Mains pressure domestic hot water problem

[chapcot]

My house has a mains pressure domestic hot water system (no loft cistern). Seems to mostly work well except that there is backflow through the thermo shower mixer (cold water seeps back into hot water pipe when the shower is turned off). Can't understand how this happens as it's an unvented system and pressure ought to be equal on both hot and cold feeds to the shower. Also where does the displaced hot water go to? One thing I have noticed is that the mains pressure reducing valve is fitted on the cold feed to the hot water tank after the point where the mains feeds to the cold taps. Is this correct and could it be the reason for the unequal pressure? Should I just fit a check valve to the hot feed to the shower?
Any advice gratefully received.

 

[jeff the gasman]

That will cure it
Jeff

 

[chapcot]

Fitted new check valve on hot supply this afternoon (yes it is the right way round) but unbelievably when the shower is turned off cold water still creeps back through the check valve and back up the hot pipe. I removed the valve to make sure there was no debris and that it was working and then refitted - but problem still persists. How can that possibly happen ????????

 

[kidgreen61]

Probably best to stick a double check valve on your HW cylinder outlet

 

[nortonj]

Maybe the hot water is set at 3 bar as it should be but the pressure on the cold to your shower is much higher.

 

[chapcot]

Many thanks to all of you who offered advice - much appreciated. In the end it did turn out to be debris (solder crumbs) in the hot feed. When I disconnected the check valve the first time everything seemed to be fine but I think the debris must have fallen out as I disconnected the hot supply from the valve without me noticing. On taking it apart for a second time there was a strip of solder jammed in the valve. Removed the solder flushed through the hot feed and all is now fine. The laws of physics can't be broken.
Thanks once again to you all.