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

Hi there

I would really appreciate help with this.

The thermostat for the downstairs heating in our house has never
worked, so I took a look at it today. There are 3 wires going to it:
green/yellow, blue and brown.

Does that mean that the heating doesn't work either - or has the stat been
by-passed in some way?


The heating does actually work. As long as the face plate of the thermostat
is on, the heating works. If it's taken off, then it doesn't.
I don't know why that's the case! The only thing I can think of is that
maybe one of the wires is a "satisfied" wire rather than "call to heat"
hence why it always thinks the heating should be on?


The thermostat is a Drayton RTS1, which has 4 terminals: Neutral,
Live, 3 (Call for heat) and Earth.

Currently, the wiring goes: Brown to Live, Blue to Neutral, and
Green/Yellow to Earth.

This leaves nothing going to the 3 (Call for heat), which I guess is
why the thermostat isn't working!

Sounds pretty likely!

I'm guessing that one of the following is the case:
1) the green/yellow wire may actually be the 3 (Call for heat)
2) the green/yellow is actually neutral, and the blue is the 3 (call
for heat)
but I need to know how to check this without fusing the system! Also
this would leave no earth - is this likely?

I've got a digi multimeter, and there's 230 Volts coming down the
brown (when checked against either the blue or the green/yellow), so
that's in right, but how do I find out how to rewire the other two?

Many thanks for any suggestions.
xena

The type of stat which you have *needs* 4 wires, and can't safely be
installed with 3. A common short-cut employed by some plumbers is to use
the
green/yellow wire for the switched live (which you refer to as the "call
to
heat"). This is unsafe for two reasons. Firstly the stat needs to be
eathed,
but isn't when this is done. Secondly, a wire which any reasonable person
would expect to be earthed is, in fact, live!

If you want to do this properly, you have 3 choices:
1) replace the 3-core cable with 4-core
2) run a seperate 4th wire from the stat to whatever it connects to
(possibly a junction box in the airing cupboard)
3) replace the stat with one which needs less wires. For example, modern
electronic programmable stats only need *two* wires - since they don't
need
to be earthed and they don't have a built in accelerator heater needing a
neutral return.


Um, of all the above options, I prefer number 3! I think the RTS7 works
with 2 wires, so maybe I'll get that.

In order to find out exactly how yours is wired, you need to find the
other
end of the cable, and see what each of the cores is connected to. If you
can
identify the type of system you have in terms of one of the "plans" shown
in
http://content.honeywell.com/uk/homes/systems.htm you can look at your
stat
connections with reference to the appropriate wiring diagram.

Cheers, I'll have a look at that now.

Many thanks for your help, Set Square )
I'll post back with how I get on.