2 questions

[JohnD]

I think JohnD has it sussed.

I reckon it's the mixer tap at fault and would prefer to fit a better one (or make the builder provide and fit one under warranty). I suspect it may be a ceramic disk-type tap, probably from Italy, and a UK approved one would be better. But I'm no expert.

You could fit non-return valves to make up for the bad tap, but this doesn't seem like the right solution to me.

If you find your main stop-cock, the cold water will stop. There may be a valve on the cold water supply to your cylinder that will turn off the hot supply (ths valve may be on a pipe coming out of the bottom of the cold-water tank in the loft); if not, once you've turned off the main stop-cock, you can drain the tank in the loft using the bath taps (bit wasteful).

 

[Softus]

You have a working service valve, so there's no point in replacing it.

You have a faulty kitchen mixer tap - replace it.

 

[roofrack]

ok.
i thought the service valves would only let water travel one way, my mistake.

Will get new taps and probably try through the builder first
we will see how that goes!
Cheers folks

 

[Softus]

Will get new taps and probably try through the builder first

The builders I know are not renowned for their plumbing expertise.

 

[BoxBasher]

I stand corrected.

Its not the service valve that needs replacing, just ad a check valve in to stop backflow.