Normally, EVs are sized to 10% of the cylinder volume and precharged to 3.0bar, the pressure reducing valve is set to the same, if the full contents are heated from 15C to 60C the final pressure will be 3.8bar, if the PRV is letting past and the pressure rose to even 4.5bar then the final pressure on reheating will be greater than 6.0bar and will lift the expansion relief valve.
Sorry John, l know what pressure it should be at. I was curious as to whether the vessel had any charge in it? After reading Harrys last post I think he meant relief valve and not reducing valve. Burst and flat vessels are by far the most common cause of relief valves being activated and component failure. Harry, it would be worth checking the EV if you haven’t already.
I had written a reply but it must've forgotten to post it!
So all of this has been a learning experience for me. I do DIY plumbing but never touched pipework around the boiler/cylinder area.
In total 3 things were faulty.
Firstly the PRV valve on the cylinder was constantly letting by. I had a big company come out and at that time I new nothing about unvented pipework. They charged the vessel up to 3 bar as it only had 1 bar in it. It hadn't been serviced in years (that will change). And they said the cylinder PRV was faulty and wanted £250 to fit a new one not including the £80 call out. Having seen the PRV online for £20 I thought that price was a bit steep so I found a local bloke who did it for £60. They saw I do a lot of DIY so explained how it works a bit.
After the PRV was replaced there was still water going through the tundish so I spoke to the plumber and they said it could be the Expansion Relief Valve on the multibloc. The plumber said that I could do it so I did some research and changed that.
After those two were replaced the ERV was still letting by but only when the water was being heated up. I noticed the manual opening knob was very very easy to twist to let out more water so I suspected it was getting close to it's rated 6 bar. Checked the pressure of the EV and it was indeed just below 6. I also checked that the diaphragm wasn't blow or the valve wasn't leaking. Relieving all cylinder pressure the EV returned to 3 bar. But when I closed tap and put mains back on the EV would creep up to 4.5 bar without the boiler coming on.
So spoke to the plumber and someone else on this forum and they said it could be the pressure relief valve that was letting by. Made sense, so changed the multibloc. While doing so I made sure the EV was still at a pre-charge of 3 bar. Looking at the old valve and seat on the pressure reducing valve half of it was eroded away so that was 100% an issue.
The cylinder is 300l, and EV is 22l. I posted a thread on this forum about this having read about the 10% rule of thumb but someone worked it out and reassured me the pressure in the EV wouldn't be too high at 65°C.
All in all a good learning experience for a DIY'er.