systems live longer than forced-air (30-50 years vs 15-25) but cost more to install. Sizing right is critical — oversized boilers short-cycle and never properly modulate.
Worked example
For a 2,000 ft² home in a cool climate (45 BTU/ft²) with radiator distribution, 95% AFUE modulating condensing boiler:
Output BTU: 2,000 × 45 = 90,000 BTU/hr. Input BTU: 90,000 ÷ 0.95 = 94,737 BTU/hr.
Next standard size: 100,000 BTU input. (Standard: 60K, 80K, 100K, 120K, 140K, 175K.)
At $4,000-$8,000 for the boiler + $3,000-$8,000 install: total $7,000-$16,000 — significantly more than equivalent forced air. Hydronic distribution (radiators, baseboard, PEX manifolds) adds another $5K-$15K.
For radiant floor in the same home: output 90,000 × 0.9 = 81,000 → input 85,263 → 100K boiler still. Radiant runs at lower water temps (90-120°F vs 160-180°F for radiators) which means a condensing boiler stays in condensing mode — better real-world efficiency than radiator systems on the same boiler.
Common mistakes & waste factors
Oversizing for "headroom." Cast-iron boilers especially don't modulate — they cycle on/off at full output. Oversized = short-cycling = wasted fuel and shorter life.
Using forced-air sizing logic on hydronic. Boilers are quoted in INPUT BTU; AC and furnace are quoted in OUTPUT BTU (cooling) or INPUT (gas). Always convert to output for comparing equipment to load.
Ignoring the radiant floor heat-loss benefit. Radiant floors emit at lower water temps because the whole floor radiates instead of a small radiator surface. Sizing radiant systems at full radiator load oversizes by 10-15%.
Buying 82% atmospheric vent in cold climates. The 95% modulating condensing upgrade typically pays back in 4-7 years on Northern winters and the equipment lasts longer because it operates at lower flue temps.
Rules of thumb
Climate factor: mild 30, mild-moderate 35, moderate 40, cool 45, cold 50, very cold 60 BTU/ft².
Standard boiler input sizes: 60K, 80K, 100K, 120K, 140K, 175K BTU/hr.
AFUE: 82% (cast-iron, atmospheric vent), 87% (mid-efficiency), 90-95% (modulating condensing).
Radiant floor: deduct 10% from radiator load.
Input BTU = Output BTU ÷ AFUE.
Hydronic life: 30-50 years for cast-iron, 20-30 years for modulating condensing (the heat exchanger is the limiting part).
Venting: 82% uses Type B-vent or chimney; 90%+ uses PVC sidewall vent.
Common questions
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