Amazing disappearing/reappearing air in radiators

5 Jun 2007
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United Kingdom
I hope someone out there can help me, I'm just about pulling what's left of my hair out with this problem!!! :confused:

I can bleed the upstairs radiators fine when the pump is off - there is a good squirt of water when the bleed valve is open. BUT as soon as the pump comes on, air magically appears in the upstairs rads and there seems to be very little oomph behind it to force it out. If I leave the bleed open, all kinds of wierd and wonderful noises start, beginning with a bubbling noise in the radiator and then a howling sound from the pipework going into the boiler. The downstairs radiators can be bled with the pump on or off, but the build up of air seems to stop the furthest ones from working when I haven't bled the upstairs rads for a while.

I've tried a chemical clean (when I drained it out, the water looked clear), and bleeding one rad at a time with all the others isolated on both the flow and return, but still this mysterious air returns. :evil:

I've got a gravity HW/pumped CH system. The pump is a Grundfos selectric on position 2 out of three. The boiler is a fairly old Potterton.

Any help would be greatly appreciated.
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Maybe you have a leaky joint on the inlet side of the pump ?

As there's a degree of suction here you probably wont see any leakage of water.

I had this problem on my system - there was a leaky joint on one of the Aerjec ports.

Thanks for getting back to me so quickly!

I shall tighten my nuts tonight :D I can't believe it could be something so simple :oops:

If I can't tighten it any more, would internal leak sealant do the trick?

Can't speak for internal sealant as I'm no heating engineer (electronics is my forte) - the peeps on this forum are very good though and I've no doubt that one of them will advise.

What I should have made clearer is that the leak, assuming that this is the problem, isn't necessarily at the pump - it could be any of the joints on the return side of the system :(

I can sympathise with you - I too have an old (but golden - at the moment) Potterton and had the same symptoms for a good while until I located the air ingress. In my case it led to the boiler overheat trip operating on a depressingly regular basis for a while and my better half hates being cold !.

Good luck !
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Crikey... the pipes into/out of the pump disappear under the floorboards immediately :(

How did you track you're leaky joint down? Was it a case of tracking the pipe into the pump back and checking each joint? Or is there an easier way?

My better half also does not like being cold, but she's revising at the moment and so definitely would not appreciate me ripping up floorboards!!!
Yes - that was it really, just careful inspection of the joints.

I'm sure I've read here that it's best not to bleed with the pump running by the way.

One other thing to check is that you have sufficient water in the header tank. If for some reason the level is too low it may be sucking in air when you're bleeding the system, particularly if the pump is running at the time.

Getting all of the air out of my system was a bit of a nightmare, it took time and a lot of patience.

Before you go ripping up any floorboards I'd wait and see what the professionals on here say as they will probably have some suggestions, or maybe they'll tell me I'm talking complete rubbish ;)

Sounds like your pump is on the return??? If so it is 'sucking in' somewhere. Go in the loft with a jam jar of water and immerse then vent pipe in the jam jar then get someone to turn ong heating and hot water seperatley and watch jam jar water level to see if it is sucked into system.
gas4you said:
Go in the loft with a jam jar of water and immerse then vent pipe in the jam jar then get someone to turn ong heating and hot water seperatley and watch jam jar water level to see if it is sucked into system.

Thanks for the advice gas4you... will do so tonight.

Taking a stab in the dark here (and going back to my dim and distant past of GCSE Science) if the vent is sucking in air then does that mean it is the path of least resistance for the water and thus that there is a blockage constricting the system. Since I've run chemical cleaner through it, and the water drained out clear, am I safe in assuming that one of the remaining original radiators (prob at least 20 yrs old) is corroded inside and that this is causing the problem? :idea: :?:

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