Gainsborough Shower Valve Stops Combi Firing For Hot Water - Confused!

[plumbandplumber]

I'm helping my brother who has a strange fault. Not long ago, he had a new Vaillant combi boiler fitted. Not long after, the hot water wouldn't come on when a tap was opened. He phoned the plumber, who suggested that he isolates the water supplies to his Gainsborough shower valve.

He did this, and the hot water started working as normal. He as been unable to get the plumber back, so I want to help him to get his shower working again.

Firstly, what is likely to be wrong with the shower valve? Why does it stop the hot water on the boiler firing up? How can this be corrected?

Any help and advice greatly appreciated.

 

[Chalked]

Probably mixing internally through the valve. Then when you open a tap, the water is taken from the cold side of the mixer instead of through the combi.
It's not stopping it from firing, it's bypasdong it completely.
A check valve on the hot side will stop this. Or replace the shower mixer.

 

[plumbandplumber]

Many thanks. That makes sense. Will try a non-return valve.

 

[Chalked]

Many thanks. That makes sense. Will try a non-return valve.

Just to clarify. The check valve needs to be fitted on the hot supply to the shower mixer only.

 

[plumbandplumber]

OK, will definitely put the check valve on hot side to shower valve.

Thanks again.

 

[Allgoode]

Check valve on the hot supply to shower is the way forwards for this.
Every thing I fit when carrying out bathroom work always has a check v/v fitted to prevent bypass/backflow,
Hope this helps with you issue,
Regards,
Allgoode