"Set Square" wrote in message
...

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!

The RTS 1 is double insulated and does not need earthed.

L Reid