Bath/shower mixer hot to bath but goes cold when put to shower?

[Ben-gee]

Going to look at thermostatic Hansgrohe ecomax today, the customer says it’s fine on bath fill but goes cold when put onto shower.
I thought both went through the thermostatic cartridge and am unsure how what see says could happen.
It used to work fine, I can only think cartridge is on its last legs and doesn’t function at the reduced pressure that occurs when going up to the shower head?
Any ideas on what I should be looking for?

 

[Aire]

I'd be checking pressures, when using the bath side of the tap, the resistance will be minimum but while trying to force water through the shower head, the resistance will be much greater and will allow the cold to over power the hot if pressures aren't balanced.

 

[SimonG]

Put shower head in bath to prove what Craig said.

 

[Ben-gee]

Clowns had put it in with tank hot and mains cold, I’m more surprised that it ever worked than that it doesn’t work now.
Other thing is that when you turn it on nothing comes out for 15 secs then it’s ok. I believe the non- return valve on the hot has been compromised by the cold main over the years and is harder to open than it should be, I think it’s been forced further into its housing than it is designed to.
However this could be false.
I hate picking up on someone else’s crappy work...