On 2007-12-25 18:18:40 +0000, "Roger Mills" said:

In an earlier contribution to this discussion,
Andy Hall wrote:


The main thing is that I want to be able to have HW only or CH only
or both at the same time if a S plan system can do this then thats
what is right for me

You can do that, although often controllers work in a hot water
priority mode.
A strange answer to this particular point! Some plans - notably W-Plan - do
indeed give priority to HW, but as far as I am aware, all zones - space
heating or hot water - in S-Plan systems have *equal* priority,

That depends on the implementation.

If you have separate thermostats and timeswitches perhaps. However,
it is possible for thermostats and time control to be combined or even
that the valves are controlled by or integral to the boiler anyway.
Under these circumstances, the HW can be given priority.






With 3 port valve systems, the hot water takes priority. This means
that if there is a demand from the cylinder, then the valve opens in
that direction, and the boiler and pump run.

Not necessarily. It is true for W-Plan systems, which use a 3-port diverter
valve - but Y-Plan systems use a 3-port mid-position valve which simply sits
at its mid position when there are simultaneous HW and CH demands.

Which is not a very good idea in a modern setup where lots of heat can
be transferred rapidly to the cylinder.