"Set Square" wrote in message
...
In an earlier contribution to this discussion,
xena wrote:


Ok, a bit more information. I've found the cable that goes from the
thermostat downstairs to the heating system upstairs, and posted a
pic he http://www.ukosborne.co.uk/images/wiring.jpg
with the relevant bits labelled up.

What a nightmare! Do all the wires going into each white block join
together
inside? If so, God knows what they have done - you have a very colourful
mixture!

Yes, the the wires inside each white block appear to be connected through
the bar inside the block.

You or somebody said that this stat contains a relay. I have a hunch or
theory - which may be rubbish - but may be worth checking out. Is the stat
*supposed* to turn on a zone valve which - in turn - turns on the boiler
and
pump (as per S-Plan in the Honeywell document)? If so, and if there's a
relay coil between L & N in the stat, the relay coil may be (incorrectly!)
wired in series with the zone valve motor - which may *just* provide
enough
current to motor the valve to the open position. When you remove the stat
from its baseplate, you would break this link, and the zone valve would
shut. Can you get someone to stand by the zone valve when you plug the
stat
in to see whether it does anything? Which cable(s) in your junction box go
to the downstairs zone valve, and what are they connected to?

Here's a pic of the system:
http://www.ukosborne.co.uk/images/system.jpg

Is the zone valve likely to be the silver box at the bottom of the pic
marked "Downstairs heating"?
I'm home alone til hubby comes home later, but we can try listening for
sounds then.

The wire from the silver box at the bottom of the pic marked "Downstairs
heating" is the first black wire in
http://www.ukosborne.co.uk/images/wiring.jpg
and has five colours coming out of it. If I'm going in the right direction,
let me know and I'll see exactly where the wires go.

Thankful, as ever!
xena