Where oh where is this air coming from?

[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.