System boiler external expansion vessel pressure

12 Oct 2016
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United Kingdom
I have a boiler (Worchester Greenstar Danesmoor System Utility 18/25) which is plumbed to a large Akvaterm cyclinder.

When the system is cold, the pressure gauge on the expansion vessel reads 0.75 bar. When the system has been running for a while the vessel gauge reads anywhere between 1.5 bar to 1.75 bar. My understanding is that when the system is cold the gauge should read 1 bar and when it is running it should read 1.5 bar. Is that correct?

I'm guessing that I need to pump a little more air into the expansion vessel in order to have it sitting at 1 bar when the system is cold. However, if my logic is correct, if I add air to the vessel using the schrader valve when the system is cold in order to bring the pressure up to 1 bar, surely afterwards, when the system is running hot the reading is likely to be higher than 1.75 bar considering that before I pumped air into the vessel it is already sitting at around 1.75 bar when the system is running, which is too high.

Therefore, if my logic is correct, does that mean I need to add air to the vessel to bring it to 1 bar when it is cold but then drain some water out of it to make sure that the pressure (when the system is running) is no more than 1.5 bar?

I hope what I've tried to explain makes some sense. If anyone is able to advise I would appreciate. Any questions just ask.

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so what is your concern? does prv letting by? or boiler shutting off? don't bother touching it nothing wrong with it. why just get a local plumber to service your boiler and ask the expansion vessel as well
The lower the fill pressure on the EV, the lower the usable capacity. The PRV doesn't operate until it hits 3 bar, so I wouldn't worry about it too much, but the next time the system is drained down, check the EV and pump it up if necessary.

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