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Discuss Water hammer caused possibly by cold feed crossing into hot feed in the Plumbing Forum | Plumbing Advice area at PlumbersForums.net

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5
Have a water hammer that can't get to the bottom of and have searched everywhere. Hammer occurs when hot tap 1st runs the noise comes from the hot water tank cupboard thought it was the expansion vessel so had it replace went away for a short time but returned. Had the vessel checked again and was not this. Did notice that when the hot water is isolated and the hot tap is run the hot runs out and then cold water flows. Advise was could be shower valves letting the cold into the hot system. Can't isolate the shower cold fed though and have 2 showers with thermostatic valves. Ran both 1 let cold out no matter where the tap was on the position 1 ran for a second and quickly stopped as it seems to work out there was no hot feed. Do I change the valve ? Also told to fit check valves but not where to put them to help the issue as can get to the shower cold feed pipes
 
Messages
1,953
Can you provide a few photos of your pipework and tank, please?

Also, a more detailed description of the noise. (E.g. a series of bangs, like a hooter, etc.)

Keen to help but 'water hammer' has a lot of possible causes...
 
Messages
1,953
Hmm. To me that sounds like the standing pressure in the DHW store is is too high. Possibly because there is not enough air in the expansion tank (what pressure was it precharged to when it was installed?) possibly due to cross-flow from an unbalanced cold feed.

The cold feed to the shower should be at the same pressure as the feed to the tank. If, however, the former has been picked off before the presssure reduction valve and there is crossflow then that will lead to the pressure in the tank being higher than it should be.

My advice would be to get a plumber with a G3 ticket to fix the cross-flow issue and if that doesn't fix the hammer they are qualified to check everything related to the DHW tank while they're at it. Also get them to check that the way the pressure relief pipework has been done complies with the regulations while they're at it.

Don't try to DIY this, please, these systems are dangerous if they are not installed and commissioned correctly so you need someone who knows what they are doing to investigate.
 
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5
Thanks had 2 g3 guys in so far. Checked the pressure balance qas OK and checked the expansion vessel and re pressurised it but was fine. Oddly when I turn off the hot water drain the pipes which then has cold come out and turn back the hammer goes away for a week. Do you think I should replace the thermostatic valve in the shower ? Or fit check valves as that was part of the advise
 

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