Sealed System Pressure Loss

27 Sep 2007
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United Kingdom
Newly registered on site !

Been struggling with this one for some time - no responses from 'other' plumbing forum !


Sealed System CH loses pressure on cooling down.


Boiler is Gloworm 60FF with Gloworm Sealed System kit.
DHW cylinder is Heatrae Sadia MegaFlow - but this shouldn't matter ?

When non operational CH holds Water Pressure @ 1bar.
Expansion Vessel holds pressure. This is set at 1bar when cold and only static head pressure in CH circuit. EV does not exhibit water leakage from Schraeder valve.
No (obvious) air in system.


When using CH pressure builds rapidly towards 3 bar and starts discharge trickle from PRV drain (there is no water loss from PRV when cold). Also I have not noticed sudden discharge (with the expected accompanying bang) from the PRV.
On cooling CH pressure has dropped to 0.2 bar (or less) and a lot of air has been admitted to system necessitating top up via filling loop. During this time the EV has NOT lost pressure at Schraeder valve and there is no noticeable increase in pressure monitored at the EV Schraeder valve.

Not sure what auto air vent is doing - boiler instructions say that auto air vent cap should be 'slackened off but not removed' (What is the auto air vent cap - the pics don't explain) and it says it should not be retightened.

The pressure loss DOES NOT occur when DHW only circuit is in use - I'm guessing that the expansion of the relatively small volume of water is tolerated ?

Any ideas and help would be greatly appreciated,


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Sounds like the expansion vessel has had it. Needs recharging or replacing if water comes out of the valve when you check it.
Thanks for very quick replies, but there is no water from Schraeder valve on EV. just hissing air (or inert gas - which I think they are when new?)

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There must be something wrong with it cause it is clearly not accepting expansion of system water, and thats why its blowing off.

what boiler is it,(ignore that if in first post)
Thanks for info. Just trying to eliminate all other possibilities because changing the EV's gonna be a big job - It's behind the boiler; between it and the wall !


Glow-worm Micron 40FF, 50FF, 60FF, 70FF, 80FF, 100FF, 120FF

Please note that this Safety Notice has been updated to include the 70FF and 80FF.

During routine quality inspection procedures it has been discovered that a manufacturing defect may be present in the above Glow-worm Micron boilers manufactured between 25th October 2006 and 2nd May 2007. All Micron boilers are tested after manufacture, but the defect does not become apparent until installation when, in some circumstances, the gas manifold can leak gas when the boiler is running.

We are in the process of carrying out a free of charge safety inspection / modification to all of the relevant Micron boilers for which we possess installation addresses. We are seeking your help to identify further boiler installations.

Please urgently call our free helpline 0800 783 8554 (Monday - Friday, 8am - 6pm. Saturday - Sunday, 9am - 12pm) with the installation address details of any of the above Micron boiler models that have been installed since the 25th October 2006. We will then contact the user and arrange for an engineer to carry out an safety inspection / modification of the boiler free of charge.

You will be asked for the boiler type, model and installation date when you phone, so please have these to hand

Thought I Should point out the above as dont know if yours is the Micron.

The EV is where I would be looking and a possible blockage to it :eek:
Rather than change it .....

add another expansion vessel in a more accessible position!
Thanks yet again !

The Bolier is indeed a Micron 60FF - but of 2002 maunfacture - so not affected by that. Checking for a blockage in flexi pipe should be easier - I'll try that first and report back.

Just one more thing what's the auto air vent cap that should 'not be retightened' I can find nothing that's obviously loose. Could this be related ?

Thanks Baxpoti, problem here is nowhere (close) to put it due to orig installation by builder. Is there a limit on length of flexi pipe to EV?

Doesn't have to be next to the boiler. How about the airing cupboard for example?
Thanks to everyone who has offered advice.

Fitted new Expansion Vessel and problem solved; still not sure what was (is) wrong with old one though !

Thanks Again, Andy :D

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