Where oh where is this air coming from?

[Ric2013]

Help please.

My heating system at home is hardly an urgent issue. That said, I've had to admit defeat on how on earth air is getting in! Any ideas welcome.

The original pipework on this system was a bit messy and there were some redundant runs, but basically sound as far as I can see. There were points where the flow from the boiler to the vent dropped below horizontal, but I think I have got rid of these. Air has been a problem for some time, and a year ago I removed some of the bad and obsolete pipework and re-plumbed it in a more rational way. This has helped to some degree.

There is no evidence of leakage on the system that I have seen.

The boiler is 25' horizontal distance from the close coupled vent arrangement. System is a Y plan with heat only boiler on ground floor, horizontal(ish) primary runs in first floor void to close coupled vent arrangement and 3 port valve in airing cupboard on first floor. The heating and DHW returns merge just before the Magnaclean, and I have fitted the Magnaclean above this confluence so that the Magnaclean is accessible and so it acts as the highest point on the return (doubling as an air vent point).

Boiler (Potterton Netaheat Profile 40e) kettles badly, and probably needs descaling. Every time I run the boiler, there is air in the Magnaclean which I bleed out. There is air in the Magnaclean even if I leave the system to cool before bleeding the air, so that rules out it being steam. The air does not appear to be hydrogen. The air is only present if the system has been run. System was refilled with fresh inhibitor 3 months ago (after trying out the efficacy of MC3 cleaner) and has been bled several times a week since, so this surely must be new air, not residual from filling! Quantity is probably about a teaspoon of air for every time I heat the cylinder up.

I'm clutching at straws now. Could the possible bubbles of steam passing the vent be somehow dragging in air? Could they be dragging in air through a micro-weep somewhere hidden? Could the boiler heat exchanger be letting in air somehow if a seal is defective? Could the compression unions on the boiler tails be letting in air?

I spend £300 a year on gas, so the idea of buying a new efficient boiler and replumbing the entire system has limited appeal. I'm not so much worried about curing the problem as understanding what is wrong... but curing the problem would be nice.

Thank you if you are still reading 🙂

Ric

 

[Ric2013]

It could be pulling in air on the vacuum side of the circulating pump, past the washer. I had one like that a few years ago where I found the most minute of weeps on the pump valve. It had fibre washers and I changed them to rubber and the air ingress problem went away.

Just an idea!

That would be the logical explanation - only part of the system that is under vacuum.

 

[Stigster]

That would be the logical explanation - only part of the system that is under vacuum.

I have been able to replicate this fault for the college apprentices I support. I used clear hoses and rigged them up to a circulating pump. I then scratched a fibre washer with my pocket knife and installed it on the vacuum side of the pump. We could see a steady stream of minute bubbles being pulled into the hose.

 

[zzzjim]

Silly thoughts is there still some contaminant / or chemical reaction thats only happening somewhere with high temperature ?
( Still like sound of small surge caused / intermittent temp dependant vacuum leak ..somewhere )

 

[Ric2013]

Hi, thanks Stigster. It was a rubber washer but wasn't all that tight. Took it out, and put it back after a clean and a smear of "Hylomar". Will report back in a few days.

zzz - I don't follow what you mean in your comments in brackets. Can you elaborate please?

 

[sammathias]

Sorry if anyone has already said this but there seems to be no bypass after the pump? Pump shouldnt be set on 3. Boiler stat shutting on and off correctly?? If you shut heating off can you hear motorized valve shutting down...no air heard by pump and vent/coldfeed??

 

[zzzjim]

.... I don't follow what you mean in your comments in brackets....

a) May happen at pump surge start-up
b) Be a slow steady process during certain conditions.
A vacuum is forming and drawing air occasionally, would have to be at a point of lower pressure / or speed change (size changes) .. like some-one elses scratched fibre washer -demo . A minuscule leak that may depend on expansion / contraction

 

[Ric2013]

Sorry if anyone has already said this but there seems to be no bypass after the pump? Pump shouldnt be set on 3. Boiler stat shutting on and off correctly?? If you shut heating off can you hear motorized valve shutting down...no air heard by pump and vent/coldfeed??

At present there is no official bypass, but the boiler can only fire when the UFH is running - the radiators come on at the same time (no separate zone as such: 'wrong', I know). Before there was UFH, the room served by the UFH has rads on manual valves and the air situation was no better. Y plan, so always somewhere for pump over-run to go.

Pump is on proportional pressure curve 3, not speed 3. The 3 pp curves are not that different each from the other.

Boiler cycles correctly, albeit with quite a high hysterisis. I think the gas rate is too high (but not by much), but last RGI to work here refused to set it down for me on the grounds that it would most likely cause the diaphragm in the gas valve to fail for not much gain.

Y plan, so motorised valve stays in CH mode until call for DHW.

After I removed the pump, air could be heard travelling up the vent. In normal use, with boiler kettling, you do hear the occasional bubble running up the vent, and sometimes a bubble manages to get pulled into the pump.

 

[Ric2013]

a) May happen at pump surge start-up
b) Be a slow steady process during certain conditions.
A vacuum is forming and drawing air occasionally, would have to be at a point of lower pressure / or speed change (size changes) .. like some-one elses scratched fibre washer -demo . A minuscule leak that may depend on expansion / contraction

I think we can rule out pump surge on start up to be the problem as the air seems to be only when there is heat/ boiler kettling UNLESS there is a miniscule leak that develops when the system is hot. (Also I would rule it out due to having semi plugged the vent in earlier experiment (see above) and confirm it would not appear to be sucking in air).
I'm wondering whether the steam bubbles collapsing in the flow could be causing a vacuum where the boiler is connected up with compression fittings, or if the boiler heat exchanger itself could have a miniscule leak with the same effect? Potterton (Netaheat) Profile 40e...

 

[sammathias]

3 port mid position valve or zone valve?

 

[Ric2013]

3 port mid position valve or zone valve?

3 port mid-position valve to select between CH, DHW, and both.

 

[sammathias]

sorry don't know if I phrased that correctly. some 3 port valves are mid position. i.e always an open circuit. but some are a closed circuit and only open with a demand...

 

[Ric2013]

Yes, always (at least) one circuit open. Merry Christmas!

 

[Ric2013]

Right, I said I'd get back to you.

I followed up the suggestion above which was a dodgy washer on the pump and applied Hylomar to my existing rubber washer (which wasn't all that tightly done up, as I think I stated above. Also, to be pedantic, I took the gland nut off the pump gate valve and shoved some PTFE behind it in the usual manner (even though the graphite packing seemed to be in good order).

After a couple of weeks there was no more air. Boiler still kettles horribly, but I can move on to powerflush or whatever for that. At least we seem to have solved the air problem, which will, of course, reduce further corrosion of the system.

Thanks for your suggestions and for finally forcing me to check the bloody obvious, because, logically, a system like this can only normally draw in air between the pump and the neutral point, but I couldn't see anything wrong with that section - must have been though!!!