How to stop a shower back feeding?

[Jason Moore]

Hi
Can anyone please give advice on our Grohe Smart shower. It initially starts off hot for 30 seconds then goes luke warm, initially tried a new cartridge and it’s still the same and we have also checked the inlets and both clear. We now thing it has something to do with the water feed?

Test Data:
Filled 2 buckets from hot and cold sink taps - they were the same time so therefore it appears to be the same pressure.

Removed the cartridge from the shower and let the mixed water pour out, it starts hot (cold supply delayed) but then quickly goes luke warm when mixed? So the cartridge appears to be fed by luke warm water after one minute and it is unable to go hot again?

The water also comes out the inlets with equal pressure.

hot sink tap temp =55C
Cold sink tap temp =17C

We have a pressurised hot tank fed from the cold.

Is it possible for these Grohe cartridge showers to work in a pressurised unvented hot water system?
Any help grateful received?

 

[Ben-gee]

Is the cold feed from a balanced supply?
The hot water cylinder will have an inlet control group , part of what this does is to lower the pressure of the incoming mains if required. If your cold feed to the shower is taken from before this it could potentially be at a higher pressure than the hot.

 

[John.g]

hot sink tap temp =55C
Cold sink tap temp =17C

With the cartridge removed then the mixed flow should be 36C, if equal flows = equal pressures?.

 

[Devisham]

Any other TMVs on the system? Are check valves in place on the pipework?