I am considering replacing a failed electric shower with a set of bath taps with integral shower head. Cold water is off the mains and hot is from a vokera combi boiler. Hot feed to tap is 22mm fitted with a flexible pipe and cold is 15mm, also with a flexible pipe. If I fit the new tap/shower mixer to the bath and the existing pipes will there be any risk that the restriction of the shower head will cause the cold water to enter the hot line and flow back to the boiler. Would it be necessarry or advisable to fit non return valve to the tap inlets. Sorry if this is a dumb question.