On Thu, 10 Jan 2008 16:05:49 -0000, "Roger Mills"
wrote:

The actuator of a 3-port valve has more components (extra microswitch,
diode, etc) than the 2-port equivalent and is more prone to failure. Mine
last about 2 years on average.

That's not very long. It's not very reassuring.

They also have the disadvantage that
they can only control 2 zones - usually one HW and one CH - whereas you can
have multiple zones with 2-port valves just by adding more valves.

Couldn't you have a 3-port for CH or HW, with the CH port connected to
another 3-way for upstairs (A), downstairs (B), or both (A+B)?

For more than two zones, I see your point.

3-port valves have the *advantage* that they always provide a flow path (unless
*all* your radiators are fitted with TRVs) - so you're less likely to need a
by-pass circuit to deal with the pump over-run phase, if required by your
boiler.

Someone else mentioned this "pump over run". What is it and how do I
find out if I have it?

It needs to be wired according to the wiring diagram for Y-Plan systems -
see the Y-Plan section of http://content.honeywell.com/uk/homes/systems.htm.
In either the HW-only or HW+CH (position) the boiler and pump are fed from
the timer's HW output, via the cylinder stat. In the CH-only position, they
are fed by the actuator's orange wire output which is internally switched to
the grey wire input.

I think taht's the point I was missing: I hadn't realised that in
addition to the orange wire they were also connected to the stat. I
knew there had to be two connections!

Something that escaped me until I thought more about it was this:

I thought what if the valve is set to CH+HW but the tank fills up,
won't the pump stop pumping? When I thought more I realised that when
the stat is satisfied it turns the valve to CH only.

Thanks again.