Water hammer from unvented cylinder

[Johntheo5]

Yeap back again this morning. But it was after I opened the hot tap the cold started doing it again

Well, I suppose time to put your plumbing skills to use again.

You could, as suggested above, supply the "balanced" cold from the mains PRV but may be advisable to remove this and then T it off the mains to supply the cold only, this PRV will then just supply the cold and the ICG PRV will supply the UVC.
Or, as you suggested, install a shock arrestor in the existing balanced cold, however the balanced cold is connected indirectly to a 18L arrestor... the HW cylinder EV, if the hammer is in the balanced cold then the rising pressure will just open the IGV NRV and the EV should then absorb the spike (theoretically anyhow), of course, if it does reach the observed 8bar then the ERV (expansion relief valve) which is set to 6bar should be feathering as well, you might see some signs of this in the tundish. If installing the arrestor in the balanced cold doesn't work, you could install it before the mains supply just before the ICG PRV. Its your choice but repiping from the mains via its own PRV migh be the easiest "solution".

 

[Madrab]

TBH I've never liked shock arresters, all they do is mask the issue IMO and they do need to be maintained, so for me it's just another component that can cause a problem, adds another level of complexity and it's then potentially another one to diagnose.

If the hammer you are describing is that single click when the water is turned off and if the outlet creating it is a 1/4 turn valve on the balanced feed then it's almost a certainty that it's a valve in the control group lifting due to the concussion from the sudden stop of the water flow. If it is that there are 2 options as I see it - change the type of outlet(s) to compression/turn down valves and see if that cures it or bypass the balanced feed altogether.

 

[Taconite5]

Well, I suppose time to put your plumbing skills to use again.

You could, as suggested above, supply the "balanced" cold from the mains PRV but may be advisable to remove this and then T it off the mains to supply the cold only, this PRV will then just supply the cold and the ICG PRV will supply the UVC.
Or, as you suggested, install a shock arrestor in the existing balanced cold, however the balanced cold is connected indirectly to a 18L arrestor... the HW cylinder EV, if the hammer is in the balanced cold then the rising pressure will just open the IGV NRV and the EV should then absorb the spike (theoretically anyhow), of course, if it does reach the observed 8bar then the ERV (expansion relief valve) which is set to 6bar should be feathering as well, you might see some signs of this in the tundish. If installing the arrestor in the balanced cold doesn't work, you could install it before the mains supply just before the ICG PRV. Its your choice but repiping from the mains via its own PRV migh be the easiest "solution".

Ok so one question, if it is a pressure spike causing the hammer you're saying it should go through the NRV and the EV takes up the shock. But what if the HW connection after the NRV is also pressurised - isn't that why we have EVs? Just trying to understand this part.

There's no tundish activity - thats been fine

I get what you're saying about removing the balanced cold connection, I just don't get how other houses aren't having this same problem.

 

[Taconite5]

TBH I've never liked shock arresters, all they do is mask the issue IMO and they do need to be maintained, so for me it's just another component that can cause a problem, adds another level of complexity and it's then potentially another one to diagnose.

If the hammer you are describing is that single click when the water is turned off and if the outlet creating it is a 1/4 turn valve on the balanced feed then it's almost a certainty that it's a valve in the control group lifting due to the concussion from the sudden stop of the water flow. If it is that there are 2 options as I see it - change the type of outlet(s) to compression/turn down valves and see if that cures it or bypass the balanced feed altogether.

Yeah agreed - seen your other posts on arrestors.

It depends on the time of day. Sometimes it's just one click on tap closing. Other times it's with open and closing. The lever taps in the house seem to produce the worst sounds. But then it doesn't explain why this doesn't happen when hot is flowing if it's drawing cold into the cylinder.

 

[dilalio]

There is a blending valve. Never need 100% sure what this does.

Yes hard piped.

Blending valve mixes DHW with cold at the outlets to prevent scalding!

This is a potential culprit!

 

[Johntheo5]

Ok so one question, if it is a pressure spike causing the hammer you're saying it should go through the NRV and the EV takes up the shock. But what if the HW connection after the NRV is also pressurised - isn't that why we have EVs? Just trying to understand this part.

There's no tundish activity - thats been fine

I get what you're saying about removing the balanced cold connection, I just don't get how other houses aren't having this same problem.

The EV will only act as a shock arrestor for the balanced cold if the PRV remains opened during the "shocking" events which it may or may not, that's why I suggest putting the arrestor before the PRV.

The only reason for a Expansion Vessel, a EV, is exactly that, it will accomodate the expansion of the water in the UVC while its heating, if its diaphragm fails or if it loses its air end pressure then the ERV, expansion relief valve, will lift at 6bar (on your UVC), if, for any reason that doesn't lift, the pressure will further increase until the T&PRV, temperature and pressure relief valve which is attached directly to the cylinder, lifts at (probably) 8bar
on your UVC.

 

[Taconite5]

Blending valve mixes DHW with cold at the outlets to prevent scalding!

This is a potential culprit!

Wait so it's not just hot directly from the HW cylinder? I actually thought that mixer was something to do with the shower as it was under the bath!

 

[Taconite5]

The EV will only act as a shock arrestor for the balanced cold if the PRV remains opened during the "shocking" events which it may or may not, that's why I suggest putting the arrestor before the PRV.

The only reason for a Expansion Vessel, a EV, is exactly that, it will accomodate the expansion of the water in the UVC while its heating, if its diaphragm fails or if it loses its air end pressure then the ERV, expansion relief valve, will lift at 6bar (on your UVC), if, for any reason that doesn't lift, the pressure will further increase until the T&PRV, temperature and pressure relief valve which is attached directly to the cylinder, lifts at (probably) 8bar
on your UVC.

