"Ghazali" wrote
| I am thinking of running both the basin mixer and bath shower/mixer
| cold taps off of the mains, whilst still having the hot from the hot
| cylinder.
| 1. A kithchen sink mixer also usually runs cold off the mains and hot
| from the storage cylinder, yet I never hear pple complaining about
| 'pressure imbalance' for washing up purposes. Why is it any different
| for a shower.

A kitchen mixer does not mix the H&C waters inside the tap, if you look
closely at the kitchen mixer tap outlet you should see two spouts (often
concentric, with the cold on the outside so the tap remains cool to touch).
The "mix" happens in the air outside the shower.

A shower mixes the H&C inside the unit, and with a pressure imbalance the
high pressure cold can push the low pressure hot back, so you don't get a
mix.

Also, from a safety point of view, you usually only have your hands under
the kitchen tap and can pull them back quickly if it's too hot. With a
shower, you're confined inside the shower cubicle and your whole body is
vulnerable. Most people also expect a kitchen tap to be hot-hot.

| 2. I usually have an early morning shower, at a time when no-one else
| is using any other water supply in the house, so there is no risk of
| any sudden temperature/flow change from, say, another tap being used
| elsewhere.

That may be your current lifestyle, but what happens if you have visitors or
change your routine for any reason.

| 4. If the water is coming through a shower mixer (i.e. pre-mixed), why
| would a pressure imbalance between hot and cold have any noticeable
| effect?

See first point.

Bear in mind too that if you have your pump sucking from the mains (which
may not be allowed under water regs) and you have low mains flow/pressure,
and you are running the shower, the loft tank will be emptying and its ball
valve will open to refill it. However, if the pump is sucking all the mains
water for the shower, no mains water will go up to refill the tank and then
the HW cylinder, air will get into the pipes (because the ball valve is
open) and you'll have the possibility of another airlock.

If the pump suction and the layout of the pipes is such that the hot water
cylinder is sucked dry, that may do it no good (those cylinders have almost
no structural strength without the weight of water inside them) and as the
heat from your boiler will have nowhere to go the boiler might overheat. All
of this is somewhat hypothetical and worst-case, but it's not sounding like
a good idea so far.

What you really want to do is have H & C waters at similar pressure going
into a *thermostatic* mixing valve. The best thing really is (although I'm
not an expert) to sort out that cold feed piping.

Owain