Central heating sucking air

So this central heating thing, lots of noise. New system, it is. Gravity HW and pumped CH, with an immersion tank heated either by the immersion heater (obviously) or the indirect coil passing through the tank.
This noise sounds like rushing water, and I can bleed all the air from the rads, and then have more in there 30 mins later. So I'm assuming that it's drawing air in from somewhere. The only open part of the system is the feeder tank, but that's full of water, and the air separator pipe, which overhangs the feed tank, but is not submerged.
I plan to extend the air separator pipe a little, so that the end is submerged, so that if it is sucking air back that way (which I imagine it shouldn't) then there'll be no air to suck back, but air can still get out.
Any other suggestions? Any ideas on how to find where the air is getting in, if it isn't through the air separator pipe?
Matt
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MattG wrote:

As its a new system it will take some time for all the gas held in suspension in the water to come out. Also for anything that is going to corrode in the short term to get it over and done with and finish liberating its gas.
I take it you have flushed the system and have an inhibitor in the water?

Careful you don't end up creating a loop where you are pumping heated water through your header tank!

I had this problem on our system when we moved in. The most effective fix was to move the vent pipe from the suction side of the pump (it was on a tee only 4 inches from the pump) to the pressure side. It seemed that every time the pump started it sucked a gob full of air into the system via the vent pipe.
Combining it with the header tank feed can work well - so that the feed connects to a tee in the vent pipe about 2' above the pump.
(select a non proportional font and learn why art was never my strong point!)
______ ___/ | [ / ] | [tank] |<- vent [____]-| | | | Feed->|___| | | | |<- old vent pipe position | | =================[ << pump << ]============
--
Cheers,

John.

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John Rumm wrote:

<noisy ch>

Yep, done that.

Well this is only a few inches from the suction side of the pump, so you could well be right.

<snip pic>
That looks like a good plan. I would have thought that the air separator would not allow air to be drawn back through, but it seems as though that is where it is coming from. I shall try your suggestion.
Thanks loads,
Matt
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I'm a little confused here, please could you help? In your diagram above it looks to me as though all you've done is move the vent pipe from very close to the "output" side of the pump to further away. Am I wrong here.
Any further info gratefully received as I have the problems described but my setup is already like your "new" positions and I was thinking of moving it to the other side of the pump to fix it!
I could have sworn I'd seen this suggested elsewhere in the N/G but I'm probably wrong.
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Mike Norris wrote:

Take a look at: http://www.google.co.uk/groups?hl=en&lr=&ie=UTF-8&oe=UTF-8&safe=off&selm=0VEOb.17029%24tQ6.707721%40wards.force9.net
You'll see the diagram properly then.
The vent pipe has been move from the input to the output side. If yours is alreadly like that, then I hope someone else can advise, as I can't help, sorry.
Matt
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Matt,
Thanks for the clarification, and a blisteringly fast response! I'd only just gone to make a cup of tea after uploading my question and there was your answere when I came back. I love this newsgroup.
Downside is mine is already like that, although to be accurate, it has the same configuration but the combined feed and vent are a long way from the pump. The boiler and pump in a downstairs utility room, the T off being upstairs and next to hot water cylinder, a piperun of some 25 feet or so probably. Maybe this has some bearing. I'll look further, but again many thanks.
Mike
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MattG wrote:

http://www.google.co.uk/groups?hl=en&lr=&ie=UTF-8&oe=UTF-8&safe=off&selm=0VEOb.17029%24tQ6.707721%40wards.force9.net
Hence my comment about needing a non proportional font - sorry perhaps I should have spelt that out a bit more - try configuring your newsreader to use Courier.
Must admit that google trick is a handy one to remember!.

You could also try running the pump at a lower speed (assuming it is not already on the lowest) and see if that reduces the problem.
You could check the feed from the header tank is not obstructed in any way - so that it can meet the quick demand for a bit of extra water if required when the pump starts.
With our system originally it was just at pump startup air got drawn in - you could hear the air circulating in a "lump" which would get more and more dispersed over the following minutes as the heating ran.
Another down side is it meant that the water in the circuit got periodically oxygenated and never had time to go stagnant. This resulted in more corrosion. So for example we got through a couple of three port valves as they would seize up every few years.
--
Cheers,

John.

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I thought of doing that to ours (which has the same layout), but thought I couldn't as the pipework provided the pressure relief path from the boiler. By moving the pipe the pump would be obstructing this path.
Have I misunderstood the need for a clear path?
IanC
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Ian Clowes wrote:

Not having seen the technology used in CH pumps I could not say for sure - on ours there is a separate pressure relief valve mounted shortly after the hot water outlet on the boiler.
--
Cheers,

John.

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John Rumm wrote:

Done, worked a treat. Thanks, much appreciated.
Matt
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HW
heater
out.
in,
Could be a blocked feed pipe from the header tank. In this case bleeding air from one place will only draw water from another. Mine gets blocked with bits from the header tank Cleaning it out is a job I really must get round to!!
Peter Scott
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