How to Bleed a Radiator (and the Boiler Check Most Guides Skip)

The heating’s on, but one room never quite warms up. Put your hand on the radiator and you notice it’s cold at the top and warm at the bottom. That’s usually trapped air, and getting it out is one of the easiest home jobs there is.

What most people don’t realise is that bleeding radiators can make your boiler pressure drop. Skip the check afterwards and you can end up with a boiler that switches itself off, and the heating stops working everywhere. Here’s how to do the whole job properly, step by step.

Does your radiator actually need bleeding?

Turn the heating on and give the radiators time to warm up. Then carefully feel each one, top and bottom (they can be hot, so go gently).

• Cold at the top, warm at the bottom: classic trapped air. Bleeding should fix it.
• Gurgling or clanking noises: also often a sign of air in the system.
• Cold at the bottom, warm at the top: that’s different. Worcester Bosch says it could mean sludge has built up, and you may need your system cleaned rather than bled (>Worcester Bosch).
• Warm all over: leave it alone. British Gas advises not bleeding a radiator that doesn’t need it (>British Gas).

Make a note of which radiators need doing. British Gas also suggests bleeding them after long periods without use, such as at the end of summer, which makes autumn the ideal time.

What you’ll need

• A radiator key. They’re cheap from DIY shops and hardware stores. Some modern valves take a flat-head screwdriver instead.
• An old cloth or towel, to catch drips.
• A small container, in case more water comes out than expected.

That’s it.

Step 1: Turn the heating off and let it cool

This is the safety step people skip. Switch the heating off and wait for the radiators to cool. British Gas is blunt about it: with the heating on, the water and air inside can be dangerously hot, and you could scald yourself (>British Gas).

Southwark Council’s guidance also suggests switching off the central heating and hot water on the timer or programmer (>Southwark Council).

Step 2: Find the bleed valve

Look at the top corner of the radiator. You’re looking for a small valve with a square nut in the middle. It’s usually at the top on one side, but it can be on the side or back.

Put your cloth underneath it.

Step 3: Open it slowly

Fit the key over the square nut and turn it anti-clockwise, gently. You only need a small turn: a quarter to half a turn is often enough. Southwark Council says no more than two turns, because the screw should never come out completely.

You should hear a hiss. That’s the trapped air escaping.

Step 4: Close it when water appears

Keep the key in place until the hissing stops and water starts to come out steadily. Then turn the key clockwise to close the valve.

Tighten it firmly, but don’t force it. Over-tightening can damage the valve.

Wipe up any drips. The water inside heating systems is often dark or dirty, so keep it off carpets.

Step 5: Repeat for each cold radiator

Work your way round all the radiators you noted. Some people start with the ones furthest from the boiler or on the ground floor, but the main manufacturer and energy company guides we checked don’t insist on any particular order.

Step 6: Check your boiler pressure (the step most people forget)

Every time you let air out, the pressure in a sealed heating system drops a little. Bleed a few radiators and it can fall low enough to cause problems.

Most modern combi and system boilers have a pressure gauge, either a dial or a digital display on the front of the boiler.

British Gas advises that with the heating off, pressure should be between 1 and 1.5 bar, and around 2 bar when the heating is on (>British Gas boiler pressure guide). Many gauges have a green zone to make this easier.

If the pressure is too low

You’ll need to top it up, usually with the filling loop: a short pipe with one or two valves, often underneath the boiler. British Gas’s general steps for a combi boiler are:

1. Switch the boiler off and let it cool.
2. Check both ends of the filling loop are securely attached.
3. Open the valve (or both valves) slowly. You should hear water going in.
4. Watch the gauge until it reaches 1 to 1.5 bar.
5. Close the valves.
6. Switch the boiler back on, pressing reset if needed.

Boilers vary, so check your boiler’s manual first. If you’re unsure, or your boiler doesn’t look like this, ask a qualified heating engineer. If you rent, contact your landlord or housing provider, as many prefer to handle this themselves.

If the pressure keeps dropping

Needing to top up a couple of times a year is normal, according to British Gas. But if the pressure keeps falling soon after you’ve topped it up, there may be a leak or a fault. Get a Gas Safe registered engineer to look at it, and don’t keep refilling it.

Step 7: Turn the heating back on and check

Switch the heating back on, give it time to warm up, then go round the radiators again. They should now be warm all over.

When to call in a professional

Bleeding isn’t a cure-all. Get help if:

• a radiator is still cold after bleeding, or is cold at the bottom (possible sludge);
• the valve is seized, damaged or won’t close;
• your boiler pressure keeps dropping;
• you have an older open-vented system with a tank in the loft and aren’t sure how it works.

Anything involving the boiler’s gas parts must only be done by a Gas Safe registered engineer.

Told straight

Bleeding a radiator is a ten-minute job with a cheap key and an old towel, and it can make a cold room noticeably warmer. Do it with the heating off, open the valve gently, close it when water appears, and always check your boiler pressure afterwards. That last step is the one that saves a cold, confusing evening.

Steps checked against British Gas, Worcester Bosch and Southwark Council guidance in October 2026. Boilers vary, so always follow your own manual.