Why air ends up at the top
A Department of Energy report on hot-water heating systems explains that heating water drives dissolved air out as tiny bubbles. That air keeps hot water from touching the radiator's metal, which cuts heat output, and it also causes noise and can jam valves at the circulating pump. Systems remove air two ways: an air eliminator that uses air's buoyancy to catch and release bubbles, and bleed valves at the radiators that technicians use to purge it.
First, make sure it's hot water and not steam
This advice is for hot-water systems. Steam systems also use radiators, and the two can look alike. The Building America Solution Center's guide to steam radiator valves says one-pipe steam is common in older buildings, especially those built before 1970, and that the air vents on its radiators have to work properly for steam to reach every room. On a steam radiator, that vent is part of normal operation, not something to bleed. A sight glass on the boiler showing a water level is a steam part, according to the DOE field guide cited below. If you aren't sure which system you have, ask a heating contractor to identify it before anyone opens a vent. If your house has forced air instead, our guide to a furnace that turns on and off every few minutes covers a common complaint there.
One radiator or several?
Walk the house while the thermostat is calling for heat and write down which radiators are cold at the top, which are cold all over, and which heat normally. Don't open any fittings to check. Also look for drips or staining at radiator valves and pipe connections, since repairing water leaks is the first step in the DOE's hot-water service list.
If only one radiator is cold at the top and the rest are fine, trapped air in that radiator is the likely cause. If several are cold at the top, or the problem returns within a few weeks of bleeding, ask for system checks. The DOE weatherization field guide lists them for hot-water systems: bleed air from radiators and piping, confirm the fill valve is feeding water in, verify that the pumps, low-water cutoff, automatic feed control and high-pressure controls work, and check the expansion tank for the right air pressure.
A radiator that's cold all over is a different problem. Hot water isn't reaching it at all, so ask the technician to check its valve, any zone valve that controls that area, and the circulating pump rather than just bleeding it.
Bleeding: how often and who does it
The DOE's Energy Saver guide recommends removing trapped air from hot-water radiators once or twice a heating season and calling a professional if you're unsure how. If nobody has shown you on your system, have the technician do it the first time and walk you through it: which vent to use, whether the boiler should be off and cooled first, and how the system refills. The field guide notes most systems have an automatic fill valve, and a manual fill valve has to be open to push water in and air out.
What to tell and ask the heating technician
Look for a heating contractor who regularly services hot-water boilers and radiators, and use our contractor hiring checklist before you book. Tell them which radiators are affected, the pattern you saw, and whether bleeding has helped before.
Then ask: Is this one radiator or the whole system? Did you check the expansion tank pressure, the fill valve and the air eliminator? What reading or observation supports your diagnosis? If they recommend replacing a part, ask what failed and how they'll confirm the radiator heats fully afterward. The DOE field guide also says the boiler's high-limit control should shut it off at a water temperature of 200°F or less, which is a reasonable thing to ask them to confirm.
If the conversation turns to replacing the boiler with a heat pump, our guide to what a heat pump quote should explain about sizing covers what to ask before you sign.