How to Bleed a Radiator, and What to Do When It Won't Bleed

To bleed a radiator, switch the heating off and let the radiator cool, fit a radiator key to the small bleed screw at one of the top corners, and turn it a quarter to half a turn anticlockwise. Air hisses out; when water starts to spit steadily, turn the key back clockwise to close it. Then check the boiler pressure. The whole thing takes a few minutes per radiator. Below: the details that trip people up, from the wrong key to a screw that will not move.

How to Bleed a Radiator, Step by Step

Bleeding lets out air that has collected at the top of the radiator, so hot water can fill the whole panel again. These are the steps.

1. Switch the heating off at the programmer or thermostat and give the radiators time to cool. With the pump stopped, air rises to the top where the bleed screw is, and you avoid a spurt of scalding water.
2. Find the bleed screw. It is a small square spindle or slotted screw set into a round cap, usually at one of the top corners on the end of the radiator.
3. Protect the floor and wall. Hold a folded cloth under the screw and have a small container ready. System water is often dark and it stains.
4. Fit the key and turn it anticlockwise a quarter of a turn, or half a turn at most. That is enough. The screw does not need to come far out, and if you keep turning it can come out altogether.
5. Let the air out. You will hear a hiss. Wait until the hiss stops and water comes out in a steady dribble rather than spitting.
6. Close the screw by turning the key clockwise until it is snug. Firm, not forced: the seat inside is brass and it strips easily.
7. Wipe it dry and check it a minute later for weeping.

If several radiators need doing, the order matters: downstairs first, starting furthest from the boiler, then upstairs. Our guide on which radiator to bleed first, and whether it should be hot or cold explains why.

What You Need, and Which Key Fits

Most radiators need a standard brass radiator key with a square socket, sold at any hardware shop for very little. Some newer radiators have a slotted bleed screw that takes a flat screwdriver instead.

• Standard square key. Fits the square spindle on most steel panel radiators. The socket should sit fully over the spindle; if it only grips the corners, it will round them off.
• Slotted screw. Many modern radiators have a slot across the bleed screw. Use a flat screwdriver that fills the slot.
• Towel rails and designer radiators. The bleed point is often a small screw or plug at the top of one upright, sometimes hidden under a decorative cap. It may need a small hex key.
• Column and cast iron radiators. These usually take a standard key, but on older ones the bleed valve can be on the back or on a top plug rather than the end.

Pliers and grips are the wrong tool. They slip, round the spindle, and make the next bleed harder.

What You Should See and Hear

A normal bleed is a hiss of air for a few seconds, then a spit of water, then a steady trickle. How long the hiss lasts tells you how much air was in there.

• A short hiss, then water. A little air, now gone. That is the normal result.
• A long hiss. A lot of air. After you close it, run the heating for a while, let it cool, and check that radiator again, because more air often works its way up.
• Water straight away, no hiss. There was no air in that radiator. If it was cold, the cause is something else, covered below.
• Black or rusty brown water. This is sludge and corrosion from inside the system. A bit of discolouration is common in older systems. Thick black water is a sign the system water has lost its corrosion protection.

After Bleeding: Check the Pressure

On a combi or system boiler, check the pressure gauge on the boiler after bleeding, because every bit of air you let out lowers it. If it has dropped below the range in your boiler manual, top it up through the filling loop.

This is the step that catches people out. Bleed three or four radiators on a sealed system and the pressure can fall far enough for the boiler to lock out, which looks like a new fault but is only low pressure. Our guide to repressurising a heating system yourself shows how to use the filling loop, and when not to. Older open-vented systems with a small feed and expansion tank in the loft refill themselves and have no gauge to watch.

Once the pressure is right, switch the heating back on and feel each radiator once it has had time to warm through. It should be warm from top to bottom.

When the Bleed Screw Won't Cooperate

Most problems during bleeding come from the valve itself, not the radiator. Here is what each one usually means.

The screw will not turn

Bleed screws are often painted over or seized from years without use. Check you are turning anticlockwise, and that the key sits fully on the spindle. Scoring round the paint line with a craft knife can free it. Do not lean on it with grips: a snapped or rounded bleed screw means the valve has to be replaced, and that means isolating and draining the radiator first.

Nothing comes out at all

No air and no water usually means one of three things.

• The radiator is isolated. The TRV is turned to off or its pin is stuck down, or the lockshield valve at the other end is closed.
• The system pressure is too low to push water up to that radiator, often after earlier bleeding. On an open-vented system, the loft tank may have run dry.
• The bleed port is blocked with sludge. Opening the screw a little further, carefully, sometimes clears it.

It keeps dripping after you close it

Nip it up a little more. If it still weeps, the seal or seat is worn and the bleed valve needs replacing, which is a small job but needs the radiator isolated and drained.

The screw comes right out

Push it straight back in and turn it clockwise while holding a cloth over it. On a sealed system the pressure behind it is only modest, so it is messy rather than dangerous once the radiator is cool.

When Bleeding Isn't the Answer

Bleeding fixes one fault, trapped air, and nothing else. If a radiator is still cold after a proper bleed, look at the pattern of the cold.

• Cold at the bottom, warm at the top. That is sludge settled in the bottom of the panel, not air. See our guide to a radiator cold at the top or bottom.
• Cold all over. Usually a TRV pin stuck down after the summer, or a lockshield that has been closed.
• Needing a bleed every week or two. Something is producing gas or letting air in. Corrosion inside steel radiators makes hydrogen, which is why it can hiss back within days. Never test the gas from a bleed screw with a flame.
• A whole floor stays cold. That points to an airlock in the pipework or a balancing problem, not one radiator.

When to Call an Engineer

Bleeding a radiator is a normal homeowner job and you should not need to pay anyone to do it. Call an engineer when bleeding keeps being needed, when the water is thick and black, or when radiators stay cold after a proper bleed and a pressure check.

Those signs point to the water inside the system rather than to air, and the fix is usually a clean and a fresh dose of inhibitor, sometimes a power flush with a magnetic filter fitted afterwards. Another round of bleeding will not help.

If your radiators still are not heating properly after bleeding, book a free survey. We will find out whether it is air, sludge, valves or balance, and give you a fixed quote before any work starts.