Understanding a Honeywell CM907

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I've moved into a new house and I'd like to upgrade to a smart thermostat, such as the Draton Wiser. Self installation seems do-able but I don't understand my current Honeywell thermostat CM907.

Most smart thermostat install guides show many wires coming into the backplate of the existing thermostat but mine has just a live and neutral (see image). My Honeywell is programmable and controls the heating so I don't think theres another control board in the house. I also can't find a wiring centre that most install guides show as being connected to the thermostat.

Am I getting confused by install guides for older, more complex systems, or do I need to locate some vital hardware? Any help would be appreciated

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It's a Live and Switch Live, the Neutral wire having been used for the switch. What kind of system do you have? What's your boiler make/model?
 
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Hi Muggles, thanks for the response. I have a Vaillant ecoTec plus 831

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Here is the wiring diagram for the system I am considering. Would I be correct in saying the live A would become 1 COM, and the switch live B would become 3 ON?
 
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