Central heating airlock?

[ss]

History: All was ok previously, I then decorated one room and took 2 rads off, reinstalled the rads and the rads do heat up, I bled off some air after a couple of weeks. The rads still heat up. However sometimes when the heating turns on there is woooosh! at one of those radiators, quite loud, it only lasts a second or so but I seem to think for whatever reason it is air in the system. I have tried adjusting the balance at the lockshield valve from almost off to full on...no difference.

It is a combi boiler and as said all rads heating ok.

Any thoughts.

[Brian Gaff]

Where is the highest radiator? Brian

[ss]

I have checked the highest radiator and a little air came out, not much about 1 seconds worth. This rad is on the 1st floor same as the other 2 I had removed.

[Harry Bloomfield]

ss was thinking very hard :

The liklehood is that there is still some air trapped in there still. You could try turning other radiator valves off, to force maximum flow to those two radiators along with the air.

[John Rumm]

Probably just a pocket of air that is getting swept along as the pump starts. Eventually you should be able to bleed it from one rad or another.

[leenowell]

Are you bleeding the system whilst the heating is off?

[ss]

When heating on.

[leenowell]

That might be your issue. Try switching it off and ensure the pump has stopped (it will run on for a bit)before you bleed it. Then bleed all the rads to be sure. Start from the lowest rad and work way up.

[John Rumm]

A small pocket of air may be just swept back into suspension each time a pump runs. So allowing some time to bubble out before bleeding may help.

[ss]

That sounds as if that could be the problem, I`ll give it a couple of weeks and see how it goes. At least the full system works ok so not a major issue.

[John Rumm]

I really meant just let it "rest" for a couple of hours, then bleed all the rads before turning it back on. That should get most of it.

[Fredxxx]

A pump can generally overcome an 'airlock'.

I would close the radiators off with the one furthest away left on. empty this one of air.

Once air is in a radiator it can't get back into the 'system'.

Then open close each at a time bleeding the rads as you go.

Sometimes some air persists and just takes time to escape.

[John Rumm]

If all the rads are warming up correctly, then I don't think this sounds like an airlock.

[Brian Reay]

Good advice.

Another trick, close all the 'working' radiators' and jam a hose into the header tank outlet. A 'blast' of mains pressure water will sometimes clear an air lock or any debris.

[Rob Morley]

More likely the air collects above the pump, gets pumped down when the pump switches on (hence the swishy sound) then rises above the pump again when it turns off - the air never reaches a radiator.