Boiler expansion vessel replacement

[benperry]

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....

[Ed Sirett]

Check out the sealedCH FAQ below. Ideally the additional vessel should be on the return pipework rather than the flow pipework. Leave existing vessel in place unless it's leaking water.

[besp]

Thanks Ed.

So no chance the leaking flow switch has anything to do with it?

Out of curiosity, why is the return better than flow for vessel placement?

Cheers.

[Ed Sirett]

I'd reckon totally independent problem.

Invariably the boiler will have its pump on the return. The circuit pressure is set by the vessel's pressure at that point in the circuit. We do not want the pressure to be low or negative at anypoint in the circuit as this could encourage air into the circuit.

In the case of a system which is insufficiently topped up, tall with the boiler at a low point (not uncommon in houses) and the pump before the vessel we could have a bad worst case.

What's more the latest Vaillants (ok yours in a GW) have a particular sensitivity, they monitor the pressure rise when the pumps starts up.

If the pressure does not rise fast enough they lockout with "F.75". This can be caused by a number of things but in particular the placement of an extra expansion vessel on the flow.

[besp]

Thanks a lot Ed.