How to Bleed a Radiator and Get the Pressure Back Up

Run the heating for twenty minutes, then walk the house with your hand. Slide it down the face of each radiator, top to bottom. A radiator holding air is hot along the bottom and cold across the top, because the hot water fills the bottom and the air sits above it doing nothing.

Air is a poor conductor, so that cold band is dead space. A small pocket barely matters. Half a panel of air and the room never warms up. Note which radiators fail the test before you switch anything off.

Switch the heating off at the programmer and give the system twenty to thirty minutes to cool. Hot water under pressure will scald you, and a cold system also reads honestly on the pressure gauge later.

A radiator bleed key is a stamped metal key with a square socket, and it costs about the same as a coffee. Newer radiators take a flat-head screwdriver in the same square. Proper DIY makes the point that the hardest part of this whole job is finding the key, so put it somewhere obvious when you are done.

The bleed valve sits at the top corner of the radiator, usually the end away from the pipework. Look for a round brass or chrome fitting with a square pin in the middle of it. The radiator here has a white plastic spout that swivels, so you can aim the outlet down into a jug instead of at the wallpaper.

Hold the jug under the spout and lay a cloth on the floor. Water that has been sitting in a heating system comes out black and it stains.

Push the key onto the square pin until it seats flat. Turn it counterclockwise, written anticlockwise on British guides, and go a quarter turn at most. You are cracking the valve open, not unscrewing it. Wind it too far and the pin can come out in your hand with the system still full of water.

Air starts hissing straight away if there is any in there. Keep the key on the valve the whole time and your other hand on the jug. Do not walk away from an open valve.

Let the hiss run. The pitch drops as the pocket empties, then the valve spits and a steady dribble of water follows. That is the signal that the air is out.

Turn the key clockwise the moment you see water and close the valve finger tight. No wrench, no force. The seat inside is soft, and overtightening is how a two minute job turns into a weeping valve. Wipe the spout and the panel dry so nothing sits on the paint.

Keep the key in your hand and do the whole house in one pass. Start downstairs at the radiator furthest from the boiler and work back toward it, then go upstairs and repeat. Air travels to the highest and furthest points, so that order clears the system instead of shuffling air from one panel into another.

Every radiator is the same three moves. Jug underneath, crack it open, close it the moment water runs.

Bleeding takes water out of a sealed system as well as air, and a sealed system does not top itself up. Every radiator you open drops the pressure a little, so the boiler is the next stop.

Read the dial. Cold, it should sit between 1 and 1.5 bar. This one has dropped under 1 bar after the bleed. Plenty of boilers refuse to fire below 1 bar, which is how a house ends up stone cold within an hour of someone bleeding the radiators.

The filling loop is a short braided hose under the boiler with a valve at each end. One end sits on the mains at around 3 bar, the other feeds the heating circuit. Opening both pushes mains water into the sealed system.

Open one valve fully, then ease the second one open a little while you watch the gauge. Bring the needle up slowly to about 1.5 bar, then close both valves properly. Leave one seeping and the pressure creeps to 2.5 or 3 bar over the next few days, which is its own problem.