In an earlier contribution to this discussion,
Pete wrote:

If your gravity HW system heats the water fast enough, your best
solution -
rather than going fully pumped - is to convert it to a C-Plan. [See
http://content.honeywell.com/uk/homes/systems.htm ]

For this you will need a 2-port zone valve and a tank stat on the hot
water
cylinder (if you don't already have one). This gives truly
independent control over CH and HW, and also provides a boiler
interlock so that the boiler only fires when either or both circuits
require heat - and shuts down
when both demands are satisfied (i.e. when both stats are open) thus
saving
fuel.

Looks like order a "2 port valve then"

An I right in thinking that to wire I need to:

Connect thermostat HW On to: Valve Brown
Permanent Live to: Valve Grey
thermostat CH On: Valve White and Pump L

Boiler L to Valve Orange

plus neutral & earth


Thus, boiler gets power when valve activated (via grey)
Boiler gets power when valve off (via valve white)

Currently the boiler has a connection for the pump. I assume that
this should be left so that the pump can continue to run when the tank
theremostat is cycling.

The wire colours may vary, but if you study the Honeywell wiring diagram
referenced earlier, and wire it *functionally* the same way, you should ok.

As Christian has said, make sure that the boiler doesn't mind zone valves -
and that you install the valve in a position which doesn't interfere with
the un-interrupted vent and fill functions. A good place is in the HW flow
pipe very close to the hot cylinder *after* the point where the vent pipe
connects.

I'm a bit puzzled by your reference to a pump connection on the boiler. This
normally only applies to fully pumped systems, and keeps the pump running
for a bit after the boiler has stopped firing in order to stop the residual
heat in the boiler's metalwork from causing it to overheat. With a gravity
system, the boiler needs to be able to take care of itself in HW-only mode -
where the pump doesn't run at all. Are you *sure* it is a gravity system? If
so, please clarify how the pump is currently connected and what, if
anything, the boiler's pump connection is used for.
--
Cheers,
Set Square
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