"Set Square" wrote in message
...
In an earlier contribution to this discussion,
BigWallop wrote:


So a new and dedicated pipe from the cold tank would continue to
supply cold water, even with the cold tank empty? Oh right.

You're not listening!

With a power shower going full tilt, you will potentially be removing
water
from the cold tank faster than the mains supply can replenish it. So the
level will drop. When the level drops below a take-off point, that
take-off
point obviously won't supply any more water.

The take-off point for the cold feed to the hot cylinder - which in turn
pushes hot water out of the cylinder to the shower pump - needs to be
*higher* up the cold tank than the take-off point for the cold feed to the
shower pump. Then, when the level drops, it will cause the hot feed to the
shower to stop first, with the cold feed still running. If it were the
other
way round, you could have scaldingly hot water coming out of the shower.

I hope that makes it clear!

Cheers,
Set Square


So you're saying it would be better if the cold supply was direct from the
mains? Or is your own contradiction a part of not understanding the
workings of creating a head of water to a shower head?

You say that the cold water storage tank may empty if the shower used:

" With a power shower going full tilt, you will potentially be removing
water
from the cold tank faster than the mains supply can replenish it."

Then you say to take yet another cold water supply from the cold water
storage tank:

"The take-off point for the cold feed to the hot cylinder - which in turn
pushes hot water out of the cylinder to the shower pump - needs to be
*higher* up the cold tank than the take-off point for the cold feed to the
shower pump."

Or am I missing your point here?

There are already two outlets taken from the cold water storage tank. One
supplies the hot water, and the other supplies the cold. Both outlets will
be near the bottom, if not actually through the bottom, of the cold water
storage tank. Therefore both outlets will be supplying their respective
appliances with an equal amount of head (pressure).

A pump for a system that is going to be sited slightly higher than a normal
appliance, in this case the shower, only needs to supply a slight increase
in the pressure of the supplied water to create the correct head (pressure)
to make the whole thing work properly.

A shower pump isn't a massive taker of water, that's why they are only
supplied with narrow bore pipes. In fact, the shower will be using only
around a quarter the amount of water flow rate that running both huge 3/4''
taps on the bath will. Even if the pump were to be supplied with 28 mm bore
pipework on both the inlet and outlet sides, the pump would still only be
able to draw about a quarter the amount of water that the pipes can supply
to it.

I advise using the cold supply in and the hot supply out of the hot water
storage cylinder because, when the shower is in use the cold supply feeding
the rest of the house will be able to supply the taps without creating any
pressure difference to the shower supply. It also creates the situation of
having both cold and hot supplies at an equal pressure to both sides of the
pump, and so make things easier to balance at the mixer valve.