TheScullster wrote:
Hi all

Does anyone have a link to a comparison of these systems.
Struggling to see the advantage of S Plan, although I have seen a number of
posters supporting this.

Wiring diagrams & stuff here;
http://content.honeywell.com/uk/home...UNDIAL%20PLANS

Strangely, I spent a few hours yesterday and a few more converting my
heating from Y-plan to S-Plan.

The Honeywell site doesn't list any of the advantages or disadvantages
of one of their systems over another. The one that has come to my
attention is that the Y-plan 3-port valve is always open to one port.
If the power is off, it is open to the HWS indirect. If you have an
unvented HWS storage cylinder, you can't use a 3-port valve system.
Well, you can, but you have to use a 3-port valve AND a 2-port spring
return valve on the HWS port, which is a bit pointless. Better to use
the S-plan system.

Secondly, having the HWS port of the 3-port valve open on power-off is
not usually a problem because the boiler will be off. If you've got a
boiler with a large water capacity or a thermal store, then it will be
a problem because you will get gravity circulation over-heating the HWS
cylinder when the system is off.