Replacing honeywell thermostat with CM907

[armitageb]

I have a Honeywell 4200 which I want to replace with a Honeywell CM907. Looking at the old therostat there are 3 wires on points 1 2 & 3. The diagram in the thermostat shows 1 as live, 2 Neutral and 3 underload. If I put a curcuit tester on 1 and 2 I get 240V with the bolier on and no current with it off. If I then put it on 1 & 3 I get 240V but when you turn down the thermostat the voltage stops when the thermostat falls below room temperature.

We have a gravity hotwater and pumped central heating system with flowcontrol valves when the hesting is off to allow only hot water.

Can anyone please advise how the wires 1 2 3 translate to ABC on the CM907. I've looked at other posts but none cover this.

The instructions for the CM907 show 3 wires required for a pump and valve other systems only require 2.

The original wiring diagram for the heating system shows 1 from the programmer and 2 to the CH Valve and Pump & 3 going to the CH valve.

Thanks

 

[doitall]

1 to A, 3 to B, make the neutral safe.

 

[armitageb]

1 to A, 3 to B, make the neutral safe.

 

[armitageb]

I have added further information regarding this issue.

 

[denso13]

Looking at the old therostat there are 3 wires on points 1 2 & 3. The diagram in the thermostat shows 1 as live, 2 Neutral and 3 underload.

The new one is A - live, B switched live (underload) and C - satisfied. If you are changing one for the other you would do as doitall has already said.