In an earlier contribution to this discussion,
Sam wrote:


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)?

Not without a lot of complication in the wiring department, bearing in mind
the way in which - unlike 2-port valves - some of the wiring is shared by
motor driving and boiler/pump running functions. Far better to use 3 x
2-port valves which will probably cost very little more than 2 x 3-port
valves. The 3-port valve is designed for one very specific application, and
doesn't readily lend itself to departures from the standard Y-Plan setup.


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

Most modern-ish boilers - which hold relatively small quantities of water -
require it. Old heavy cast iron boilers usually don't. When the boiler is in
full flight and the room stat decides to turn it off, the flame is cut
instantly but the water goes on getting hotter due to the residual heat in
the metal parts. There needs to be a flow of water through the boiler for
long enough to carry away this residual heat - otherwise the boiler
literally boils. This is achieved by allowing the boiler to control the pump
rather than controlling the pump directly by wiring it in parallel with the
boiler. If your boiler needs it, there should be something about it in the
installation manual. Also, if you look at the boiler's wiring connections,
there will be a permanently live input - in addition to the switched live -
and there will be pump connection terminals.

For pump over-run to work, there must always be a flow path open for the
water to follow. A 3-port valve always has at least one port open. A 2-port
valve doesn't - and all (2 or 3) could be closed when all demands are
satisfied. You would then need a by-pass circuit - preferably using an
automatic by-pass valve - to provide that flow path.
--
Cheers,
Roger
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