Hopefully easy question on Honeywell T40 thermostat

[RoryJ1978]

I've replaced my old honeywell controller and thermostat with a Hive system. Its all running and working, BUT when I move house, I intend to take the Hive with me (sorry, scottish genes). So I'm keeping the thermostat wires in the wall and will reconnect before I sell.

So, the old honeywell wired thermostat I've just connected wires 1 (live) and 3 and it all works - can someone explain what wire 2 does please? (currently not used in my system).

Attached are pics of wired thermostat before I started messing and helpful wiring diagram inside thermostat.

Just worried as I'm not using wire 2 any more - it must be there for a reason?!

 

[ericmark]

Wire 2 is neutral and it is used to reduce the slewing range of the thermostat, that's the difference in temperature at which is switches on and which it switches off. There is a small resistor which warms up the thermostat to reduce the slewing range not required with electronic types.