"Christian McArdle" wrote in message
.. .
Doesn't negative head also imply that the pump needs to be able to suck
water into its inlet from a lower level, without cavitating?

No. Negative head pumps should be installed below the level of the cold
tank, ideally adjacent to the hot water cylinder. The negative head
applies
to the outlet, not the pump location. It would be quite difficult to
design
a shower pump that could prime itself by sucking water up by a column of
air, as the impellors required to pump water are quite different to those
used to provide high pressure air suction. Also, the lubrication and
cooling
behaviour is very different.

It is also usually unnecessary and provides a strict limit on the pressure
possible, in that it could only provide a maximum of 1 bar at zero head
pump
installation level, reducing with altitude.


Ah yes he's right, I realise. Damn Christians - always right ). Apologies
for any misleading from my first post.


--
Bob Mannix
(anti-spam is as easy as 1-2-3 - not)