How to bleed a radiator, step by step

How do you know a radiator needs bleeding?

Air gets trapped in a central heating system over time. Because air rises, it collects at the top of your radiators and stops hot water filling them, so they heat your home less well.

Turn the heating on, give the radiators time to warm up, then carefully feel each one. Signs a radiator needs bleeding include:

• It's warm at the bottom but cold at the top.
• It takes a long time to heat up, or never gets properly hot.
• You hear gurgling or rattling in the radiator or pipes when the heating comes on.

If a radiator is the other way round, hot at the top and cold at the bottom, bleeding won't help. That usually points to sludge, which is covered further down.

What you need to bleed a radiator

• A radiator key. The Energy Saving Trust puts the cost at about £3. Some modern bleed valves take a flat-head screwdriver or an Allen key instead.
• An old towel or cloth to catch drips.
• A small bowl or jug to hold under the valve.
• Gloves, in case the radiator is still warm.

The bleed valve is a small square or slotted screw in the top corner of the radiator, usually at one end.

How to bleed a radiator in six steps

1. Turn the heating off and wait for the radiators to cool. Glow-worm suggests about an hour, or two to be safe. The water inside can be hot enough to scald.
2. Lay the towel under the valve and hold the bowl or jug beneath it.
3. Fit the key onto the bleed screw and turn it slowly anti-clockwise. A quarter to half a turn is enough. You'll hear a hissing sound as the air escapes.
4. When the hissing stops and water starts to come out steadily, turn the key clockwise to close the valve. Don't over-tighten it.
5. Wipe up any water and repeat on the other radiators that need it.
6. Check the pressure gauge on your boiler, top it up if needed, then turn the heating back on and check the radiators heat evenly.

Manufacturers differ slightly on the order. Glow-worm recommends doing the ground floor before upstairs, and Viessmann suggests starting with the radiator furthest from the boiler. Either way, the aim is to bleed every radiator that has air in it.

Safety: Only bleed radiators when the heating is off and they have cooled down, as the water can scald. Keep drips well away from sockets, plugs and any electrical items, and if water reaches anything electrical, keep away from it and don't touch it with wet hands. Never open the boiler casing. Anything inside the boiler is a job for a Gas Safe registered engineer.

Why check your boiler pressure after bleeding radiators?

Letting air and a little water out of the system can make the pressure drop. Worcester Bosch advises checking the boiler pressure once you've finished bleeding all the radiators and topping it up if it has fallen.

If your boiler has a pressure gauge, it will normally read around 1 to 1.5 bar when the heating is cold. British Gas says around 2 bar is normal when the heating is running. Your boiler manual gives the exact figure for your model and explains how to top it up, which on many combi boilers is done with a filling loop underneath.

Not sure how to top up the pressure, or the manual isn't clear? Stop there and ask a heating engineer rather than guessing. Topping up too much can be as much of a problem as too little.

How often should you bleed radiators?

Glow-worm recommends bleeding radiators once a year, and Viessmann suggests the start of each winter, before you rely on the heating every day. The Energy Saving Trust points out that summer is also a good time, because the heating is more likely to be off for hours and the radiators are already cool.

Outside of that, bleed a radiator whenever the top feels cold or you hear gurgling in the pipes.

Radiator still cold after bleeding?

If bleeding didn't fix it, a few other things could be going on:

• Cold at the bottom, warm at the top: Worcester Bosch says this may be sludge rather than air. Vaillant says a system that stays cold after bleeding may need flushing, which must be done by a heating engineer.
• Not heating fully: check the radiator's valve settings. A manual valve shows its setting, and a thermostatic valve may simply be turned down too low.
• Big temperature differences between rooms, or some radiators slow to heat: the system may need balancing, which means adjusting the flow of water so every radiator heats evenly.
• No heat anywhere: Vaillant says the most common reason for a cold radiator is the boiler itself not working. Check its display for a fault code and look it up in the manual.

When to call a heating engineer

Bleeding radiators is a safe job for most homeowners. Get help if:

• You need to bleed the same radiators again and again. Glow-worm says this can point to an underlying problem worth raising at the next boiler service.
• The boiler pressure keeps dropping after you've topped it up, which can mean a leak somewhere in the system.
• A bleed valve is seized, damaged or won't stop leaking once closed.
• Radiators stay cold after bleeding, or the boiler shows a fault.

For anything involving the boiler itself, use a Gas Safe registered engineer and ask to see their Gas Safe ID card before they start. For radiators, valves and pipework, a heating engineer or plumber can help.