Wait isn’t the NRV between the balanced cold and the UVC EV/cylinder feed? It’s what stops hot water going back into the balanced cold?
Water hammer from unvented cylinder
- Thread starter Taconite5
- Start date
Sure can - which bit do you want to see?
It is, but when you shut a tap taken off the balanced cold and when/if the pressure builds up very rapidly due to the water flow stopping abruptly then the PRV will close very rapidly, the pressure is then trapped in the mains up as far as the PRV but any pressure that may have built up in the balanced portion will be relieved as the NRV will open but this doesn't do anything to relieve or stop the pressure build up in the mains portion.
Thanks that diagram really helps!
I'm wondering if it's the balanced cold causing the problem then. I'm going to fit a T here and see if I can take some pressure readings.
Do you have a toilet valve that shuts off quickly?
That's a good idea, you might then take a few readings with various HW only draw off flowrates and ditto from the balanced cold.
Why is there another PRV installed?, is it because the mains pressure exceeds the max (I think) pressure ratio of 3:1, ie, a max mains pressure of say 8 to 9 bar, have you measured this mains pressure?, if its say 5.5bar or less then it might be better to remove the extra one.
Water hammer is caused by hydraulic shock, invariably where the pressure is higher and there is a valve that shuts down quickly. That quick shut down causes a pressure spike and creates a pressure wave and that will travel along and rattle any valves or loose pipework that it encounters until it dissipates - that knocking is the water hammer - invariably 1/4 turn or solenoid valves are the known culprits
The trick is to find what valve it is that is causing it, so it's down to tracking the pattern of the hammer and being able to replicate it. There may be an issue when the balanced cold is running and something then impedes or shuts down the flow and the wave is travelling back up and bounces the Pressure Reducing Valve on the control group. These PRV's are known to knock when the flow suddenly reverses, it's similar in behaviour to a directional TRV that is flowing the wrong way and the spring of the valve resists the flow and the valve bounces on the seat, creating the knock.
When it knocks can you put a large screwdriver on the PRV and hold it to your ear, you will hear the valve knock quite clearly, if that's what it is.
The trick is to find what valve it is that is causing it, so it's down to tracking the pattern of the hammer and being able to replicate it. There may be an issue when the balanced cold is running and something then impedes or shuts down the flow and the wave is travelling back up and bounces the Pressure Reducing Valve on the control group. These PRV's are known to knock when the flow suddenly reverses, it's similar in behaviour to a directional TRV that is flowing the wrong way and the spring of the valve resists the flow and the valve bounces on the seat, creating the knock.
When it knocks can you put a large screwdriver on the PRV and hold it to your ear, you will hear the valve knock quite clearly, if that's what it is.
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