On 2007-12-25 20:42:09 +0000, "Roger Mills" said:

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

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.


OK, but in that case you will have departed considerably from the standard
S-Plan definition.

I've always considered it to be defined by the valve types used rather
than a specific control logic.

Honeywell says:

The main operating requirements of Sundial Plans a

1.The controls should provide full independent temperature control of
both living space and stored domestic ho****er.

2. If there is no demand for either living space or domestic hot
water heating, the central heating boiler and pump must both be
switched off.

It doesn't say anything about whether or not one can circuit can
pre-empt the other. The timers and thermostats are essentially the
same and it has been the valves that have limited what's possible.

IOW

W plan is the most restrictive because it permits flow in only one
direction at a time.

Y plan less restrictive, but mid position valves do restrict flow.

S plan fully flexible in terms of what the valves can do.






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.

Yes, an interesting one! My impression is that diverter valves were invented
before mid-point valves, and that W-Plan has largely been superceded by
Y-Plan once mid-point valves were in common supply. Maybe with fast recovery
cylinders W-Plan systems are coming back - I don't know?!

I have a Y-Plan system with a programmable stat on the CH and usually [1]
succeed in arranging the timing so that CH and HW are not both on at the
same time.

[1] With optimum start on the CH stat, it may sometimes come on before the
HW is fully hot - but it seems to cope ok!

That can all be avoided by using separate valves and controlling them
with HW priority.