Plumbing

Banging/hammering/shuddering noise from pipes when taps opened quickly or toilet flushed

Anonymous user 23/02/2024 - 3.38 PM

Note: edited to include PRV/SRV Hello all Apologies if this has been posted already. I looked through lots of Q&As but couldn't find a question with exactly the same problem. I have given as much detail as I can so please excuse the length of this post. We have a two-part hot water system. The old part has gravity fed cold water and a hot water tank without an expansion vessel (that I can see). The new part has a modern (not gravity fed) Santon unvented hot water tank with an expansion vessel. The old part supplies one bathroom upstairs and all the downstairs. The new part feeds two bathrooms upstairs and nothing downstairs. In case it matters, all the taps are monobloc mixers and the new system has a Santon hot/cold mixing valve with (looks like) pressure reducing and relief valves and a Fernox TF1 filter. The problem is a loud hammering noise whenever we open taps quickly (either hot or cold direction) or flush toilets supplied by the new system. The noise starts loud and then dies down over 1-2 seconds. It is loudest right next to the expansion vessel and you can see the pipe shaking there. Using either the old system or central heating doesn't seem to affect noise production on the new system. I initially thought it might be air in the pipes / air chambers needed to be regenerated. I tried draining just the new system, but it doesn't seem to have made any difference. To do this, I closed the mains stopcock, opened the taps/showers from highest (showers) to lowest (bidet), waited until no more water came out, opened the mains stopcock, closed the taps from lowest to highest. All these outlets are upstairs. I did not open any of the outlets supplied by the old system. I am not sure if drained the new system properly. Specifically (1) if I should have drained the old system too and (2) if it mattered that the shower heads were on the floor (i.e. would this make them the lowest outlet)? If we open taps slowly or use the showers there is no noise, it is only when quickly opening taps or flushing toilet (sudden pressure drop?). Also, after flushing (and getting the noise and waiting 1/2 seconds for it to stop), if you open a tap while the toilet is filling it doesn't cause another noise. There are isolating valves on the outlet pipes that are all fully open. If I close any of the valves to just over halfway (around 45 degrees) the noise stops happening for that particular outlet. I could do this on all the valves I can access but I'm not sure if this is a safe/wise thing to do and it makes the flow from the taps annoyingly slow while the toilet is filling. Also the toilet would fill more slowly and I can't access the isolation valves (if present) on one of the toilets or the bath. I would really appreciate your expert advice on what is causing the problem and how to solve it. I'm an engineering nerd (specifically fluid dynamics) so if anyone is willing I would love a technical explanation to help me understand.