Water hammer

[Taconite5]

Hi,

I've noticed since the install was about 6 months old I'm getting a water hammer/knocking/clunking noise from my unvented cylinder whenever the cold taps in my house are open and closed.

I've had two G3 plumbers come out to have a look at and they say they will come back once they've spoken to the manufacturer and then I hear nothing. They both checked the charge pressure on the expansion vessel and said it was fine. The original installer said it needed a service to fix this but both plumbers said the service won't do anything.

It seems to be coming from around the multibloc/expansion vessel location.

Not that I'll be messing with any of this myself but is there anything else I can suggest to a plumber?

 

[Johntheo5]

Did the check the PRV (pressure reducing valve) pressure and setting, this is usually set to 3.0bar, you can do a few tests yourself if so inclined and you have a tyre pressure gauge.
Shut the mains isol valve before the multibloc and open a hot tap to depressurize the cylinder, attach the PG to the expansion vessel air end, it should then read the charge pressure, reopen the isol valve and open a hot tap very slightly to give a dribble of water, check the pressure again, then open a hot tap fully and again note the pressure.

 

[Taconite5]

Did the check the PRV (pressure reducing valve) pressure and setting, this is usually set to 3.0bar, you can do a few tests yourself if so inclined and you have a tyre pressure gauge.
Shut the mains isol valve before the multibloc and open a hot tap to depressurize the cylinder, attach the PG to the expansion vessel air end, it should then read the charge pressure, reopen the isol valve and open a hot tap very slightly to give a dribble of water, check the pressure again, then open a hot tap fully and again note the pressure.

Hi - they didn't. They said "this should be pre-set from the manufacturer" - it says 3bar on it.

From memory it seems to read 3 bar on the air end no matter what I do - but I can check again.

Btw random question - how does the system remove the air below the expansion vessel given it's the highest point in the system?

 

[Johntheo5]

Ideally, the PRV set pressure should be set to the (dynamic) mains pressure with a big HW demand, your pressure may only be 2bar or so, it might even only have a static pressure of 2bar (even if unlikely), even so, the air end pressure will rise while heating takes place as water is incompressible, setting the PRV dynamically of course may not cure the problem. The EV prepressure shuld also be reduced to the dynamic pressure.

The EV is generally connected to the cold mains after the PRV so even though the connecting pipe/water end will have some bit of air it won't have any huge effect on the EV pressure after reheat since the air end vol, is still very large in comparision to the "pipe end" air vol. A 200L UV cylinder may have a 20L EV, the air end vol after precharging to 3bar is still 20L, (asuming water pressure 3.0 bar or less), when/if the full vol of 200L is heated from say 15C to 60C then the expanded vol of 3.34L (1.62%) will reduce the air end vol to 16.76L. (and a final pressure of 3.77bar,)

 

[Taconite5]

Ideally, the PRV set pressure should be set to the (dynamic) mains pressure with a big HW demand, your pressure may only be 2bar or so, it might even only have a static pressure of 2bar (even if unlikely), even so, the air end pressure will rise while heating takes place as water is incompressible, setting the PRV dynamically of course may not cure the problem. The EV prepressure shuld also be reduced to the dynamic pressure.

The EV is generally connected to the cold mains after the PRV so even though the connecting pipe/water end will have some bit of air it won't have any huge effect on the EV pressure after reheat since the air end vol, is still very large in comparision to the "pipe end" air vol. A 200L UV cylinder may have a 20L EV, the air end vol after precharging to 3bar is still 20L, (asuming water pressure 3.0 bar or less), when/if the full vol of 200L is heated from say 15C to 60C then the expanded vol of 3.34L (1.62%) will reduce the air end vol to 16.76L. (and a final pressure of 3.77bar,)

OK did the test above it shows 3 bar consistently no matter what I did

 

[Johntheo5]

Not much more you can do so, it is of limited value as it only proves that the mains pressure after the PRV is 3.0bar or less, only other suggestion is to ensure no one runs off any hot water when the cylinder is reheating and towards the end of the reheat period check the pressure again, it shouldn't rise to more than say 3.5bar to 3.8bar.

 

[Taconite5]

Not much more you can do so, it is of limited value as it only proves that the mains pressure after the PRV is 3.0bar or less, only other suggestion is to ensure no one runs off any hot water when the cylinder is reheating and towards the end of the reheat period check the pressure again, it shouldn't rise to more than say 3.5bar to 3.8bar.

Is there any scenarios where the mutlibloc would cause hammer?

 

[Johntheo5]

One would think that opening and closing cold water taps shouldn't cause the multibloc to then cause a problem, you could install a shock arrestor on the cold water supply, but you need to know the mains pressure in order to set the arrestor vessel charge pressure

 

[Taconite5]

One would think that opening and closing cold water taps shouldn't cause the multibloc to then cause a problem, you could install a shock arrestor on the cold water supply, but you need to know the mains pressure in order to set the arrestor vessel charge pressure

Thanks - what I don't get isn't that what the EV is suppose to do?

 

[Johntheo5]

Thanks - what I don't get isn't that what the EV is suppose to do?

The Cylinder EV is purely for accomodating the expansion volume when the cylinder heats up, it may help to dampen water hammer., there is a non return valve in that multibloc which prevents any backflow through it, a shock arrestor (EV) should have a unristricted connecting pipe to the mains or whereever.

Can/have you tried opening/shutting cold water taps with the mains isol valve before the multibloc shut off?.

But if the cold water is taken off as a balanced feed from the multibloc then above test doesn't apply.

 

[Taconite5]

The Cylinder EV is purely for accomodating the expansion volume when the cylinder heats up, it may help to dampen water hammer., there is a non return valve in that multibloc which prevents any backflow through it, a shock arrestor (EV) should have a unristricted connecting pipe to the mains or whereever.

Can/have you tried opening/shutting cold water taps with the mains isol valve before the multibloc shut off?.

I see. So there is a 3bar shock arrestor after our main stopcock which enters under the kitchen sink. But the cold inlet for the cylinder is upstairs in a cupboard. Are you suggesting a shock arrestor before or after the multibloc?

Yes - that just cuts the water off coming through the taps if I do that.

 

[Johntheo5]

So you've seen my edited post above?, ie the balanced cold is taken from the multibloc?

 

[Taconite5]

So you've seen my edited post above?, ie the balanced cold is taken from the multibloc?

Sorry - yes it is.

 

[Johntheo5]

Sorry - yes it is.

It can be very difficult sometimes to pick the right location for the shock arrestor, have you checked the one under the sink, ensure that there is air at the air end and that the diaphragm hasn't failed and it's full of water, also test it's pressure, if possible with mains isolated and depressurized and again with the mains deisolated.

 

[Taconite5]

It can be very difficult sometimes to pick the right location for the shock arrestor, have you checked the one under the sink, ensure that there is air at the air end and that the diaphragm hasn't failed and it's full of water, also test it's pressure, if possible with mains isolated and depressurized and again with the mains deisolated.

Yeah checked that!

I was just having a play now and I'm wondering if the hammer is coming from the multibloc - you said there's a one way valve in there? Could these make noises?