How Boiler Heat Works — Radiators, Radiant Floors, and Hydronics

A furnace heats air and pushes it through ducts; a boiler heats water and pumps it through a closed loop of piping to radiators, baseboard convectors, or in-floor tubing. The heat you feel is radiant and convective — steady, quiet, and even, with none of the dust movement or draft of blown air.

Because water carries heat far more densely than air, hydronic systems deliver a lot of comfort through small pipes — which is exactly why homes built before ductwork was standard (common in Beaverdale, Drake, Sherman Hill, and other older metro neighborhoods) were designed around them.

• Burner and heat exchanger — where fuel becomes hot water; modern condensing boilers reach high efficiency the same way condensing furnaces do.
• Circulator pump — moves water through the loop; a failed pump means cold radiators with a running boiler.
• Expansion tank — absorbs the pressure swing as water heats; a waterlogged tank causes pressure faults and relief-valve drips.
• Air in the loop — the classic cause of a cold radiator (and gurgling pipes); bleeding restores flow.
• Zone valves — homes zoned by floor or wing rely on these small motorized valves; one stuck valve means one cold zone.

An annual professional check: combustion analysis, pressure and expansion tank verification, safety control tests, and a look at venting and the condensate loop on high-efficiency models. Homeowners can bleed radiators in fall and watch system pressure on the gauge. The reward for this modest routine is longevity — cast-iron boilers routinely outlast two or three furnaces.

One honest note: boilers heat, but they don't cool. Boiler homes in the metro typically add cooling with a separate central system where ducts exist — or, more often, ductless mini-splits.