How to Bleed a Radiator: Step-by-Step Guide

What you will need

You need very little to bleed a radiator: a radiator bleed key (a small, inexpensive square-socket key sold in most hardware shops and supermarkets), a cloth or small towel, and a small container such as a cup to catch any drips of water. Some modern radiators and TRV heads use a flat-blade screwdriver instead of a key, so check the bleed valve before you start.

It is best to do this when the heating has been off for at least half an hour so the radiators have cooled down. Bleeding a hot radiator is possible but increases the risk of a small splash of hot water catching your hand, so cooler is safer and more comfortable.

Step 1: Identify the bleed valve

The bleed valve is a small square-shaped fitting, usually at the top corner of the radiator, opposite the TRV side or sometimes on either end. It has a small hole in the centre through which air and water escape. Fit the radiator key onto the square section, or the screwdriver into the slot, and have your cloth positioned just below it before you start.

Step 2: Open the valve slowly

Turn the key anticlockwise a quarter turn at a time. You should hear a hissing sound as trapped air escapes; this is the sign it is working. Keep the cloth in place, as a fine spray or occasional drip is normal. Never open the valve fully or quickly, as this makes it harder to control and messier to close again.

Continue holding the valve open until the hissing stops and a steady trickle of water appears with no more air in it. At that point, close the valve firmly but without overtightening, since the seal only needs light pressure to hold. Wipe away any water from the radiator and floor.

Step 3: Work through the house in the right order

Bleed radiators starting on the ground floor and working upwards, and within each floor start with the radiator closest to the boiler and finish with the one furthest away. This order follows the direction air naturally collects as it rises through the system and gives the most reliable results.

If you are only dealing with one problem radiator, such as a cold spot in a bedroom, you can bleed that one on its own without doing the whole house, though it is good practice to check all radiators once or twice a year, typically at the start and partway through the heating season.

Step 4: Check the boiler pressure afterwards

Every time you release air (and a little water) from a radiator, the overall pressure in the sealed central heating system drops slightly. Once you have finished bleeding, check the pressure gauge on the boiler; most domestic boilers should sit between 1 and 1.5 bar when the system is cold.

If pressure has dropped below this, top it up using the filling loop according to your boiler's instructions, usually by opening a valve or pair of valves slowly until the gauge reaches the correct level, then closing them again firmly. Running the system with low pressure can cause the boiler to lock out or heat unevenly.

Why air keeps coming back

A radiator that needs bleeding once or twice a year, especially at the start of the heating season, is entirely normal. However, if air returns within days or weeks of bleeding, something is drawing air into the sealed system. Common causes include a faulty automatic air vent, a pump seal letting in air, or a poorly positioned feed and expansion tank on older gravity-fed systems.

In these cases, repeated bleeding only treats the symptom. It is worth having a heating engineer check the system for the underlying cause, particularly if you are topping up the pressure frequently as well, since that combination often points to a small leak somewhere in the pipework rather than the radiators themselves.

Frequently asked questions

How often should I bleed my radiators?

Most homes benefit from bleeding radiators once at the start of the heating season, typically in autumn, and again partway through winter if any cold spots reappear. Bleeding more often than that suggests an underlying air ingress problem worth investigating.

Is it dangerous to bleed a radiator with the heating on?

It is safer to bleed radiators once they have cooled, as the water and any escaping steam can otherwise be uncomfortably hot. If you do need to bleed one while warm, work slowly and keep your hand well clear of the valve opening.

What if water keeps coming out but there's no air?

If you open the valve and only water comes out immediately with no hissing, the radiator likely did not have an air problem in the first place, and the cold spot is more likely caused by sludge, a closed valve, or a seized TRV rather than trapped air.

Do I need to turn the heating off before bleeding radiators?

It is not strictly essential, but it is easier and safer with the heating off and radiators cool, since this avoids hot water or steam spraying out when you open the valve.

Can bleeding a radiator fix a boiler fault?

No. Bleeding only removes air from the radiators and pipework; it will not fix a faulty pump, valve, or boiler component. If bleeding does not resolve cold radiators, or if the boiler itself is showing a fault code, a qualified heating engineer should take a look, and any gas-related work must be carried out by a Gas Safe registered engineer.