DIY Doctor UK DIY Forums

Hi, I'd like to replace my Honeywell ST9400C programmer and Honeywell T40 thermostat with a new Nest.

It should be relatively straight forward, but the wiring diagrams of each aren't obvious and the programmer seems to have a lot of wires.... any ideas?

Thanks

Okay, so I can see that the L/N has 3 wires as:

1 - there is a power switch to provide/cease power to the bolier and programmer
2 - the second set goes to the bolier to provide power there
3 - the 3rd set goes to the old thermostat

So the 3rd set move over to T1/T2 on the heatlink

Terminals 1/2 on the programmer have no connection. T3 is Hot Water on ad T4 is Heating on, so these go to 6/3 on the heatlink.

So that just leaves me with the 'Common' connection on the heatlink (5/2 on the heatlink)??????