Newly installed bathroom uses Cherish Bath Shower Mixer £216 as supplied by Wickes. The thermostatic control works fine running a bath but when switching to shower closes down the boiler and runs cold after a short period. I should mention that the combi boiler is a brand new (expensive!) British Gas installation with brand new pipework throughout bungalow (nothing in the roof or gravity fed. The boiler is situated a couple of feet away in the next room.
I'm wondering whether the Mixer is faulty (the twist tap just goes round and round without a fully open 'stop' point) or whether the problem lies in the increased pressure needed to supply the shower head 'backing up' in some way. How could this be - surely the hot and cold, both coming straight off the mains will be at the same(ish) pressure.
If any of you professionals out there can shed some light on this problem I'd be very grateful to hear your views.
It's a Worcester Bosch Greenstar 30: I take your point about BG's expensive labour but I went with them as I needed their all inclusive removal of all legacy pipework, tanks, immersion cupboard, old pipework and old radiators giving a nothing-in-the-roof setup.
The boiler demand light goes out after a couple of minutes sometimes sooner, but only happens when switched from bath to shower use. Bath and other mixer taps work as they should.