How to bleed a radiator: a ten minute checklist before the cold sets in

A radiator that is hot at the bottom and cold at the top has air trapped inside it, and the fix takes about ten minutes. This checklist explains how to bleed radiator valves safely, which radiators to do first, how to tell trapped air from sludge, and how to top the boiler pressure back up afterwards, because a system that has just been bled almost always needs it. Do it now, before the first cold snap, and the heating will be ready when you need it.

What you need before you start

You do not need a plumber for this job, but you do need a few things to hand. Most of them are already in the house.

• A radiator bleed key, sold at any hardware shop for a pound or two. Some modern radiators have a slotted valve that takes a flat screwdriver instead.
• An old cloth or a small container to catch the drips. The water that comes out is often dark and will stain carpet.
• Kitchen roll and a torch if the valve is behind a cover or tucked under a windowsill.
• A note of the boiler pressure gauge reading before you start, so you can see how far it drops.

If the bleed valve is seized, stop there. Forcing it can shear the pin and turn a ten minute job into a call-out.

Is it air, or something else?

Bleeding only fixes one problem: air at the top of the radiator. Put a hand on the panel while the heating is running. Hot at the bottom and cold across the top is air. Cold at the bottom and hot at the top is sludge, a layer of rust and debris that bleeding will not shift and that usually needs a flush or a magnetic filter from a heating engineer.

A radiator that is cold all over while the others are hot is usually a valve problem. Check the thermostatic valve is open and the pin underneath the head is not stuck down. If one radiator is always last to warm, the system may need balancing rather than bleeding. Only the first case belongs on this checklist.

How to bleed radiator valves: the ten minute checklist

1. Turn the heating off and let the radiators cool. Bleeding a hot system sprays scalding water and pulls more air in as the pump runs.
2. Find the bleed valve. It is a small square pin inside a round nut at one top corner of the radiator.
3. Hold the cloth under the valve and fit the key. Turn it anticlockwise, no more than a quarter to a half turn. You should hear a hiss.
4. Wait while the air escapes. Keep the key in place and keep the cloth ready.
5. Close the valve clockwise the moment water comes out in a steady dribble rather than a spit. Do not overtighten.
6. Wipe the valve dry and check it again a minute later for weeping.
7. Move to the next radiator and repeat.
8. Check the boiler pressure gauge before turning the heating back on.

If a valve hisses for more than a minute, it is still doing its job; a large double panel can hold a surprising amount of air.

Which radiators first, and how often

The order you bleed radiator valves in matters. Air rises, so it collects in the radiators furthest from the boiler and highest in the house. Start on the ground floor with the radiator nearest the boiler and work outwards, then go upstairs and do the same, so air pushed along by the first radiators is caught by the later ones.

Once a year, in October or early November, is enough for most homes. Bleed again any time a radiator develops a cold top, or after the system has been drained for a repair. If you are bleeding the same radiator every few weeks, air is getting in somewhere, often through a weeping joint or a failing pump seal, and that is a job for an engineer.

The Energy Saving Trust puts heating at the largest share of a typical household’s energy bill, which is why a radiator working at half its surface area matters. Its heating advice covers the other winter checks worth doing at the same time.

Topping the boiler pressure back up afterwards

Every pocket of air you release is replaced by water from the system, so the pressure falls. On a combi or sealed system boiler, the gauge should sit at about 1 to 1.5 bar when the system is cold. If it has dropped below 1 bar, top it up using the filling loop, which is the braided hose or built-in lever under the boiler. Open it slowly, watch the gauge climb to around 1.2 bar, then close it fully.

Do not overfill. A reading above 2.5 bar cold will push water out through the pressure relief pipe outside, and above 3 bar most boilers lock out. If the pressure keeps falling in the days after you top up, read our explainer on why a boiler keeps losing pressure. Older open-vented systems with a small tank in the loft refill themselves and have no gauge to worry about.

Once the pressure is right, run the heating for half an hour and walk round the house again. Any radiator still cold at the top can be bled a second time while the system is warm, carefully, with a thick cloth, because the pressure behind the valve will be higher. That second pass usually finishes the job for the season, and it is a good moment to run through the rest of our autumn heating checks.

When to call a professional

Bleeding radiators and topping up the pressure are safe for any householder. Anything beyond that, including opening the boiler casing, replacing a pump, fitting a filter or flushing the system, is work for a heating engineer, and any work on the gas side must be done by someone on the Gas Safe Register, whose ID card you should check. If you smell gas at any point, leave the building and call the National Gas Emergency Service on 0800 111 999.

If you have worked through the bleed radiator checklist and a radiator is still cold, or the pressure will not hold, our free help desk can find a heating engineer with the right credentials. Tell us what you have tried through our request help page. The team reviews every enquiry and confirms availability and credentials before anything is agreed, so nothing is booked until they come back to you.

Guidance is general and refers to the authority that sets each rule. Costs are indicative ranges; always get written quotes. Trade availability and credentials are confirmed when you enquire.