In an earlier contribution to this discussion,
Army wrote:

Zoning is beyond my current level
of knowledge but I'm trying to plug that particular gap right now!
I'm guessing that I need to introduce a 3-port mid position valve to
the flow end of the CH and connect to it (a) the original house stat
and (b) a new stat to be located in the conservatory.

You *could* do it with a 3-port valve but, as others have said, you'd be
better off with two 2-port valves.

Have a look at the 'plans' and their associated wiring diagrams in
http://content.honeywell.com/uk/homes/systems.htm

None of them exactly match your setup - but you should get some clues.

The Y-Plan is the one with the 3-port mid-position valve. It is intended to
give independent control of HW and CH in a setup with stored hot water,
rather than a combi. Note that the tank stat in the HW circuit has a
changeover switch with 3 contacts rather than a simple on/off switch. If you
use this setup to control two heating zones rather than HW + CH, it *will*
work - but whichever heating zone is in place of HW *must* have a changeover
thermostat.

The S-Plan (and S-Plan-Plus) are the ones which use 2-port valves. You can
have as many zones as you like, with one valve per zone. The motor of each
valve is controlled by a timer and stat for the appropriate zone or, even
better, by a programmable stat. The secondary contacts on each valve - which
close when the valve opens - are all connected in parallel and used to fire
up the boiler and run the pump whenever one or more valves are open. The
S-Plan shown in the Honeywell diagram (see URL above) shows a HW zone and a
CH zone - but will work just as well for two heating zones.
--
Cheers,
Set Square
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