I think you're required to have double non-return valves in both the hot
& cold supplies to the shower, to avoid exactly this potential issue.


In article . com, prb95
@hotmail.com says...
replies to my mixer shower fitting questions has prompted more.
Currently my bath has a mixer tap, hot from tank, cold from mains. If I
turn the cold on full (with the hot still on) it runs cold (obviously)
but the hot then takes a good while to come back, could this be due to
high pressure cold forcing the hot back to the tank, and is this a
problem? I was planning to pipe cold from the header tank, is there any
alternative so i can use the mains feed, such as pressure equalising
valves (such as the ones Screwfix sell), non-return valves etc? Thanks
for all help