Guide · Heating how-to
Bleeding a radiator lets trapped air out so hot water can fill the whole panel. It takes a few minutes and a radiator key, and is one of the few heating jobs most homeowners can do themselves.
The tell-tale sign is a radiator that is warm at the bottom but cold at the top, often with gurgling as the heating comes on. Air rises, so upstairs radiators and the ones furthest from the boiler are usually the first to need it. A radiator cold at the bottom is a different problem, explained in our radiator cold at the bottom guide.
Turn the heating off and let the radiators cool. Hot water can scald and the system is under more pressure when warm. Find the bleed valve, a small square or slotted fitting at one top corner of the radiator.
Hold the cloth under the valve, fit the key and turn it slowly anticlockwise, a quarter to half a turn. You will hear a hiss as air escapes. When the hiss stops and a steady trickle of water appears, close the valve firmly but do not overtighten it.
Work from the radiator nearest the boiler to the furthest, downstairs first, then upstairs.
On a sealed combi or system boiler, bleeding lets out a little water, so pressure drops. If the gauge has fallen below the level in your boiler's instructions, top it up using the filling loop and close it fully afterwards; our guide on how to repressurise a boiler shows each step.
If the boiler is not working after bleeding radiators, low pressure is the most likely reason. Top it up and reset the boiler. Older vented systems with a tank in the loft top themselves up automatically.
If cold water comes straight out with no hiss, there was no air in that radiator, and something else is stopping it heating. Close the valve and see our guide to radiators not heating up.
If the bleed valve will not close, keeps dripping or the boiler starts leaking afterwards, put a container underneath and call us. A bleed valve that has seized or a pressure relief valve that has opened needs an engineer. If radiators need bleeding every few weeks, air is getting into the system somewhere and it should be checked.
If the radiators are now full of water but some rooms are still cooler than others, the system may need balancing. Our guide on how to balance radiators walks you through it.
If bleeding has not solved it, call or WhatsApp 07429 632804, read about our central heating work, or use the quote form.
Common questions
Clear answers before you arrange a visit.
A small metal key that fits the square bleed valve on a radiator. Most hardware shops sell them, and some radiators take a flat screwdriver instead.
Only when they need it, typically when they are cold at the top. Needing to do it often suggests air is getting in and the system should be checked.
Off, and the radiators cool. That avoids scalding and gives a more accurate result.
Bleeding lets water out of a sealed system. Top it up with the filling loop to the level shown in your boiler's instructions.
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