Air drawn into sealed system

I've been having a problem with air in a sealed system, not huge amounts but enough to make the system noisy. It only seems to be noticable when the hot water is on by itself, and i'm not getting much air in the rads.

- Definately air not hydrogen, and there is plenty of inhibitor in system.

- Quite a large system, 30KW condensing boiler, 6m head Grundfos Alpha pump set to max. Pressure vessel is between boiler & pump intake.

- Zone valves are on returns, not feeds.

- Boiler, pump, zone valves etc all downstairs, and there is quite a long run to HW cylinder (17m of pipework approx).

- System pressure seems to remain constant, and does not drop over time.

I've pressurised system to 3bar and checked for leaking compression joints, rad valves etc, everything seems fine.

How long roughly does it normally take for air in the system to clear after a refill on a sealed system? (note there is no air separator)

I'm pretty sure air is being drawn in, any suggestions to what the most likely causes could be?

Reply to
Mary Hinge
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Is there a way of removing the air from all the highest places in the system (obviously each radiator has a bleed point) but what about in the airing cupboard?

A few weeks is what I find before every last bubble has found its way to a rad.

If the pressure is not rising then I doubt air is being drawn into the system.

Reply to
Ed Sirett

Yes there is an auto air vent in the airing cupboard.

Maybe the pressure isn't rising as i'm being to efficient at removing the air as it accumulates? (via a manual air vent above the boiler).

Having said that i haven't bled the rads for a few weeks so don't know what is in them at the moment.

Reply to
Mary Hinge

Is the auto-vent functioning?

Reply to
Ed Sirett

Most definately working.

Air does tend to also get trapped on pipework around the boiler, since the feed & return drop down 2m immediately before/after the boiler if you get my drift, i just bleed these using the manual air vents?

Is it really necessary to have auto air vents wherever air tends to accumulate to ensure all the air comes out of a sealed system? I sort of assumed that after a short period it would just end up in the rads where it can be bled out, leaving an air-free system.

My gut feeling is air is being pulled in around the area of the pump inlet, is this possible and how common is it?

Thanks,

MH

Reply to
Mary Hinge

The whole system is under pressure. It is possible apparently for a system to draw in air even though it is under pressure through small leaks.

The auto air vent is a convenience really rather than an absolute necessity, it can even be closed once all the air is out.

If I were faced with the system professionally then assuming I haven't missed anything in your posts then I would go for sealing it up with Fernox LS-I or equivalent.

Reply to
Ed Sirett

Thanks for the replies :o)

I did bleed a reasonable amount of air from one of the rads yesterday (it seems to acculumate in just one rad due to the system layout), and noticed the pressure may have dropped very slightly, so it could be a small leak somewhere on the pipework that is not visible.

I just surprised that air can still sneak in on a pressurised system, and that it can make quite a noticeable amount of noise. I'm guessing this would be a very small, slow leak due to the lack of immediately noticable pressure drop. The problem did go away about 6 months ago (after the system was pressurised), but seems to have come back after a couple of rads were added to the system (all the new pipework looks watertight).

I'll give the leak sealer a bash and see if that resolves, I assume this stuff is quite effective and has solved similar issues in the past?

Many Thanks,

MH

Reply to
Mary Hinge

Two litres of Betz-Dearborn's finest Sentinel leak sealing fluid and the problem persists :o(

Any other suggestions? This is really starting to get me down...

Cheers,

MH

Reply to
Mary Hinge

How long has it been in? Expect to get some air out for the next few weeks - even a system which is completely air/water tight will take some time to get all the air out. Have you closed the auto vent yet?

Reply to
Ed Sirett

Ok thanks - i'll give it a fews weeks and see what happens - yes the AAV is still open.

Cheers,

MH

Reply to
Mary Hinge

Update: I stumbled across this thread

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(warning this is a long read!)

Basically this guy's problem was very similar to mine (I also have a 6m head pump set to max), in fact i had tried many of the steps he had (conversion to combined feed & vent etc) also without success, before going sealed which resolved the air issue, until we had some work done in the house five months or so later.

After reading this thread I did recall that after the plumber converted to a sealed system i did notice the cold pressure was quite high, near

1.5 bar, whereas I had been pressurising to around 1.25 bar cold. So a couple of nights ago I increased the cold pressure to 1.5 bar and hey-presto, problem solved immediately, no more air.

Whilst i'm relieved that the problem has gone away, I am perplexed as to how you could have a leak that doesn't let water out, but plenty of air in unless you pressurise to 1.5 bar? Bear in mind this issue wasn't a small amount of air, but what seemed like loads, ie the system made noises you associate with venting a refilled system every time I had the hot water circuit on by itself.

Any ideas? I don't really feel like I've solved the problem, just found a work-around.

Reply to
Mary Hinge

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