combi boiler/mixer shower/water regs

Either the mixer tap itself (by dint of its design) must prevent the possibility of hot water going back up the cold pipe and backfeeding the supply, or a double-check valve should be fitted further upstream in the cold supply to make sure hot water can't flow back up the cold pipe.

(In theory this unlikely situation could occur if the hot supply remains under pressure and the cold pipe develops a vacuum. There is then the possibility that the public main could be contaminated with your dodgy water causing widespread public poisoning. Not really a very likely problem with mixer taps but more important on say a outside tap where a hosepipe dangles into a garden pond which could result in frogspawn etc being sucked up into the water main... not too clever.)

One additional detail: The mains pressure going low (not a real vacuum, but anyway) can happen in real life - it's not just theoretical. If the Fire Service turn up and start pumping water out of the mains ...

On 5 May 2006 03:34:16 -0700 someone who may be "Wingedcat" wrote this:-

Not very likely if hot and cold are fed from the same supply, as in a house with mains pressure hot and cold water.

Provided that the shower attachment is restrained so that it cannot be submerged in bath water which could be sucked back into the mains.

IIRC the concern about this was prompted by a farmer who had a hose pipe immersed in a large tank of weedkiller when the mains pressure reduced and the weedkiller back-siphoned into the mains.