Air in Sealed Central Heating System

[RichCF]

I'm hoping someone can help as this has been driving me mad for some time.

I'm getting air in the central heating system, enough to mean that the top bar of a 6ft towel rail is completely cold within a week and needs bleeding.

Some information:

- Boiler is WB Greenstar 34CDi Classic Combi
- System pressure is ~1.3bar cold rising to just under 2bar at 70°C
- It is definitely air not hydrogen
- No visible leaks found anywhere (including inside the boiler itself)
- No sign that PRV has been leaking
- Expansion vessel has been checked and is fine at 0.8bar
- AAV in the boiler was swapped by WB engineer as a precaution
- Plumber still suspected leaks (perhaps in screed) so 2 bottles of Fernox - F4 have been added (made no difference)
- System pressure is stable i.e. not falling
- The air seems to collect in the tall towel rail in the bathroom (not a surprise - it's the highest point)

I can often hear the air bubbling into the towel rail.

This might be a red herring but perhaps someone will think it's relevant - the bubbles can be heard after running the hot water for a decent period e.g. a shower or filling the hand basin.

The pattern is:

- Run hot water
- Turn off hot water
- Pump runs for a bit (maybe a minute)
- Pump stops
- After a couple of seconds bubbles heard (seems the longer you've run the water for, the more air gets in)

It's pretty reliable i.e. this pattern happens every morning when I've finished in the shower.

Any suggestions gratefully received!

Thanks.

 

[Terry lambert]

Is there an additional expansion vessel installed on the system ?

 

[RichCF]

Is there an additional expansion vessel installed on the system ?

Nope.

(thanks for the reply)

 

[45yearsagasman]

Collect the "air" in an upturned empty can and see if it ignites. It's safe to do and will prove the presence of hydrogen or not.

 

[RichCF]

Collect the "air" in an upturned empty can and see if it ignites. It's safe to do and will prove the presence of hydrogen or not.

Thanks for the reply.

Already tested - it's not hydrogen.

 

[Agile]

A lighter flame held against the bleed screw usually demonstrates if its hydrogen.

Has the AAV cap screw been closed to avoid drawing air in ?

Tony

 

[RichCF]

Thanks for the reply.

A lighter flame held against the bleed screw usually demonstrates if its hydrogen.

Yep, I've done that too - it definitely won't light and/or pop etc.

Has the AAV cap screw been closed to avoid drawing air in ?

Tony

AAV cap is currently open.

WB engineer came out two weeks ago to check the boiler, I asked him whether the AAV could be the source - he said it was fine but changed it for a new one anyway.

 

[Agile]

The AAV is designed to have a positive pressure inside it. make it negative and air will be sucked in.

Close it and see what the result is.

Tony

 

[RichCF]

The AAV is designed to have a positive pressure inside it. make it negative and air will be sucked in.

Close it and see what the result is.

Tony

I've been wondering whether it is the source but being a layman wasn't sure whether it could be

Any issues likely to arise from running the system with it closed?

If it is the AAV, what's the long term solution? (keep replacing them until I get one that doesn't do it?)

Thanks

 

[Agile]

Just close it to see.

Its not a fault with the AAV but how they work.

IF air is being drawn in then its a system fault which would probably need repiping. Easier to just leave it closed IF that cures it.

They only have a purpose when a system has just been refilled.

Tony

 

[45yearsagasman]

I wish someone would explain to me how an auto air vent would allow air into a sealed system @ 2bar pressure, but not show signs of water leaking.

 

[RichCF]

Just close it to see.

Its not a fault with the AAV but how they work.

IF air is being drawn in then its a system fault which would probably need repiping. Easier to just leave it closed IF that cures it.

They only have a purpose when a system has just been refilled.

Tony

Thanks.

I'll give it a try and report back.

Out of interest, are there any examples of the kind of "system faults" that could cause this effect? (just in case I recognise any of them on my system)

 

[RichCF]

I wish someone would explain to me how an auto air vent would allow air into a sealed system @ 2bar pressure, but not show signs of water leaking.

I wish I knew how air could get into a sealed system full stop!

I suppose somewhere in the system the pressure is (temporarily) low enough to draw air in?

Over Christmas when we were away for a few days and only the CH was running (i.e. no DHW use) there didn't appear to be any air build-up.

It's just a theory at the moment but it seems to happen directly after using the DHW.

 

[dcawkwell]

Possible loosing water due expansion vessel failure which is a very frequent problem.
It will be dumping the water outside.

Possible cavitation cause by poor circulation if you have all thermostatic valves on the system.

 

[RichCF]

Possible loosing water due expansion vessel failure which is a very frequent problem.
It will be dumping the water outside.

Expansion vessel checked (0.8bar) and no leaks outside via PRV.

Possible cavitation cause by poor circulation if you have all thermostatic valves on the system.

Did wonder about that but it does seem to be related to DHW use i.e. leaving CH only running for several days doesn't seem to introduce any air but I'm not 100% sure if it was a fluke over Christmas.