Unvented system - Very loud vibration when drawing hot water.

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EV connection OK.
Not sure what you mean by return, if its the return from the heating coil to the boiler then could be hot/warm.
 
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Ok thanks

It's just labelled Return on the cylinder. Close to the Cold on the cylinder.
 
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In the morning, if you are going to do my suggested test, first have a look to see that the exp.valve isn't dripping, if the PRV is leaking past then if it increases the filling pressure to ~ 4.5bar (would require 4.9L leakpast overnight) the reheat will rise the pressure close to the 6bar exp.valve lifting pressure, that in itself shouldn't cause vibration on the rapid opening of a hot tap but its a starting point anyway to drop the pressure gently and then open the HW tap rapidly.
 
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As suggested in John' description

Pipe heading up (purple) is the balanced cold feed out to the outlets
Pipe going out to the left (blue) is the cold water supply to the unvented cylinder - is that a red handle gate valve on that pipe, if so then there shouldn't be any valve on that pipe after the control set/PRV - if there is take the head off it to avoid anyone turning that off. It shouldn't be there in the first place as it can interfere with the UV safety systems if someone inadvertently shut it down
Pipe heading out the back (green) is the pressure relief valve.

The coil (CH) return wil be warm when the cylinder is warming up and will get hotter as the cylinder reaches temperature
The secondary return will be hot as the pump circulates the HW around the system.
 
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A couple of things sound a little odd so, what you can do, is get hold of the installation instructions (online) and follow the pipes to satisfy yourself that it's connected up correctly. You'll see that gate valve as said, is wrong, and there should be an isolating valve on the inlet. (full flow ball valve is best).
Seeing all that PTFE tape in "amateur" places is disconcerting!
If it's the pressure reducing valve making the noise you should be able to feel it a little it any time, even if you can't hear it. Water pressure varies through the day/night which will affect that. A glamorous assistant at the stop cock may be necessary for testing, as you're missing the inlet valve.
Don't worry about precise pressure calculations. Mains pressure variations and unknown inlet and outlet pipe resistances make a difference, and they aren't very useful anyway (especially if they're not right ;) ). The optimum precharge pressure is debatable. Some manufacturers say some specific amount, say 0.3 bar below the incoming mains pressure, but mine varies between 1.7 and 4.2 bar,...etc. Below 3bar dynamic is normal at peak times.
 
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Its a pity that unvented cylinders don't come with a pressure gauge as standard like pressurized gas/oil boilers, I know that the house owner can top up a gas/oil boiler EV and that a UV cylinder is "topped up" with the mains and any air end charge readings/resetting have to be done by a G3 and he can find a tapping point somewhere to attach his gauge, if necessary.
 

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EV, with air on the upper side of a rubber diaphragm.

The vessel you refer to is only used on heating systems. Vessel in question has a bladder which is surrounded by a cushion of air ( expansion medium)
 

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Also - that pinkish circulator pump should be dead horizontal, because they don't have endthrust bearings. Doesn't appear to be.

Grundfos instructions suggest otherwise. Shaft can be dead horizontal or bleed point to be like shown in the picture is correct also.
 
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The vessel you refer to is only used on heating systems. Vessel in question has a bladder which is surrounded by a cushion of air ( expansion medium)
The only one, Red, 12L, that I removed from a oil fired boiler just had a bladder that was sandwiched with a flange at the water end, it doesn't seem a great arrangement as the bladder is then pushed back (crushed?) against the water end on precharging and will only be partially refilled with 2.4L of water with a 1.5Bar filling pressure (1.0Bar precharge pressure), I'm surprised they have a reasonably long life but obviously if inserted the "right" way, from the air end (if it was possible) the the EV might corrode fairly rapidly as water would now be in contact with the mild steel interior, instead of nitrogen and eventually air.
I have seen photos of others (white & red?) where a diaphragm is sandwiched between two "halves" of the EV, seems a better arrangement but maybe only used on the larger EVs?.
 

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The only one, Red, 12L, that I removed from a oil fired boiler just had a bladder that was sandwiched with a flange at the water end, it doesn't seem a great arrangement as the bladder is then pushed back (crushed?) against the water end on precharging and will only be partially refilled with 2.4L of water with a 1.5Bar filling pressure (1.0Bar precharge pressure), I'm surprised they have a reasonably long life but obviously if inserted the "right" way, from the air end (if it was possible) the the EV might corrode fairly rapidly as water would now be in contact with the mild steel interior, instead of nitrogen and eventually air.
I have seen photos of others (white & red?) where a diaphragm is sandwiched between two "halves" of the EV, seems a better arrangement but maybe only used on the larger EVs?.
Vessel used on a heating system cannot/ should not be used on an invented cylinder.

In a heating system the system water is already polluted. Water from the hot water cylinder needs to be clean as possible, not discoloured when wrong vessel used.
 
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Choice of materials for HW vs CH I suppose, a HW (potable water) EV can be used for CH . I'm thinking that a diaphragm type will give (should) smoother operation when a hot tap is opened suddenly after a reheating period, the one in this thread does look like the diaphragm type?.
 
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Yep basically it's down to materials used - diaphram is different type of rubber and steel vessel for unvented is either coated internally or made from stainless.
 
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The other option used more these days due to the advance in materials used in for EV's is they use a temp resistant EPDM bladder. With these it's the bladder that fills with water and as it expands it compresses the gas in the space within the vessel. That way the water itself never comes into contact with the water minimising corrosion and the bladders are usually replaceable.
 

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