In short
Turn the heating off and let the radiators cool. Hold a cloth under the bleed valve at the top corner, turn the radiator key a quarter turn until air hisses, and close it as soon as water appears. On a combi or sealed system, check the boiler pressure afterwards and top it up if it has dropped.
When should I bleed a radiator?
Bleed a radiator when it is cold at the top and warm at the bottom, or when you can hear trickling or gurgling in it. That means air has collected inside. Bleeding will not help a radiator that is cold at the bottom; that is usually sludge. See why radiators go cold.
What you need
- A radiator key (from a DIY shop) or, on some valves, a flat screwdriver.
- An old cloth or towel, and a small container.
How to bleed a radiator, step by step
- Turn the heating off and wait until the radiators are cool. Hot water can scald, and a running pump can draw air in.
- Find the bleed valve: a small square or slotted valve at one top corner.
- Hold the cloth under it and fit the key.
- Turn the key anticlockwise slowly, about a quarter to half a turn. You will hear air hiss out. Do not unscrew it fully.
- Close it as soon as water appears, turning clockwise until snug. Do not over-tighten.
- Repeat on other radiators that need it, usually starting downstairs and finishing upstairs.
- Check the boiler pressure (see below), then turn the heating back on and check the radiators heat fully.
Do I need to top up the boiler afterwards?
On a combi or sealed system boiler, often yes. Letting air out lowers the pressure. With the boiler cold, check the gauge against the figure in your user guide, which is around 1 to 1.5 bar on most sealed boilers, and top up with the filling loop as the guide explains. Close the filling loop valves fully afterwards. If you have a regular boiler with tanks in the loft, the system refills itself from the feed and expansion tank.
What if I keep having to bleed radiators?
Needing to bleed every few weeks is not normal. Air may be getting in through a fault in the system, or gas may be forming from corrosion inside, which also produces sludge. A heating engineer can find the cause, and check the inhibitor level in the system water.
What CombiCert would check
If radiators keep filling with air, we check for leaks, the expansion vessel and pressure behaviour on a sealed system, and whether sludge or a failing component is letting air back in. Occasional bleeding after a drain-down is normal; repeated bleeding is a fault that needs finding.
When to call a Gas Safe engineer
Call an engineer if bleeding does not warm the radiator, if water will not come out, if the bleed valve is seized or leaking, or if the pressure keeps dropping after you top up. See why is my boiler losing pressure? Never take the boiler case off to reach anything inside: that is Gas Safe registered work only. Our central heating engineers can help.
Need a hand? CombiCert in Telford
CombiCert Ltd is a Gas Safe registered (621146) gas and heating company based at The Old School House, Park Lane, Telford TF3 4TG, working across Telford and Shropshire for homes and businesses. We are open Monday to Saturday, 08:00 to 19:45 (closed Sunday).
Boiler repairs and services from £90 + VAT. Commercial work by quote.
See Central heating engineer in Telford, or call or WhatsApp 07393 220694, or email hello@combicert.co.uk.
We cover Telford and nearby, including Wellington, Madeley, Dawley, Oakengates, Shifnal, Newport, Ironbridge and Shrewsbury.
Sources
Frequently asked questions
Should the heating be on or off when bleeding radiators?
Off, and the radiators cool. Hot water can scald you as it comes out of the bleed valve, and with the pump running it can draw air in rather than letting it out. Turn the heating off, wait until the radiators are cool to the touch, then bleed them one at a time.
Which radiator should I bleed first?
A common approach is to start with the radiators downstairs, furthest from the boiler, and work towards the upstairs ones, because air tends to rise. In practice, bleed any radiator that is cold at the top. Check the boiler pressure when you have finished, and top up if it has dropped.
Why does my boiler pressure drop after bleeding radiators?
Letting air out of a sealed system reduces the pressure, because the space the air took up is now filled by the water already in the system. With the boiler cold, check the gauge and top up to the figure in your user guide using the filling loop, then close it fully.
What if water will not come out when I bleed a radiator?
If nothing comes out, the valve may be blocked with debris or the system pressure may be too low. Check the boiler gauge first. If the pressure is fine and the valve still will not release air or water, close it and ask a heating engineer to look at the valve and the system.
This answer is general information, not a substitute for an on-site assessment by a Gas Safe registered engineer. If you smell gas, call 0800 111 999.