No hot water to shower

[kuotone]

We cannot get any hot water from our shower(mixer-over bath) when heating is off-boiler fires when hot tap at sink (next to shower) is on, but nothing happens when bath/shower tap on. only solution is to put CH on when we want to have shower. Any ideas why this is happening? The boiler is a museum grade 'ideal sprint' and due to be replaced next month, so do not want to have to call out plumber if we can fix this ourselves. Thanks in advance.

[The Builder]

...er....... you haven't got the shower connected in to the radiator circuit have you Kuo?

[kuotone]

no-its been working perfectly for last 18 months, summer and winter

[gucciman]

its almost certainly the mixer tap at fault rather than the boiler as mixer taps are notorious for causing problems with combi boilers. im a gas engineer rather than a plumber so i don't fully understand whats going on but i believe from a conversation with one of the boiler makers tech helpline staff a while back its something to do with the cold water backfeeding up the hot water pipework

[gasman]

You should have Double Check Non Return Valve's Fitted to the pipes supplying the tap. This is actually regulations to fit them to any mixer taps to prevent backflow and cross contamination. You may even find that the shower works properly if you have them fitted.