How to bleed a radiator, step by step

When a radiator needs bleeding

Air collects at the highest point of each radiator. The classic sign is a radiator that is warm along the bottom but cool across the top. You may also hear gurgling or trickling in the radiator or pipes when the heating first comes on, or notice that one or two upstairs radiators take much longer to warm up than the rest.

Early autumn, when the heating is switched on for the first time in months, is the most common time to bleed. Air that has gathered over the summer shows itself as soon as the pump starts moving water again. It is also normal to need a round of bleeding after any work that drained part of the system, such as a new radiator, a valve change or a boiler replacement. If you are taking a radiator off to decorate, our guide on how to drain a radiator explains how to do it without emptying the whole system.

Bleeding only releases air. It will not help a radiator that is cold along the bottom, cold all over, or cold because its valve is stuck shut. Those faults need a different fix, so check the pattern before you start.

What you need

Most radiators use a standard square radiator key, which any hardware shop sells. Some modern radiators have a slotted bleed screw that takes a flat screwdriver instead. Have everything to hand before you open the valve, because air and water can come out faster than you expect.

• A radiator key or a flat-bladed screwdriver, depending on the valve
• An old towel or cloth to wrap around the valve
• A small container or tray for drips
• Access to the boiler pressure gauge and the filling loop, if you have a sealed system

How to bleed a radiator: the steps

Work calmly and keep the valve open only as long as air is coming out. The whole job usually takes a minute or two per radiator.

• Switch the heating off at the programmer or thermostat and wait until the radiators are cool enough to touch comfortably.
• Find the bleed valve. It is a small round fitting with a square centre or a slot, at one of the top corners of the radiator.
• Hold the cloth under and around the valve, fit the key and turn it slowly anticlockwise, roughly a quarter to half a turn. Do not unscrew it all the way.
• Listen for the hiss of air. Keep the key in place and wait.
• When the hissing stops and water begins to run out steadily rather than spitting, turn the key clockwise to close the valve. Snug it up by hand; do not force it.
• Wipe the valve dry and check it is not weeping.
• Move on to the next radiator that needs bleeding, then check the boiler pressure.

Never bleed a radiator with the heating running and hot. The water inside can be very hot, and a running pump can push water out of the valve with force.

Which radiator to bleed first

If several radiators need doing, start on the lowest floor and work upwards, beginning with the radiator furthest from the boiler on each floor. Air rises through the system, so finishing with the upstairs radiators lets you catch the air that has moved up while you were working below.

Bleed with the radiators cold rather than hot. A cold system is safer to handle, and the air has had time to settle at the top of each radiator instead of being carried round the pipes by the pump. After bleeding, run the heating for a while, let it cool again and recheck any radiator that was badly airlocked.

Check and top up the boiler pressure

Most homes with a combi or system boiler have a sealed heating system with a pressure gauge on or under the boiler. Each time you release air, the pressure in that system falls a little. After bleeding several radiators it can drop low enough for the boiler to stop and show a low-pressure warning.

Look at the gauge with the heating cold. Manufacturers usually give a cold figure of around 1 to 1.5 bar, but your boiler manual gives the exact range for your model. If the pressure is low, top it up through the filling loop, following the instructions for your boiler, and close the loop fully when you reach the right figure.

Older homes with a regular boiler, a hot water cylinder and a small tank in the loft usually have an open-vented system. These refill themselves from the loft tank, so there is no gauge to watch and no filling loop to use.

If you have to top up the pressure more than once or twice a year, the system is losing water somewhere. That is a leak to find, not a job for more bleeding.

What you might see, and what it means

Most bleeds are uneventful. These are the situations worth paying attention to.

When bleeding does not fix the radiator

A radiator that stays cold along the bottom after bleeding usually has sludge settled in its lower channels. A radiator that is cold all over while its pipes are hot may have a thermostatic valve pin stuck down, a closed lockshield valve or poor balancing across the system. If every radiator is lukewarm, the problem is more likely to be at the boiler or pump than at the radiators. Stuck valve pins are common when the heating first goes back on, and our guide to thermostatic radiator valve settings shows how to free them.

Needing to bleed the same radiator every week or two is a warning sign in its own right. Common causes are a small leak on a sealed system, corrosion producing gas inside steel radiators, or a pump and vent arrangement that draws air in on an open-vented system. Each of these gets worse if it is ignored, and repeated fresh water in the system speeds up corrosion.

Get help with radiators that will not warm up

Loyal Plumbers repairs radiators, valves and central heating systems across London and everywhere inside the M25, with 10 vans working 24 hours a day. If you have bled your radiators and they are still cold, the pressure keeps dropping, or a bleed valve will not seal, call us or send an enquiry.

It helps to tell us which radiators are affected, what the pressure gauge reads when the heating is cold, how often you have had to bleed, and whether the water from the valve was clear or dark. That lets the engineer arrive with the right parts for a valve repair, a leak investigation or a system clean.