On Thu, 22 Jan 2009 12:21:41 +0000, The Natural Philosopher wrote:

That allows the stats that drive the valve motors to be on a different
live..useful as you may want to link into the local ring main for that.

Ooo, no, please. The entire heating system should really be fed via a
single DP isolator switch. You don't want bits here and there live when
other bits or not. Apart from the safety aspect when working on the system
it's quite likely to confuse people if it acts strangely if a ring main
trips.

As this system is likely to "evolve" I'd run all the contacts of any
device back to a central wiring cabinet and do the appropiate cross
connections there. I've done that here and it makes changing things very
simple, like adding pump over run or possibly replacing the relays with a
PLC.

--
Cheers
Dave.