Our central heating thermostat went kaput. It is a simple Honeywell T40 and it is a "two-wire" item.
I couldn't get a direct replacement so chose a Siemens RAA20 which is also a "two-wire" item.
So far, so good. But on opening up the broken Honeywell, while the wiring diagram inside the cover shows two incoming wires, L and N, the wire from the controller has *four* cores and they are all connected.
Green/yellow goes to earth terminal (E). Blue goes to neutral terminal (2). Yellow goes to terminal 3. Red goes to live terminal 1.
So I thought to myself, how can this be a "two-wire" item?
Just to complicate matters, the replacement Siemens RAA20 is also described as a "two-wire" item, but it has *three* terminals. L is clearly live, but there are two other terminals Y1 and Y2.
Now I am totally confused. Can anyone please tell me which of the four original wires (red, blue, yellow/green and yellow) goes to which of the three terminals of the "two-wire" Siemens RAA20?
Please help! It's getting cold in here!