Hi Set Square, Lurch et all,
OP here, many thanks for all your helpful posts!
Set Square, your post at 17:16 on Oct-01 was very helpful thank you, in particular your reference to the honeywell site was really usefull. I've studied the wiring diagrams and that side of things is fairly clear now. In order to further my education, a couple of questions spring to mind if I may be so bold...
(1) Why is S-plan favoured over y-plan? Y Plan seems to be considerably cheaper to implement (assuming a changeover stat is not overly expensive) and neater (ie less clutter under the boiler and disruption to pipework). Also, regarding failures, the Y plan is never 'closed' so no chance of the pump being activated when both circuits are off. (Clearly the boiler has a bypass if the above were to occur on the s-plan). I guess I must be missing something here.
(2) What is a changeover stat?
(3) Why can the motorised valve(s) not be installed on the return rather than the flow?
BTW, I don't have TRVs on any of the rads. We have lived in this house for a year now and the system seems to hang together quite nicely without them. In addition, most of the rads have microbore into 'same-end' rad valves and I don't think I've seen TRVs in that configuration before.
Thanks all
Mike