Endless Air In Central Heating - How?

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Stuart Henderson

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Hope someone can help. I regularly get significant air in my CH that I can bleed out only for it to return a week or so later. The obvious answer is there is a leak somewhere but the pressure in the system does not drop until such time as I bleed the rads.
If I ignore it / leave it for long enough, the situation escalates from noise rads / pipes to a banging boiler as the air works its way through. But again - at no point does it seem to be leaking water or losing pressure - its a constant 1.2 bar when cold. Only place I can think the air is coming from is the expansion vessel? Is this a thing? Had a plumber take a look - suggested all was ok and just keep bleeding .... can't be right and I don't think I'll see him again for the dust he kicked up leaving so fast.
All help gratefully received.
 
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See (carefully) if what's coming out burns. If it does, it's hydrogen caused by corrosion which is indicative that there is a lot of oxygen in the system and not enough inhibitor.

New steel will create gas as an oxide layer is formed on it... can take a number of months.

If it's a low temperature system (e.g. UFL) it could be biological issues, in which case a product like X700 might help.

Also check the system pH level, especially if you have aluminium radiators.
 
The hydrogen flame is very pale, but very hot, so check carefully. Any red rust (Fe203) inside the rad reacts with water to make FE304 (black) + H2, which slowly builds up at the top of a radiator.
 
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Terrywookfit said:
Have you got an AAV in the suction side of your pump.
Click to expand...

Terrywookfit, there is an aav in the boiler but not on the pipework. Interestingly there is a 15mm vertical deadleg on the flow at the point the CH and DHW valves are. In some pictures I've seen this is common but tends to have an aav on it. Mine does not. Should it?
 

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It is a manual vent.
Are you venting the system when the pump and boiler are turned off.
Which boiler do you have and where is the boiler relative to the rest of system.
 
Terrywookfit said:
It is a manual vent.
Are you venting the system when the pump and boiler are turned off.
Which boiler do you have and where is the boiler relative to the rest of system.
Click to expand...

Yup. Vent when off and cold. Its a vaillant 637 system boiler. Its about 3m away from the tank and valves on the same floor. The manual vent thing, new one on me, should I just crack that open and see if there is air in there?
 
Stuart Henderson said:
Yup. Vent when off and cold. Its a vaillant 637 system boiler. Its about 3m away from the tank and valves on the same floor. The manual vent thing, new one on me, should I just crack that open and see if there is air in there?
Click to expand...

So I tried to burn the gas - it blew the match out so not Hydrogen. I cracked open the manual vent and got soaked - so no air there. I have noticed the pressure us rising higher by the day - expansion vessel issues?
 
Terrywookfit said:
It is a manual vent.
Are you venting the system when the pump and boiler are turned off.
Which boiler do you have and where is the boiler relative to the rest of system.
Click to expand...

So I tried to burn the gas - it blew the match out so not Hydrogen. I cracked open the manual vent and got soaked - so no air there. I have noticed the pressure us rising higher by the day - expansion vessel issues?
 
If you have needed to repressure the system periodically, maybe the expansion vessel membrane has failed and it's air has been transferring to the system water? And it's now out of charge so when heated the pressure rises.
 
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