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Hi all. This is my first post, and I'm a homeowner and not a heating engineer or plumber, so apologies if my knowledge is a bit off.

To give a bit of history we had a problem with water hammer in our pipes so had a pressure reducing valve fitted under the kitchen sink. The local water company had measured the water pressure coming in to the property at 4.5 bar, and when the valve was fitted and set at 2 bar, the water hammer problems stopped. So far so good.

After a while we started to get what was seemingly randomly timed loud vibrations in our pipes. It soon became apparent it only happened when opening the kitchen sink mixer tap, but not all the time. Over time it settled into a pattern of hot water going on, the pressure reading on the valve slowly rising from 2 to over 4 bar, and then when the kitchen tap was turned on, a loud vibration, and the pressure going back down to 2 bar. But again the noise didn't happen every time the pressure dropped. I'm not sure about this, but having looked at a few websites, the noise may have been the valve trying to handle the backflow pressure out of the system?

Anyway the noise is a problem, but the main concern was the rise in pressure (get rid of the cause, and get rid of the effect at the same time). Having looked into possible causes we settled on the white expansion tank on the unvented cylinder upstairs not doing it's job. So we had a heating engineer come out to have a look at it (under our annual maintenance contract). He found that there was no pressure in the tank, and recharged it. Since then, not only has the pressure risen as before when the hot water is on, but it also slowly rises to 3.5 to 4 bar when the hot water is off. Again using the kitchen tap at this time may result in a loud vibration and pressure drop to 2 bar. The central heating isn't being used at the moment.

So we're planning to get an engineer back, but I'm trying to understand what's going on before we do. The engineer spotted some info on the side of the expansion tank, eg. 6 bar max pressure, and also a picture showing 3.5 bar. So he pressurised the tank to 3.5 bar. Should he have actually pressurised the tank to 2 bar to match the pressure reducing valve setting? My basic understanding would be that when the hot water is off, and the water in the system cools and lowers pressure, the higher air pressure in the tank will push water out of the tank, and raise the water pressure in the pipes above the 2 bar we want. I'm also wondering if the 3.5 bar icon refers to the pressure the tank was shipped with after manufacture, not a suggested pressure. It's not easy to tell as the pictures on the side of the tank are partly obscured by the centre seam of the tank. Or does this behaviour suggest something is wrong with the tank?

Not really having any experience in this area what I'm thinking could well be completely wrong, but on the other hand I'd really appreciate any comments or suggestions that I can put to the engineer. Ideally having an annual maintenance contract should mean that we can depend on their engineers' advice, but I'm not sure what the first engineer did was actually the right thing to do. Thanks, and sorry it's a bit long!
 
Did he drain all the water out the expansion vessel before he re set the pressure in it to spec ?
 
I am wondering is the plumber G3 qualified for working on unvented cylinders and their parts?
The pressure reducing valve you have had later installed seperately could be faulty, or wrong type. The pressure on mains cold should stay no higher than what is set. If the PRV allows the static pressure to go too high, you could find the symptons you have of pipes rattling when a tap is opened and closed.
The unvented cylinder pressure is different in that it will go slightly above its static pressure when the water inside heats and expands, even when expansion vessel doing its job properly.
 
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Best - I don’t know if the plumber who fitted the PRV is G3 qualified unfortunately. He fitted the PRV and changed some taps for us, and didn’t do anything else. I can attach a picture of the PRV if it would help? It has RBM on the face of the gauge. Excuse my ignorance here, but what could be the right (or wrong) type of PRV for the situation?

The heating maintenance contract is with British Gas, so I’d hope their engineers are appropriately qualified. But like I say, I have this query on whether the pressure was set too high in the expansion tank on their first visit. I guess a British Gas engineer should also be able to advise if the PRV is appropriate for the system?

Last Plumber - Why do say that the engineer sounds a bit suspect? Just curious. Particularly as we're paying for the maintenance cover, and would hope we're getting reliable info!

Thanks for taking the time to reply by the way.
 
Best - I don’t know if the plumber who fitted the PRV is G3 qualified unfortunately. He fitted the PRV and changed some taps for us, and didn’t do anything else. I can attach a picture of the PRV if it would help? It has RBM on the face of the gauge. Excuse my ignorance here, but what could be the right (or wrong) type of PRV for the situation?

The heating maintenance contract is with British Gas, so I’d hope their engineers are appropriately qualified. But like I say, I have this query on whether the pressure was set too high in the expansion tank on their first visit. I guess a British Gas engineer should also be able to advise if the PRV is appropriate for the system?

Last Plumber - Why do say that the engineer sounds a bit suspect? Just curious. Particularly as we're paying for the maintenance cover, and would hope we're getting reliable info!

Thanks for taking the time to reply by the way.

A picture would be great
 
I've attached a couple of images. A 'before' one with the pressure at about 3.5 bar, and then an 'after' one when a tap has been opened and closed. I hope you can see enough of the valve in the pictures? Otherwise I'll need to empty the cupboard and pop out the shelf.

before.jpg
after.jpg
 
The water regulator you have will regulate the pressure of water flowing. It may creep over the set pressure a little when standing but not normally by much.
Do you know if that water governor is regulating the water for all the house or is it just for the kitchen sink?
 
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!
 
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.
 
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!
 
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.
 
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
 
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