Cabling Connections for Installing Nest


Postby reddwarfcrew » Sun Oct 01, 2017 11:51 am

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
Attachments
IMG_3422.jpg
Honeywell ST9400C Programmer
Old Thermostat 2.jpg
Honeywell T40 thermostat wiring diagram
Old Thermostat 1.jpg
Honeywell T40 Thermostat wiring
Nest Heatlink.jpeg
Nest Heatlink wiring diagram
ST9400.jpeg
Honeywell ST9400C Wiring Diagram
reddwarfcrew
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Postby reddwarfcrew » Sun Oct 01, 2017 2:39 pm

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)??????
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