Why a radiator goes cold at the top
Air is lighter than water, so any air in the heating system rises and collects at the top of the radiators. That pocket of air stops hot water filling the panel, leaving a cold band across the top while the bottom stays warm.
Air gets in in a few ways. A little is released from the water every time it heats up. More enters after work on the system, such as a new radiator or a drain down. On sealed systems, regular topping up of the pressure brings in fresh water with dissolved air. Corrosion inside the system also produces hydrogen gas, which collects in the same way.
If the radiator is warm at the top but cold at the bottom, you have a different problem, usually sludge. Our guide to radiators cold at the bottom covers that.
Signs your radiators need bleeding
A cold band across the top of the panel is the clearest sign, but there are others worth knowing:
- gurgling, trickling or bubbling noises from the radiator or pipes when the heating starts
- a radiator that takes much longer to heat up than it used to
- the top floor radiators colder than those downstairs, because air collects at the highest points of the system
- a pump that sounds noisy or rattly when the heating runs
Any of these after work on the system, such as a new radiator, a valve change or a drain down, is a strong hint that air got in during the job.
What you need
- A radiator key. These are cheap from any DIY shop. Some modern bleed valves take a flat-head screwdriver instead.
- An old cloth or towel to catch drips.
- A small container, if you want to check the colour of the water.
How to bleed a radiator safely, step by step
- Turn the heating off at the thermostat or programmer and wait until the radiators are cool enough to touch. Water in a hot radiator can scald, and the pump can pull air back in if it is running.
- Find the bleed valve. It is a small square-ended valve at one of the top corners of the radiator, sometimes on the side.
- Hold the cloth under the valve and fit the key onto the square end.
- Turn the key anticlockwise slowly, about a quarter to half a turn. You should hear air hissing out. Do not open it further than you need to.
- Wait for water. When the hissing stops and a steady trickle of water appears, the air is out.
- Close the valve by turning the key clockwise until it is snug. Do not overtighten it, as the small valve is easy to damage.
- Repeat on any other affected radiators. On a two-storey house, start with the downstairs radiators and then do the ones upstairs.
- Check the boiler pressure before turning the heating back on (see below).
- Turn the heating back on and check the radiator heats evenly from top to bottom once it has warmed up.
If only water comes out when you open the valve, there was no air in that radiator. Close it and look for another cause, such as a closed valve or stuck TRV.
Repressurise the boiler after bleeding
Bleeding lets water out of the system, so on a combi or system boiler the pressure drops a little each time. With the system cold, the gauge should read 1 to 1.5 bar.
| Gauge reading after bleeding (cold) | What to do |
|---|---|
| 1 to 1.5 bar | Nothing, turn the heating back on |
| 0.5 to 1 bar | Top up with the filling loop to 1 to 1.5 bar |
| Below 0.5 bar | Top up, then reset the boiler if it shows a code |
| Above 2.5 bar | Bleed a little more water out to bring it down |
Our guide to repressurising a combi boiler shows how to use the filling loop. If you let too much out, the boiler may lock out with a low pressure code such as Vaillant F22 or Worcester Bosch CE 207. Topping up and resetting usually clears it.
Homes with an older regular boiler and a small feed and expansion tank in the loft refill automatically, so there is no gauge to check.
It is worth bleeding every radiator that needs it before you top up, rather than topping up after each one. That way you only use the filling loop once, and you can see the true cold reading once all the air is out.
If the radiator keeps filling with air
Needing to bleed once or twice a year is normal. Needing to bleed the same radiator every week or two is not. The common reasons are:
- A small leak. Water lost through a leak is replaced when you top up, and that fresh water brings air in with it. If the pressure keeps falling, look for drips at valves and joints.
- Corrosion. Gas that smells of rotten eggs when you bleed, or water that is black, points to corrosion producing hydrogen. The system needs cleaning and fresh inhibitor.
- A faulty automatic air vent. Some boilers and pipework have a small automatic vent that can stick.
- Pump or pipework layout problems. On older open vented systems, poor pipe layout can draw air in through the vent pipe.
Keep a note of which radiators you bleed and when. If the same one keeps needing it, that pattern is useful to an engineer, because it helps show whether the air is coming from one part of the system or from the boiler and pump area. Note how often you top up the pressure as well.
When to call an engineer
Bleeding radiators is a normal homeowner job. Call a heating engineer or Gas Safe registered engineer if:
- the same radiator fills with air again within a couple of weeks
- the bleed valve is seized, snaps or will not close
- the pressure drops again soon after you top it up
- the gas released when bleeding smells unusual, or the water is black
- the radiator is still cold at the top after bleeding and water came out straight away
If you ever smell gas near the boiler rather than a faint smell from the radiator itself, stop, open windows, turn off the gas at the meter if you can and call the National Gas Emergency Service on 0800 111 999.