"MM" wrote in message
...

I thought I'd hold off ordering anything until after everyone is back
from the Christmas/New Year break, so this morning I enquired at two
separate places this morning, the local family-run bathroom supplier;
and Plumb Center in Spalding.

Both told me the same thing independently of each other: Water regs
prevent swapping out two separate taps for a mixer tap/shower where
the cold water is at mains pressure, which mine is. Apparently, once
the mixer is installed, when the cold and hot are turned on, the high
pressure cold water will overwhelm the gravity-fed hot.

I had a similar problem with my kitchen mixer tap, I was advised to fit a
one-way valve to the hot side because when I used a hosepipe, the (mains)
cold water was pushing the hot back to the tank and out of the overflow
outside when the hosepipe "stop" valve was on and the taps fully open.


Two solutions: One is to fit a pressure reducing valve in the cold
feed, but the Plumb Center chap said this would have to be screwed
right down to minimise the pressure.

Better (both suggested this independently): Run a new length of copper
pipe from the cold water tank in the loft to the bath and cap off the
existing mains-pressure cold supply to the bath. NB: The cold water
tank currently only replenishes the copper cylinder in the airing
cupboard. Disadvantage: The new gravity-fed cold feed to the bath
would mean that the bath would take ages to fill. Not sure how much of
a disadvantage that is, although I will say that it DID take ages to
fill a decent bathful at a relative's house over Christmas. They have
gravity-feed. Only mains pressure to the kitchen cold tap.

But would it really take longer? Unless your hot water is above scalding or
you take very cold baths, you wouldn't need more than 50% cold water so with
the (gravity) cold running at the same rate as the (gravity) hot, they'd
both take the same amount of time at worst, surely?