Hi, I had exactly the same problem of knocking shower valve after we renovated our bathroom during fall 2020. That caused us a lot of stress and problems. The valve installed was a generic pressure balanced valve made in China. I was supposed to have a nice Italian valve installed but the designer finally opted for a generic valve. Each time we were doing the laundry, using the dishwasher or even flushing the toilets, there was a double knock behind the shower. Sometimes, it was very loud, especially when we were using the washing machine or late at night when the water pressure was higher. We first installed water hammer arrestors everywhere in the apartment, 14 in total but it did not resolve the problem. The colder the water was the louder was the double knock. I almost imstalled a pressure reducer but since de pressure was 60 psi, I did not. Anyhow, the condo syndicate did not allow and we did not have problems before renovating. I then got my dishwasher inspected but it was ok. We then isolated the shower valve from the rest of the plumbing by closing the two valves or soupapes in French on each side and then, the knocking miraculously stopped. After two weeks without using the shower, the plumber came to the conclusion that it was the shower cartridge. We changed the cartridge twice but the problem was still the same. Finally, I decided to install water hammer arrestors on the shower. The plumber and contractor thought that they were not required but I insisted. We made an opening in the wall from the bedroom closet behind and installed two water hammer arrestors on the shower but it did not resolve the problem. Finally, I decided to change the valve. I called at many locations and nobody could explain what was going on with my plumbing. I then called Baril Design in Montreal and the sales representative identified immediately the problem. It was the piston of the valve that was too sensitive to pressure variations . He experienced quite a few similar cases before. He proposed me a Thermostatic pressure balanced valve. Such valves have a completely different mechanism. I double checked with another plumber and he confirmed that the problem was the valve. It was not of the best quality and also, it was too small and not robust enough. He told me “I would not install that in my house”. I finally bought the thermostatic pressure balanced valve and got it installed in late March 2021. So far so good. We don’t hear any noise from the shower valve anymore. We keep our fingers crossed! Things are back to normal after spending almost 6,000 CAD$ with the plumbers, for the water hammers, for the valve and for rebuilding the wall and closet. If a better valve had been installed at first, maybe we would have saved all the stress and money and some sleeping pills!
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