How to Bleed a Radiator That Is Cold at the Top

The heat came on for the first cold night and one radiator stays cold across the top while the bottom is hot, or the pipes clank and gurgle when the boiler starts. That is air trapped in the radiator, not a broken boiler, and it does not need a service call. It needs a radiator key, a rag, and one check on the boiler pressure gauge that most people skip and then wonder why the whole house went cold the next day.

Confirm it is a hot water radiator, not steam

Look at the top corner of the radiator. A small square or slotted screw inside a round fitting is a bleed valve, which means hot water heat and this guide applies. A bullet shaped chrome or gray cylinder on the side with a tiny hole in it is a steam air vent, and a steam radiator has one pipe and no bleed screw. Do not open a steam vent with a key. A steam vent that stays cold is replaced, not bled.

Run the heat and note which radiators are cold on top

Turn the thermostat up and give the system time to heat through. Feel the top and the bottom of every radiator. Any radiator that is cold on top and warm on the bottom, or cold all over while its neighbors are hot, goes on the list. Worcester Bosch says to check every radiator in one pass rather than finding the next one tomorrow.

Turn the heating off and let the radiators cool

Switch the thermostat or the boiler to off and wait until the radiators are cold. Vaillant says to allow about an hour, or two to be safe. A hot radiator sprays hot water when the valve opens.

Open the bleed valve until water runs steadily

Start with the radiators on the lowest floor and work upward, and on each floor start with the radiator nearest the boiler. Hold a rag under the bleed valve. Put the radiator key on the square screw and turn it counterclockwise about a quarter turn. Air hisses out. Keep the valve open while it hisses, and the moment water comes out in a steady dribble instead of spitting, turn the key clockwise until it stops. Do not force it. Wipe up any drips, because heating water stains floors.

Check the boiler pressure gauge and top it up

Every bit of air let out lowers the system pressure. Go to the boiler and read the gauge. Vaillant's figure for a cold system is between 1 and 1.5 bar, which is about 15 to 22 psi on a gauge marked in pounds. If the needle is below that, open the filling loop or the fill valve under the boiler until the gauge reaches the low end of that range, then close it fully. A boiler left below its minimum pressure locks out and shows a fault code, which is the "whole house went cold" call the next morning.

Turn the heat back on and feel the radiators again

Run the heat and check every radiator you bled. Warm all over means done. If one is still cold on top, bleed it once more with the system off, because a radiator that held a lot of air may need two passes.

If the same radiator needs bleeding every few weeks. Air is getting in somewhere or the system is losing water. That is a leak, a failing expansion tank, or a corroded system, and it needs a heating technician rather than another turn of the key.

If no water comes out with the valve open. System pressure is too low to reach that radiator, which is common on a top floor. Close the valve, top the boiler up first, then bleed again.

If the gauge is well above the range after topping up. With the system off, open a bleed valve on the nearest radiator until the gauge drops back into range. An overfilled sealed system lifts the relief valve and dumps water on the floor.

Sources. How to bleed a radiator, Worcester Bosch, How to bleed a radiator, Vaillant.