What is a radiator?
Updated August 2026
A radiator is the metal unit in a room that gives up a boiler's heat to the air, fed by hot water or steam through pipes in the floor or wall. Cast-iron column radiators, flat steel panel radiators, and finned baseboard convectors all fall under the term. Despite the name, most of the heat leaves by convection: air warms against the metal, rises, and pulls cooler room air in underneath.
Where it is
They sit under windows on exterior walls, where rising warm air cancels the cold draft coming off the glass. Count the pipes to identify yours. One pipe entering the bottom plus a silver air vent on the side means steam. Two pipes, one in and one out, with a small square or slotted bleeder at the top corner, means hot water. Both types carry a hand valve at the inlet, and on a steam system that valve belongs all the way open or all the way shut, never halfway.
What it does
A cast-iron radiator holds several gallons of water and keeps giving off heat for 20 or 30 minutes after the boiler stops, which is why these systems feel steady rather than blasty. Trouble reads as a temperature map. Hot at the bottom and cold at the top on a hot-water unit means trapped air. Banging on a steam system means condensate cannot drain back, often because the radiator is pitched wrong by a quarter inch. A radiator that stays stone cold while its neighbors heat has a shut valve, a dead air vent, or sludge in the bottom sections. Why a radiator is not heating up sorts the rest.
Maintenance
Bleed hot-water radiators once at the start of the heating season, working from the lowest floor upward, and again any time one gurgles; timing and technique are in how often to bleed radiators. Steam radiators are never bled, because the air vent does that job automatically, and a vent that hisses without ever closing is finished. Vacuum the fins and the wall cavity behind each unit yearly, since a dust blanket on cast iron measurably cuts output. The rest of the season prep sits on the radiator maintenance checklist.
What it costs to replace
A replacement radiator runs A steam air vent or hot-water bleeder is $15–60; a full cast-iron radiator swap runs $500–1,200 installed.
OnOtto's photo scan identifies parts like this in your home and builds the maintenance schedule for you. See how the scan works.
Frequently asked questions
How long does a cast-iron radiator last?
The iron itself routinely goes 50 to 100 years, and plenty of 1920s radiators are still in daily service. What wears out is everything bolted to it: air vents every 5 to 10 years, packing at the valve stem, and the union nut that weeps a rusty ring onto the floor. Those parts run $15 to $60 each. Judge a salvaged radiator by whether any section has ever frozen and cracked, not by how old or how ugly it looks.
Can I paint a radiator without hurting its output?
Yes, with one exception. Ordinary latex or oil paint in any color shifts heat output by a percent or two, which nobody notices. Metallic finishes are the problem: aluminum and bronze paints lower surface emissivity and cut radiant output measurably. Scrape loose flakes, prime bare iron with a rust-inhibiting primer, and lay on thin coats while the system is cold and shut off. Skip solid wood enclosures with small openings, which trap far more heat than any paint.
Why does my radiator gurgle or knock?
Gurgling on a hot-water system is air traveling with the water, and a one minute bleed clears it. Knocking on a steam system is water sitting where steam wants to move. The cure usually costs nothing: shim the legs at the far end so the radiator tilts back toward the supply valve by about a quarter inch, letting condensate drain instead of pooling. If the banging is out in the pipes rather than the radiator, the mains are pitched wrong and that needs a steam specialist.