Replace ancient Honeywell mechanical thermostat with a Honeywell T3R

[PeeJay]

I would like to replace my ancient Honeywell mechanical thermostat with a Honeywell T3R PROGRAMMABLE THERMOSTAT

I have three wires in the old thermostat. Please see the first three photos.

1 Red
3 Yellow
2 Blue

are these

1 Live
3 Live return (Switched)
2 Neutral

Could I configure it as in the combi boiler option with a separate power supply and connect Red to A and Yellow to B as the Red wire is not always live and Blue terminated/Isolated (Not used)

or

Option 1 and leave the controller, which is a Seimens RWB29 heating on constant and control it from the T3R

Red to L and L linked to A
Yellow to B
Blue terminated/Isolated (Not used)


Many thanks for your time

 

[Knappers]

You'd find the thermostat wires (prob in airing cbd) and terminate/link out red and yellow.
Either fit relay inplace of existing timer or terminate those wires in airing cbd and fit relay there.
Patch old holes in walls and fit new thermostat in livingroom