I have an unvented hot water system.
Following a recently fitted shower I noticed our hot water all over the house was coming out tepid--->hot.The worst taps are those nearest to the shower. I cannot use the shower yet because the water won't come out hot enough!
British Gas came out today and diagnosed back feeding through the new shower.The elbow to the hot supply of the shower goes immediately cold when you turn off the shower and then run the sink tap. If I run the shower I can get hot water from the sink tap! Is this a faulty shower with a dodgy non return valve or can it be fixed by reducing the cold water pressure??
I am assuming the hot and cold water supplies should be at similar pressures. Is there a risk to the hot water cylinder if the cold is back feeding to the tank--I note that those that have had a similar problem in a gravity fed system have had header tanks overflowing! I have no way of isolating the cold supply to the shower!
Any advice appreciated!
The plumber is coming back on Monday to look at the shower mixer.

The cold is not balanced properly rather than non return valves you need a pressure reducing valve of the same pressure on the incoming main. When I do unvented i always do that.

By doing this you will mitigate against future problems with mixer taps etc.