A radiator that's cold at the top and warm at the bottom has air trapped in it, and knowing how to bleed a radiator is how the air gets out. It's one of the simplest heating jobs there is — no tools beyond a radiator key, no parts, and the whole thing takes about ten minutes per radiator.
If bleeding doesn't fix a cold radiator, the cause is something else — a valve, pressure, or a balancing issue — and the radiator not heating diagnosis works through those in order.
Bleed when a radiator is cold at the top but warm at the bottom, when it gurgles or bubbles, or after any work on the heating system — draining and refilling always leaves some air behind. Don't bleed on a schedule: radiators that heat evenly don't need it, and bleeding them just drops system pressure for no reason.
One cold radiator can also be a balancing issue rather than air, and why one room runs hotter than others covers the uneven-heat picture beyond a single radiator.
Turn the heat off and let radiators cool. Switch the heating off and wait until radiators are cool to the touch. Bleeding a hot radiator risks scalding water, and a running pump can pull air straight back in.
Find the bleed valve. It's at the top corner of the radiator, opposite the pipework end — a small square peg or slotted screw. Put the towel under it and hold the container beneath.
Open it a quarter turn. Fit the radiator key and turn slowly, counterclockwise. You'll hear a hiss as air escapes. Don't open it all the way — a quarter to a half turn is plenty, and an over-opened valve can be hard to close again.
Wait for a steady stream of water. Air first, then sputtering, then a steady stream. The first water out is often dark and dirty — that's normal sediment, and it's why the towel is there. The moment the stream runs steady with no sputtering, the air is out.
Close the valve. Turn the key back clockwise until snug. Don't overtighten — bleed valves are small brass fittings, and a snapped valve turns a ten-minute job into a repair.
Check boiler pressure. Bleeding lets water out, so system pressure drops a little. Look at the boiler gauge: most systems want about 12–15 psi cold. If it reads below that, top up through the filling loop following your boiler's manual. Boiler-fed systems lose a little pressure every time you bleed, which is one more way they differ from forced-air setups — the boiler vs furnace comparison lays out the system side.
Turn the heat back on and recheck. Let the system run for an hour, then feel the radiator: warm top to bottom means the job worked. Still cold at the top after a proper bleed points to a valve or balancing problem, not air.
Cool radiators only. Hot water under pressure scalds, and the bleed valve is the last place you want a surprise jet of it. If a valve is seized and won't turn with gentle pressure, stop — forcing it can snap the valve off and flood the room. A seized valve is a pro job, and so is any leak that keeps going after the valve is closed.
Repeated bleeding is a warning sign. If the same radiator needs bleeding every few weeks, air is getting in somewhere — a small leak or an expansion issue — and a heating pro should track it down rather than you topping up the routine.
Key, towel, container, quarter turn, steady stream, close, check pressure. That's the whole job, and a radiator that was cold at the top should heat evenly within the hour. If it doesn't, the air wasn't the problem — run the wider diagnosis before you bleed it a second time.
Only when they need it — cold at the top, gurgling, or after system work. Radiators that heat evenly don't need bleeding, and bleeding them just drops system pressure.
Off, and radiators cool. Bleeding a hot radiator risks scalding, and a running pump can pull air back into the system.
If air hisses but no water follows, system pressure may be too low to push water up to that radiator. Check the boiler gauge and top up through the filling loop per your boiler's manual, then try again.
Bleeding lets water out of the system, so pressure falls a little. Check the boiler gauge afterward and top up if it reads below the normal range, usually about 12–15 psi cold.
Some bleed valves take a flat-head screwdriver instead of a key. If yours has a square peg, a radiator key costs very little at any hardware store — don't improvise with pliers, which chew up the valve.