Expansion tank - charging and system pressure

[Last Plumber]

It could be a few things one of them of course being a faulty governor as Best said but I suspect an issue with the unvented cylinder set up or something related to both that and mains cold.
An experienced person qualified to G3 requirements will be able to solve your issue fairly easily in my opinion. It doesn't sound too difficult to be honest.
Don't be reluctant to ask about peoples qualifications or experience!

 

[bacon sarnie]

We've got an engineer coming out early next week. There's a lot more banging and vibrations in the pipes since the first visit, so I'm hoping it isn't too tricky too solve, and that the first engineer just over pressurised the expansion tank.

I totally agree with what you say about asking about experience. Though on the other hand, that doesn't always help. One comically bad example was when we had the water hammer, and mentioned to one plumber about fitting a PRV. He basically said we didn't need one, and turned on a tap full and then halfway to demonstrate the difference in flow we would experience. And that would be only difference we would see. This was a guy who claimed 35 years experience as a plumber. We didn't use him, unsurprisingly.

 

[Last Plumber]

We've got an engineer coming out early next week. There's a lot more banging and vibrations in the pipes since the first visit, so I'm hoping it isn't too tricky too solve, and that the first engineer just over pressurised the expansion tank.

I totally agree with what you say about asking about experience. Though on the other hand, that doesn't always help. One comically bad example was when we had the water hammer, and mentioned to one plumber about fitting a PRV. He basically said we didn't need one, and turned on a tap full and then halfway to demonstrate the difference in flow we would experience. And that would be only difference we would see. This was a guy who claimed 35 years experience as a plumber. We didn't use him, unsurprisingly.

Shameful!

 

[bacon sarnie]

Hi

I just thought I'd add an update to this problem from a couple of months ago. Partly as I have a question, and partly because I think what's been done could be helpful for someone in the future.

Anyway I did some investigating, and found that the general recommendation is for the potable expansion vessel to be situated on the cold water line in to the unvented cylinder. Ours was on the hot side. It had always been on that side, but we thought it would be worth someone trying moving it over to the cold side. We weren't necessarily expecting it to fix the problem (and it didn't). So then we went down the route of having the check valve just upstream of the expansion vessel replaced. That did stop the pressure rising on the cold water supply. So it seems that the old check valve had failed.

So the main issue is resolved, but we still notice that if the hot water has been on, and then a hot tap is used, there is an initial 1 to 2 second burst of high hot water pressure, before it settles down to the required flow. Is the initial high hot water pressure to be expected on an unvented system? Or is it indicating there is still an issue? Maybe the charged pressure in the expansion vessel isn't correct? As it was a new vessel it had the shipped charge of 3.5 bar. I kind of assumed the vessel would manage the pressure increase on the hot taps, even though it's on the cold side of the cylinder. But that could be a wrong assumption.

We are going to run this by our engineer, but having investigated unvented systems on the internet myself, there seems to be quite a bit of contradictory information. So any thoughts or comments are much appreciated.

 

[Best]

The air charge in vessel has to relate to the actual cold pressure on unvented unit. I don't want to say any details on open forum.
You can expect a burst of water for first second from a hot tap, but it will be made more forceful the higher the pressure in unit increases to, which, amongst other faults, can be result of the unvented expansion vessel air charge being wrong, as you suspected.
Really should have been set correctly by the G3 plumber, but I guess the qualification doesn't mean experience, myself included.
Getting slightly off the point, but just thought I would mention that IMO taps with airators suitable for high pressure, are better so to control the flow from the tap and save water