unvented water hammer problem

[Nat]

Hi all some help please.I am staying with some friends in Somerset and they have a problem with their unvented direct cylinder. When you run the hot in the bath or in the shower (but not kitchen sink or basin) after about 15 seconds there starts a load of water hammer. Really starts banging away and then stops after a bit. There is no cold taken after the pressure reducing/ strainer valve so it is not balanced. My initial thought was could.be the multi function valve. Checked strainer but was clean. Turned off cold supply to cylinder and let off pressure via the bath tap and still got the hammer noise start quickly and then slow down as the pressure dropped off so in my mind that rules out multi valve as there's no water passing through it. It seems to be coming from expansion vessel but why would it do that? Have checked at schreader valve and air does come out but I have no guage to check pressure. I am actually on holiday and obviously don't have tools with me apart from a few odd bits my hosts have. I would like.to sort this for them if possible before I leave tomorrow. Does anyone have any idea what this could be? Thanks in advance.BTW I have unvented ticket but mostly do installs hardly ever fault find.

 

[Gary71]

The expansion vessel needs to be as close to the cold inlet of the cylinder as possible, the position of the vessel and pipe work design can cause noise issues especially if the water side of the vessel is full of air.

 

[BLOD]

Measure the static pressure at a hot tap and then the dynamic pressure at the same tap then drop the expansion vessel pressure to about halfway between these two.

 

[kimbo]

In my experience I have found that the problem lies with one of two option,s the expansion vesell flexible hose, if it is that then using fixed pipework cures this.
Secondly The check valve in the combination valve causes the same symptoms and it will need changing.

 

[Nat]

Measure the static pressure at a hot tap and then the dynamic pressure at the same tap then drop the expansion vessel pressure to about halfway between these two.

Thanks blod. Not sure what you mean by 'dynamic pressure'. How do you measure this?

 

[Nat]

In my experience I have found that the problem lies with one of two option,s the expansion vesell flexible hose, if it is that then using fixed pipework cures this. Secondly The check valve in the combination valve causes the same symptoms and it will need changing.

Thanks for your reply. The expansion vessel is piped in hard with copper in 22. Also if you see what I wrote in op I think I have eliminated problem with the check valve as I turned off lever valve before the check valve and then let the pressure of the cylinder discharge from bath taps. It still made the noise and obviously there was no water flowing through the multi valve. See what I mean?

 

[BLOD]

Dynamic pressure - stick pressure gauge on a hot outlet and take reading while another hot outlet is running

 

[kimbo]

I understand what you are saying but by releasing the pressure l from the cylinder you are still applying a suction on the check valve. To eliminate it you will need to stop the water after the check valve and then you can be sure it is not that.

 

[Nat]

Have attached couple of pics. Dunno if that helps. The feed for the cylinder drops from the multivalve up top all the way down to bottom of cylinder unclipped. Initially I thought it could be that but its not. I am convinced its the expansion vessel but why would it be making that noise? Don't think I am gonna be able to do anything about it without my tools anyway but would be nice to tell them what they need to do before I go.

 

[kimbo]

Is the exp vessel taken off the cold feed before the comb valve if so that is wrong it wont take up any exp from the hot water cylinder.

 

[Nat]

Dynamic pressure - stick pressure gauge on a hot outlet and take reading while another hot outlet is running

So if the static pressure is 3 bar and dynamic is 1bar I need to pump 2 bar of pressure into the expansion vessel? Never knew this. Thanks, bliss.

 

[mikegas]

Measure the static pressure at a hot tap and then the dynamic pressure at the same tap then drop the expansion vessel pressure to about halfway between these two.

Do you set up the vessel on all unvented cylinders like this or only when you get this problem? ive often wondered because i was taught on the course to match the Prv setting (3bar) but ive since been told to match the incoming pressure like you say

 

[Nat]

Is the exp vessel taken off the cold feed before the comb valve if so that is wrong it wont take up any exp from the hot water cylinder.

No its not. See pic I just posted. I would've worried myself if I hadn't noticed that! Any other ideas kimbo?

 

[BLOD]

yes set i do them all like this - it wasn't mentioned on the course i did either
Been to quite a few noisy unventeds and 9 out of 10 times setting them up like this has worked.

 

[kimbo]

Cant see how this works.

 

[Gary71]

I have found after all the little tricks have failed me and with a expansion vessel fitted in this manor then repositioning the expansion vessel to beside the cylinder can put it to bed, it's because the vessel can have air trapped on the wet side of the expansion vessel, the air is compressed (more so left over night) and can generate noise either leaving the expansion vessel or pushing against the combination valve causing a shudder, this will repeat it's self as the air cannot clear due to pipework design. Todate any installations where I have asked to reposition the expansion vessel have had no issues after.