On Fri, 27 Jan 2006 18:01:54 +0000, Grimly Curmudgeon wrote:

We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember "John"
saying something like:

If the MCWS head is adequate to push up to the storage tank above its enough
to create your problem John

What's MCWS?

I had exactly this backfeed problem when fitting a mixer tap (French).
It mixed in the body and not at the exit. It didn't get to the stage of
overflowing the storage tank, I sussed it out immediately when the flow
of hot was freezing cold immediately after turning off the cold tap, as
the introduced cold water in the hot pipe made its way back down.

But with the open spout adjacent the head of hot water would have to be
extremely low to allow the cold to push up into the storage tank.

However the problem did turn out to be a monobloc mixer tap. It must have
a flaw in the casting because cold water was getting through to the HW
side with both hot and cold taps turned hard off (and washers and seats in
good condition). Actually you could hear the water hushing through when
you were close to the tap. Up in the storage tank you could see a little
sort of plume of turbulence in the water where warm water was getting
pushed back into the tank and mixing with the colder water.

Glad I got to the bottom of it: I was tearing my hair out,
reputation-on-the-line time!