Hi,

As a follow up to my previous post I have put a couple of pictures of
the diverter valve on a webpage in case it helps jog anyone's memory.

http://www.wiseman999.plus.com/diverter.htm

Can't help with original wiring diagram, I'm afraid. But surely, there can't
be *that* many wires?

That is true and I have some idea of how it is all connected up because
I spent a very happy half term week at the age of 10 watching it all
being installed, however I do recall that the "professional" who
installed it wired everything up with multiple lengths of identical 3
core cable.

Hence all the cables from the boiler, programmer and room stat look
identical at the point they enter the airing cupboard.

Working it out will be straightforward, however it would be much quicker
and much less tedious with a wiring diagram.

The only slight change to this would be if the boiler needs pump over-run -
which is unlikely if it's 30 years old!

I'm pretty sure it does. There are certainly more than 3 wires going to
the boiler.

Things to watch are that the programmer must have a HW OFF connection and
the cylinder stat must be the changeover type, with 2 connections.

A good point, which hadn't occurred to me. Those are also things that a
wiring diagram would help with (I should point out that I don't live
particularly close to my mum so I can't just pop over there to have a
look). What I am hoping is that I can just turn up on Sunday with a 3
port valve and wiring centre and wire it up so that at the bare minimum
she has heat that she can manually switch on and off.

Martin.