Boiler, or shower problem?

[Robert Tyrrell]

Good morning lovely people,
I went to look at a shower that wasn't working correctly yesterday, and I'm not certain if its a shower problem or a boiler problem so I'm posting here in case.
If any of the mods want to throw this into another forum then please do so, but let me know so I can follow it

My problem is this:
The shower works fine as hot, or cold - but nothing in between. It's a brand new shower (See photos) bought by the customer - installed by another so called plumber (He was with the builders company apparently, and he's left leaks all over the place - but that's another story).
Anyway, the boiler is an older Worcester combi situated directly below the bathroom and it will supply hot water at a rate of 10 lpm with a temperature of around 65°C, if the boiler control is turned right up to display 70.
I originally thought that it was a faulty thermostatic cartridge but after fitting a replacement, it's still the same and I'm now wondering if the boiler might be at fault - for some strange, unfathomable reason.
The shower is this one: Jupiter Milan Shower Thermostatic Shower Mixer Kit From Jt Spas and the company supplied no paperwork with it at all. The 'New' cartridge arrived in a jiffy bag, with no other packaging at all so it looks (Might not be I suppose) as though it's a used item.

 

[kb76]

Is the boiler firing when the shower stat is midway (mixing) assuming it's thermostat? You could have unequal pressures (hot and cold) and the mains is over powering the hot therefore the boilers flow switch won't operate

 

[Riley]

Piped back to front?

 

[Robert Tyrrell]

Definitely not piped back too front Riley - I did check that first
I suppose it could be unequal pressures kb76, but:

A - I can't get to the pipework without removing part of a wall

B - Surely if the cold pressure was too high, it would be cold water coming from the shower head not hot, despite the boiler output. The combi can't output a higher pressure than the mains can it?

C - it goes from cold to scalding and vice versa, with no in between

 

[ambrosia]

if you run a hot tap at the same time does that make a difference

also when you run a hot tap slowly does the boiler still fire

just wondering if its the flow switch