Hydronic heating — boilers feeding radiators, baseboard, or radiant floor — concentrates in cold US regions where forced air can't keep older masonry buildings comfortable. Sizing is the same climate-zone rule used for furnaces, with two adjustments: boiler AFUE ranges differently (82% cast-iron up to 95% modulating-condensing), and radiant floor systems can be sized 10-15% smaller than radiator systems on the same envelope. Here's the math and the system-type adjustments.

The boiler sizing formula

Same envelope load math as any heating system, with system and efficiency adjustments at the boiler:

output_BTU = ft² × climate_factor × system_adj input_BTU = output_BTU ÷ AFUE

Example: 2,200 ft² Boston house with cast-iron radiators, 87% AFUE mid-efficiency boiler. Output = 2,200 × 45 = 99,000 BTU/hr delivered. Input = 99,000 ÷ 0.87 = 113,793 BTU/hr nameplate. Round up to a 120,000 BTU/hr input boiler.

Same house with radiant floor instead of radiators: output drops to 99,000 × 0.9 = 89,100 BTU/hr. Round to 100,000 input. The boiler size calculator handles both.

Boiler AFUE — three classes

Boiler efficiency depends on whether it condenses the flue gas and how it modulates output:

  • 82% AFUE (cast-iron, atmospheric vent): The traditional residential boiler. Vents through a chimney, no power vent fan, no condensate drain. Reliable for 25-30 years, parts cheap, easy service. Loses ~18% of fuel up the flue.
  • 87% AFUE (mid-efficiency): Power-vented through a sidewall, no chimney needed. Standing pilot replaced by electronic ignition. Same general design as 82% but tighter combustion control.
  • 90-95% AFUE (modulating condensing): Stainless heat exchanger, condenses flue moisture, modulates from 20-100% output. Ten years of life expectancy lower than cast-iron but 12-15% fuel savings and zone-perfect comfort.

Cast-iron boilers are still bought new in 2026 because they outlast condensing units 2:1 in hard-water regions where the condensing heat exchanger fouls and fails at 12-18 years. Condensing boilers are the right answer in soft-water regions and in tight modern envelopes where their modulation range matches the actual heat load most of the year.

Radiator vs radiant — why radiant gets the discount

Radiator systems supply 160-200°F water to a small surface area (the radiator face). The water has to be hot to deliver enough BTU per square foot of radiator. The boiler runs at full output to maintain that supply temperature.

Radiant floor systems supply 90-120°F water to the entire floor area. Lower water temp means less heat loss in the supply lines and a much larger emitter surface. The boiler can run lower and modulate down. ASHRAE design data gives radiant systems about 10-15% lower design heat loss than radiator systems for the same envelope.

Modulating-condensing boilers benefit most from radiant — they run in their condensing range (return temp under 130°F) almost year-round and hit nameplate AFUE. Cast-iron boilers on radiant need a mixing valve to prevent condensation in the heat exchanger, which complicates the install.

Standard residential boiler sizes

Boilers ship in slightly different increments than furnaces, typically every 20,000-30,000 BTU input:

  • 60,000 BTU input — small house under 1,200 ft² in cold climate, or zone-only boiler
  • 80,000 BTU input — 1,500-2,000 ft²
  • 100,000 BTU input — 2,000-2,500 ft² standard
  • 120,000 BTU input — 2,500-3,200 ft² or older drafty house
  • 140,000 BTU input — large or very-cold-climate houses
  • 175,000+ BTU — multi-family or commercial residential

For houses over 3,500 ft², two smaller modulating boilers in parallel (a “mod-con array”) is more efficient than one large boiler — they sequence based on load and one can be down for service without losing heat.

What pros do differently

Pull the boiler's manual and read the venting table. Condensing boilers vent through PVC sidewall pipes; cast-iron boilers vent through metal Class B chimney. Mixing them voids the manufacturer warranty. The vent type determines where the boiler can physically go in the basement.

Size the expansion tank to the boiler output. Hydronic systems need an expansion tank rated for the system water volume and operating pressure. Use the expansion tank sizing calculator after the boiler size is set.

Run a heat-loss calc for tight or unusual envelopes. The climate-zone rule overshoots tight modern construction by 20-40%. Use the heat loss calculator to size by actual envelope U-values for any new construction or deep retrofit.

Quick FAQ

Cast-iron or condensing — which lasts longer? Cast-iron boilers commonly hit 30 years. Condensing units average 15-20. Condensing units save more in fuel cost over that life, but the upfront cost is 50-80% higher.

Can I run radiators and radiant floor on one boiler? Yes — design the system with primary-secondary piping so each loop runs at its own design temperature. Most modulating- condensing boilers handle this natively.

What size boiler for 2,500 ft² in Buffalo? 45 BTU/ft² × 2,500 = 112,500 BTU output. At 95% AFUE, ~118,000 BTU input. Round to 120,000 BTU input boiler.

Forced-air system instead? Run the furnace size calculator — same climate-zone math but with furnace-specific AFUE and standard furnace size increments.