Trapped air is often the cause of cold radiators at the top but warm at the bottom. Bleeding allows that hot air out so the water cycles through as it should. This is a quick and easy DIY job for many of us in the UK, but there are times when you probably shouldn’t give it a go yourself. When you need a Cheltenham Boiler Service, contact combi-man.com/
What you’ll need for straightforward jobs:
A radiator key (or flathead screw driver for some valves)
An old towel/cloth
A small bowl or cup
In this guide, we run through the steps you need to bleed your radiator:
1) Turn the heating off
Turn off your central heating and allow the system to cool for 20–30 minutes. It can be extremely hot, and you shouldn’t bleed a hot system as this can be dangerous.
2) Find the bleed valve
That device is typically located at the top corner of your radiator. Put a towel underneath and catch the valve.
3) Open the valve slowly
Put the key in and give it a quarter turn counterclockwise. If everything is good, you should now hear a stream of air being released with a hissing sound.
4) Wait for water
Once water starts to trickle out and hissing stops, turn the valve clockwise again so that it is closed. Don’t overtighten, but ensure the valve is closed.
5) Repeat around the house
Begin downstairs with the radiators (generally those nearest to your boiler) and move upward. If any are cool in places, make these your priority.
6) Check boiler pressure afterwards
Bleeding can reduce system pressure. Verify the pressure gauge – on many boilers, it is at about 1.0-1.5 bar when cold (confirm in your manual). If it is low, you might have to fill up the system via the filling loop.
7) Turn the heating back on
Now turn the heating on again and re-check your radiators. If you still have cold spots, this may need further bleeding or another complication entirely.
When not to bleed radiators (step forward, the plumber)
Drop in boiler pressure (radiators may leak)
Your system is sealed, and you are not comfortable adding pressure to that
The radiator is totally cold at the bottom (usually sludge, not air)
Dirty/black water comes out (could be corrosion/sludge)
You have a heat pump system or something unusual and don’t know what’s safe
Quick tip
If you find that your constantly bleeding radiators, this could be an indication of a larger problem, whether it’s down to a slow leak and air ingress or even system sludge.
When bleeding is not enough: replacing a radiator
Bleeding will not fix a radiator that has developed pin holes and started to leak. At that point a replacement is the answer, and with the right tools it is a manageable job. You will need a bleed key, two adjustable wrenches, a bowl or bucket, old towels and some PTFE tape. A like-for-like radiator avoids any changes to the pipework, while modern designs with convectors may mount differently.
Turn the heating off and let the water cool. Close the control valve, then remove the cap on the lockshield valve and turn it off, counting the turns so you can reopen it by the same amount. Hold the valve body with one wrench, loosen the nut with the other, and open the bleed valve so the radiator drains into the bowl. Disconnect the other side and lift it off its brackets.
Wrap PTFE tape round the threads of the new radiator, tighten each joint finger tight plus a three-quarter turn, then open the control and bleed valves to fill it. Close the bleed valve, reopen the lockshield and check for leaks.
Draining and refilling a sealed system
Bleeding is enough for most cold radiators, but some jobs, such as repairs or fitting a new radiator, mean draining a sealed system completely. Switch the boiler off and disconnect its power first, to lower the pressure and avoid scalding. Find the drain point, usually on the pipework by the lowest radiator in the house, attach a hose with a jubilee clip and run it downhill to an outside drain.
Open all the radiator valves, then open the drain valve with a spanner. Opening the bleed valves at the top of the radiators speeds things up. When the water stops flowing, the system is empty.
To refill, close the bleed valves and then the drain valve, remove the hose and open the filling loop. Bleed each radiator, starting downstairs and finishing upstairs so air is not trapped. Top up, check for leaks and turn the boiler back on. Bleed the radiators again as heating forces out more air, top up the pressure as needed and recheck for leaks, as joints can seep once warm.