Sorry I probably missed this point there is a mini shock arrestor before the PRV, but I think thats the one you were suggesting making a bigger volume?

That's what I'm thinking if during a heating event there is pressure build up and this is just pushing the NRV enough so it doesn't hammer. Then when you run the hot tap the pressure will reduce due to you introducing fresh water into the cylinder.

 

[dilalio]

Wait so it's not just hot directly from the HW cylinder? I actually thought that mixer was something to do with the shower as it was under the bath!

Depends but new homes now have these fitted if its under the bath you can isolate the cold and the hot should stop running as this is the anti-scald protection.

And they do fail btw, so should be tested at least annually.

Post a photo.

It's another place where hot and cold are mixed so pressure and Equilibrium is involved.

Edit: try bypassing this first!

 

[Taconite5]

Depends but new homes now have these fitted if its under the bath you can isolate the cold and the hot should stop running as this is the anti-scald protection.

And they do fail btw, so should be tested at least annually.

Post a photo.

It's another place where hot and cold are mixed so pressure and Equilibrium is involved.

Edit: try bypassing this first!

So this mixer is feeding all the hot taps in the house? Got me wondering now - as I've never checked this!

Picture might be a small wait as I need to remove the bath panel which is sealed in!

 

[denso13]

So this mixer is feeding all the hot taps in the house?

Should be the bath only, not the rest.

 

[Madrab]

But then it doesn't explain why this doesn't happen when hot is flowing if it's drawing cold into the cylinder.

Sure does - hammer will be created when the water goes from still to moving to still again. It's the abrupt water stop/start movement that creates the pressure wave. As water doesn't compress then that shockwave will travel along the water until it dissipates, that can be when it hits a valve and the lifting of the valve uses the energy or it hits something that does compress - like the air pocket in an arrestor/EV - that dissipates the energy, If the water is in motion at the time, then that compression wave won't be created in the first place.

Whilst it may be of benefit to check the TMV (agreed that it is only regulated to protect the bath's HW outlet so should be installed there). If the knock can be created by the 1/4 turn mixer taps then we already know what is creating it. Any valve on that circuit that shuts down too quickly will cause the knock. The reasons why the systems is creating knock can only be down to what is installed onto the pipework, a lot of it can be down to distances too, the longer the runs from outlet to valve, the less chance of hammer as the pressure wave will dissapaite before it gets to something it can rattle.

Ok so one question, if it is a pressure spike causing the hammer you're saying it should go through the NRV and the EV takes up the shock

It will but if it hits a valve before it gets there, it's still going to rattle that valve. EV's are primarily made for absorbing expansion, when cold water heats up, not as shock arresters, that's more of a secondary function. If shock arrestors are to be used then they should be installed as close to the offending outlet as possible, this is to absorb the shockwave as soon as it is created.

 

[Taconite5]

Sure does - hammer will be created when the water goes from still to moving to still again. It's the abrupt water stop/start movement that creates the pressure wave. As water doesn't compress then that shockwave will travel along the water until it dissipates, that can be when it hits a valve and the lifting of the valve uses the energy or it hits something that does compress - like the air pocket in an arrestor/EV - that dissipates the energy, If the water is in motion at the time, then that compression wave won't be created in the first place.

Whilst it may be of benefit to check the TMV (agreed that it is only regulated to protect the bath's HW outlet so should be installed there). If the knock can be created by the 1/4 turn mixer taps then we already know what is creating it. Any valve on that circuit that shuts down too quickly will cause the knock. The reasons why the systems is creating knock can only be down to what is installed onto the pipework, a lot of it can be down to distances too, the longer the runs from outlet to valve, the less chance of hammer as the pressure wave will dissapaite before it gets to something it can rattle.

It will but if it hits a valve before it gets there, it's still going to rattle that valve. EV's are primarily made for absorbing expansion, when cold water heats up, not as shock arresters, that's more of a secondary function. If shock arrestors are to be used then they should be installed as close to the offending outlet as possible, this is to absorb the shockwave as soon as it is created.

Sorry what I meant was if I open and close the hot water tap - you're still getting flow from through ICG but rather than going down the balanced cold connection it's going into the cylinder. It doesn't hammer when I do this. I'm confused though because someone said there's no NRV between balanced cold and ICG inlet so not sure why it's only affecting the cold taps.

I'm homing in on the idea that it might be the NRV in the ICG now...which I've already had replaced once...

 

[Madrab]

I'm homing in on the idea that it might be the NRV in the ICG now...

As suggested - if you hold a large screwdriver onto each part of the control group and create the knock then it will be very apparent what components is knocking.

NRV's are very simple valves with a very light spring holding them so I would be quite surprised if it was that creating that level of knock. The PRV's valve spring is much heavier and it's the slamming back down of the valve on the seat, driven by that strong spring, that would be more likely to create that amount of knock rather than the NRV but hey, never say never.

 

[Taconite5]

As suggested - if you hold a large screwdriver onto each part of the control group and create the knock then it will be very apparent what components is knocking.

NRV's are very simple valves with a very light spring holding them so I would be quite surprised if it was that creating that level of knock. The PRV's valve spring is much heavier and it's the slamming back down of the valve on the seat, driven by that strong spring, that would be more likely to create that amount of knock rather than the NRV but hey, never say never.

Yeah tried that - it's hard to do because the shock is so strong you can hear/feel it on the inlet pipe, balanced cold, cylinder feed and even the EV leg!