Bristan exposed shower

[muckymarshall]

Hello and i apologise if this kind of problem has already been posted!!
Basically we have a Bristan shower which looks like the prism exposed version, it was already in the house when we moved in. We also have a Alpha CB28 combi boiler. For the last two years things have worked perfectley until last week - the Bristan shower started going from hot too cold and more regular cold!! So i ordered a replacement cartridge but the new plastic ones are too small so i bought what looks like an almost identical cartridge from Showerdoc which is a threaded brass cartridge. The shower still fluctuated massivley so i pulled the unit of the wall and flushed pipes then checked non returning valves in the elbows were clear and working, i then checked the hot and cold valves on the unit were working and not sticking. Everything looked ok. Finally i somehow figured by mistake that when the shower is turned full on hot and the water is cold that the hot water inlet on the shower unit is not hot until ...... i turn the bathroom sink hot tap fully on which somehow in turn gives us a maintainable warm shower??? How is that happening and why? Is it possible its a boiler problem but all the other mixer taps work fine or is it a shower problem and my missus wants to hurt me bad!!! please help i want to smash the damn thing.

[plumbbob]

It can be a boiler fault which causes this effect. The boiler has a sensor which detects the hot water demand. If the unit is failing it may require a larger flow of water before the boiler is triggered. The shower isn't using sufficient hot water to trigger the boiler which is why it works when a tap is turned on too.

[muckymarshall]

Just to let you all know and thank you for your reply plumbob! it turned out to be the boiler diaphragm. there was a hole in the diaphragm and it took two hours to diagnose and repair for britgas to sort out at a cost of £180 on there fixed price repair plan. thank you again.