Expansion vessel replacement

11 Mar 2008
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United Kingdom
Hi all,

I have a Glowworm 80e combi with pressure issues.

Low pressure on the gauge (sub 0.5bar). Filling loop used to increase pressure. Turn heating on and working, heating off and pressure drops below level it was increased to.

Did some testing and when the heating is turned on, pressure increases. If turned off before 3bar then returns to original pressure but if reaches 3bar, the PRV kicks in and when turns off pressure drops below original value (longer the PRV on, the lower the final pressure).

So from searching about, it looks like as a blown expansion vessel. The air valve on the expansion vessel spurts out water also.

I think it's an entirely separate issue but..... I have noticed a leak from the DHW flow switch (the front cover gasket).

Any other possibility or should I fit a new expansion vessel?
Can I just fit an extra vessel by teeing of the heating loop and if so, does it matter where?
And what happens to the old vessel - does it stay as part of the circuit and remain filled with water?

Hope you can help....

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From the sounds of the flow switch problem im assuming you have a glowworm COMPACT 80e.

For this you need a cover plate, diaphragm and microswitch.

As for the expansion vessel it does sound as though the diaphragm has split. An external one is easiest if theres room obviously.

The one in the boiler is probably 10ltrs but fit a 12. Check the pre charge, most come about 1.5 bar which is too high. Reduce to just less than 1.

Fit it tee'd into the return pipe as close as possible to the boiler and you will also need to replace the PRV.
Thanks Rob.

New vessel will go in as per your advice.

Yes it's a compact 80e.

Can i check that i understand you/terminology correctly?

Flow switch is round black plastic unit. Cover plate is on the front (i.e away from the wall) of the flow switch. Micro switch is the square black plastic unit on the front of the cover plate which is removable if i pull out the securing clip.

From this i assume there is a diaphragm in the flow switch which detects when tap is opened, moves the pin through the coverplate which opperated the micro switch. The diaphragm has burst leading to the leak through the cover plate gasket.

The microswitch is still working as it's only a drip at present so a diapragm would fix the leak, the new coverplate would come with a new gasket (which isn't available seperately?) and the microswitch should be OK. Is the gasket actually called a "gland seal" and replacing this along with the cover plate is good practice even if not strictly needed? Or is it strictly needed?

Many questions I know!!!

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No its the coverplate gland which is leaking. Just good practise to change the diapragm aswell its only a quid. And chances are small amount of water has entered microswitch and not long before it affects this.

Total parts should be about £30. Takes about 2 mins. You may have noticed its held in with torx screws. t15 iirc.

Good luck :D

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