How to Bleed a Radiator

Signs Your Radiator Needs Bleeding

You usually need to bleed a radiator when you notice one or more of the following.

• Cold spots at the top: The top of the radiator stays cold while the bottom gets hot. Air sits at the top and blocks the flow of water.
• Gurgling or knocking noises: Air moving through the system can cause bubbling or banging sounds when the heating is on.
• Slow to heat up: The radiator takes much longer than others to get warm, or never gets fully hot.
• Uneven heat across the room: Some radiators are hot and others are lukewarm or cold, even when the system has been on for a while.

What You Need

You only need a few things, most of which you may already have.

• Radiator bleed key (or a flat-head screwdriver for modern radiators with a slot instead of a square nipple). Bleed keys are cheap and sold at DIY stores and online.
• Old towel or cloth to place under the bleed valve and catch any drips.
• Small container to catch water if a little comes out before you close the valve—optional but useful.

Step-by-Step Guide to Bleeding a Radiator

Follow these steps in order. Always let the system cool before you open any bleed valve.

Step 1 — Turn the heating on and identify which radiators need bleeding

Switch the heating on and wait until the radiators have had time to heat up. Feel the top and bottom of each radiator. If the top is noticeably colder than the bottom, that radiator needs bleeding.

Step 2 — Turn the heating off and wait for radiators to cool

Turn the heating off completely and wait until all radiators are cold. Bleeding with the system hot can spray hot, pressurised water and cause burns. Cooling also lets air rise to the top of each radiator where the bleed valve is.

Step 3 — Start with radiators furthest from the boiler

Work from the radiator that is furthest from the boiler toward the boiler. That way you push air out in one direction and avoid reintroducing air into radiators you have already bled.

Step 4 — Place a towel under the bleed valve

The bleed valve is a small square nipple or slotted screw at one of the top corners of the radiator (usually opposite the pipes). Put an old towel or cloth underneath to catch any water that drips when you open it.

Step 5 — Insert the bleed key and turn anti-clockwise slowly

Fit the bleed key onto the valve (or use a flat-head screwdriver if it has a slot). Turn anti-clockwise slowly—about a quarter turn is often enough. You should hear hissing as air escapes. Do not unscrew the valve fully or it may come out and water will leak.

Step 6 — When hissing stops and water appears, close the valve

Once the hissing stops and a steady trickle of water appears, turn the valve clockwise immediately to close it. Tighten it gently; over-tightening can damage the valve. Wipe any spilled water from the radiator.

Step 7 — Repeat for all radiators that need bleeding

Go through each radiator that had a cold top, working from furthest to nearest the boiler. Bleed each one in turn until only water comes out when you open the valve.

Step 8 — Check boiler pressure after bleeding

Bleeding releases water from the system, which can lower the boiler pressure. When the system is cold, check the pressure gauge on your boiler. It should be between 1 and 1.5 bar. If it is below 1 bar, repressurise the system using the filling loop (see your boiler manual). Do not overfill—staying in the green zone is enough.

Step 9 — Turn the heating back on and check all radiators

Switch the heating on again and let the system run. Check that all radiators heat up evenly from top to bottom and that gurgling has stopped. If one radiator is still cold, you may need to bleed it again or there could be another issue (see below).

What If Bleeding Doesn't Fix It?

If you have bled the radiators but some are still cold or noisy, the cause may be elsewhere. A stuck TRV (thermostatic radiator valve) pin can stop water entering the radiator—gently lifting the pin with pliers (with the head removed) can free it. Sludge or corrosion inside the system can block flow; if several radiators stay cold or the water is discoloured, a powerflush by a qualified engineer may be needed. If you are unsure, get a Gas Safe or heating engineer to check the system.

How Often Should You Bleed Radiators?

It is a good idea to bleed your radiators at least once a year, ideally before winter when you start using the heating more. You can also bleed whenever you notice cold spots at the top of a radiator, gurgling noises, or uneven heat. New systems or after a refill may need bleeding once or twice in the first few weeks as remaining air finds its way out.

Frequently Asked Questions

Can I bleed radiators with the heating on?

No. You should always turn the heating off and wait until the radiators are cold before opening a bleed valve. Hot, pressurised water can spray out and cause scalding. Let the system cool first, then bleed.

What if there is no hissing sound when I open the valve?

If you open the bleed valve and water comes out straight away with no hissing, there is no trapped air in that radiator—it does not need bleeding. Close the valve and move on. If the radiator is still cold at the top, the problem may be a stuck TRV, sludge, or a different fault; an engineer can help diagnose.

Why do my radiators keep needing bleeding?

Frequent air in the system can be caused by a small leak (allowing air to be drawn in), a faulty or incorrectly set feed-and-expansion tank, or corrosion producing hydrogen. If you have to bleed more than once or twice a year, it is worth having a heating engineer check for leaks and system condition.

Do I need a plumber to bleed a radiator?

No. Bleeding a radiator is a simple DIY task that does not involve gas work. You only need a bleed key (or screwdriver) and a cloth. If bleeding does not fix the problem, or you are not confident doing it, then a Gas Safe or heating engineer can do it and check the rest of the system.