Replace ST6400 controller w nest third gen heatlink

[reddingthompson]

hello

I have an st 6400 honeywell controller on my heating system. Looking at the nest heatlink install guide I think I have an s plan system that also includes a pump.

For the sr6400 present wiring includes live, neutral, red wire to terminal three (HW ON) and black wire to terminal four (CH ON). There are no other wires entering the st6400

I'm hoping I can just move wires from st6400 to heatlink and do not need to mess with the junction box near it where all of the signal and power wires for boiler, hot water tank, valves and pump are joined on a terminal block.

I think that for the nest heatlink connections I need provide power to L and N and to link live to 2 and 5 and then the red wire to term 6 and the black wire to term 3. I do appreciate that I need to run existing thermostat wires to T1 and T2 so I don't fry the nest thermostat.


Does this make sense? I have seen other threads on diynot that seem to indicate more wires in the st6400.

Thanks for any help

 

[Johnmdc]

What you have suggested is correct.

 

[reddingthompson]

Johndmc. Thank the you.

 

[reddingthompson]

Well I tried to start and then realised that if I just remove thermostat wiring ( which carries 240v) from terminals 5 and 7 in junction box I then leave an open circuit for the heat valve ( see attached). Do I need to link terms 5 and 7 in junction box so that power could get to heat valve when instructed and then the nest thermostat closes the CH ON ?

 

[reddingthompson]

Have now added schematic for Junction box

 

[Johnmdc]

Yes link 5 and 7 in your junction box and disconnect the 2 thermostat wires in them. Also if there is a neutral wire going to the thermostat disconnect that too.