The situation involves 2 'low-level' WC toilet cisterns, one upstairs and one on the groundfloor of a domestic residence.
The upstairs WC has had a "double service check valve" fitted to the copper pipe just before it enters the cistern ever since it was fitted in 2005 and there have never been any issues with it.
I have recently fitted a "double service check valve" on the copper pipe leading into the ground floor WC and ever since doing so there is now a groaning/straining sound inside the pipe whenever either the ground floor or the upstairs toilets are flushed.
This groaning/straining sound is coming from somewhere around the "check valves" on the pipes and lasts for about the last 5 - 10 seconds before the flushed cistern is finally full again.
The problem only started after I fitted that second "check valve" on the groundfloor WC.
I have already tried adjusting the float arm mechanism in each cistern but this has not cured it. In any event, the problem definitely seems to be in or around the check valve(s) on the pipes and not within the cistern(s).
I have also already tried taking off the new "check valve" downstairs and cleaning it inside but again the problem still persists.
I don't think there is an airlock in the pipework either - none of the basin/bath taps coming off the same pipework have been affected in any way and, furthermore, the pipework was flushed through with water while I tried cleaning the new "valve".
Can anyone suggest a likely cause for this newly-developed groaning/straining noise?
The screw in the centre will NOT allow you to turn the water on or off. Its not what its there for. Its a test point for monitoring the operation of the downstream valve. If you have fitted one of these (it is the right way round isn't it?) then you should remove it and replace with a service valve.
Thanks for your reply.
The 'double check valve' shown in your first link is what are fitted to both W.C.'s. I also have service valves fitted on both W.C.'s as well.
The reason I fitted the double check valves is because I had previously read in a number of places that they were required for toilets to prevent the possibility of back-siphonage from the cistern in the event that it malfunctions. If it transpires that they are not legally required by water regulations then I will remove them and see whether it solves the new noise problem.
In any event, thanks again for your consideration of the issue.
It's very unlikely back siphonage can occur from the cistern back into the mains because of the air gap between the outlet and the water level, (ie the outlet will not be submerged in the water), and there's definately no chance of back-siphonage from the toilet pan back into the mains water supply. Remove the NRV/double check valves - they're not needed.
It's possible to litertaure you have read was relating to a bidet ?
Thanks for your reply.
From what I can recall, the books I read did recommend check valves specifically on the toilets although, as this was some years ago, I cannot recall exactly what books they were.
In any event I will remove the valves and see how it goes. Thanks for your further consideration.