Bleeding radiators: how to do it safely

Signs your radiators need bleeding

• The radiator is cold at the top and warm at the bottom.
• It takes much longer to warm up than the others.
• You hear gurgling or trickling in the radiator or pipes.
• The whole house is slow to heat after the system has been drained or worked on.

Air gets into heating systems over time, especially after work on the system or when it has been topped up. It rises to the top of radiators and stops hot water filling them.

If the radiator is cold at the bottom and warm at the top, or cold in the middle, the cause is usually sludge rather than air. Bleeding will not help; see the section further down.

What you need

• A radiator key. Most hardware shops sell them. Some newer valves take a flat-head screwdriver instead.
• An old cloth or towel to catch drips.
• A small container if your bleed valve tends to spit water.

Is it safe to do yourself?

Yes. Bleeding radiators is a normal homeowner job and does not involve touching the boiler, gas or oil. Take two precautions:

• Turn the heating off and let the radiators cool first. The water inside can be hot enough to scald.
• Open the valve slowly and only a quarter to half a turn. You do not need to remove it.

If a bleed valve is seized, snaps, or will not stop leaking when you close it, stop and call an engineer.

How to bleed radiators, step by step

1. Turn the heating off and wait until the radiators are cool. With the pump off, the air settles at the top.
2. Find the bleed valve. It is a small square or slotted fitting at one of the top corners of the radiator.
3. Start in the right place. Begin with the radiator furthest from the boiler on the ground floor, then work towards the boiler. Then do the upstairs radiators the same way.
4. Hold the cloth under the valve, fit the key and turn it slowly anticlockwise, about a quarter to half a turn.
5. Listen. You should hear a hiss as the air escapes.
6. Close the valve when water appears. As soon as a steady trickle of water comes out with no spitting, turn the key clockwise to close it. Do not overtighten.
7. Repeat for each radiator that needs it.
8. Check the boiler pressure. On a sealed system, the gauge will often have dropped. Top up to about 1 to 1.5 bar cold, or the figure in your manual. See how to top up boiler pressure.
9. Turn the heating back on and check the radiators heat evenly. Check the bleed valves for drips once they are warm.

Oil systems with a header tank

Some older oil-fired homes have an open-vented heating system with a small feed and expansion tank in the roof space instead of a pressure gauge. Bleeding works in the same way, and the tank refills the system automatically. If the tank does not refill (for example, the ball valve is stuck), call an engineer.

When bleeding is not enough

The same radiator keeps filling with air

If you are bleeding the same radiator every few weeks, something is letting air in or creating gas inside the system. The usual causes are:

• a small leak that lets air in as the system cools;
• corrosion inside the system, which produces hydrogen gas that collects in radiators (if the gas you release could burn, it is hydrogen, which points to corrosion);
• a pump or system design problem drawing air in.

All of these need an engineer to diagnose.

Cold at the bottom: sludge

Rust and debris from inside the system settle at the bottom of radiators as sludge. It blocks the flow and can damage the boiler’s pump and heat exchanger over time. Signs include:

• radiators cold at the bottom or in the middle;
• dark or black water when you bleed;
• the boiler getting noisy (kettling) or locking out.

The fix is cleaning the system, typically a chemical clean or power flush, followed by an inhibitor to protect it, and sometimes a magnetic filter on the boiler return. Ask our engineer at your next service if you see these signs. See boiler noises too.

Only some radiators heat

If the radiators nearest the boiler are hot and the far ones lukewarm, the system may need balancing: adjusting the valves so water is shared evenly. It is a fiddly job but not a dangerous one, and an engineer can do it quickly.

No radiators heat at all

If none of the radiators warm up, the problem is likely the boiler, pump, valve or controls rather than air. Check the boiler is showing no fault, then book a repair.

How often should you bleed radiators?

Only when they need it. Many people bleed their radiators at the start of each heating season, which is a sensible habit. If you need to do it more than once or twice a year, get the system checked.

Get help

If bleeding does not fix your cold radiators, book a repair visit: £85 for the first hour, then a fixed quote before anything else. If your boiler is due its annual service, mention the radiators when you book and the engineer will take a look: gas £79, oil £89. Book online or email [email protected].