Nest & Baxi system boiler

[smoothbadger]

Installing the Nest Heat link in place of the ST9400c. Is a pretty straight forward swap one for the other, no need to alter any of the wiring at the wiring centre or boiler to do this part.

but, there's a crucial piece of information that you haven't told us. What did you do with the old analogue thermostat? If you have just disconnected it, the heating wiring will be permanently 'open circuit', even though the relay is clicking, it won't be switching anything. If on the other hand, the thermostat is still in circuit it should be set to 'maximum' otherwise it can interfere with the operation of the Nest thermostat.

In case you have just disconnected the old thermostat, here's how to decommission it properly.

1) Trace the thermostat cable from the thermostat back to find its origin. [Probably the wiring centre]

2) When you find its origin, make a note where the wires from the thermostat cable are connected, then disconnect all of the wires and remove the cable.

3) Add a wire to link the two terminals where the 'live' and 'switched live' wires going to the thermostat originated from so that they are now electrically connected. That closes the circuit. This single link is the only part of the rest of the wiring that needs to be changed. Nothing changes at the boiler. Provided of course that it was working correctly before.

Having said all of this, the hot water side of the installation should still fire the boiler up, unless of course the hot water is already heated.

thanks for this,


I took out the old analog thermostat and have no idea where it was terminated at the wire centre

oh gawd

 

[smoothbadger]

You havent connected the orange wires from the zone valve to where they should be, they should be connected to here, age 26 fig 33 on the MIs

View attachment 229773

Ok that totally makes sense. I’ve re wired so terminal 1 is now fed to the two orange wires at the wire centre. But Terminal 2 news some kind of live feed doesn’t it? Look at the manual
If so where do I feed it from

 

[ianmcd]

Ok that totally makes sense. I’ve re wired so terminal 1 is now fed to the two orange wires at the wire centre. But Terminal 2 news some kind of live feed doesn’t it? Look at the manual
If so where do I feed it from

No it doesnt but if you want to connect something in there, connect the Greys to there rather than the permanent live , it is simply a switch, was that single brown wire in your pics the two oranges at 240 V ?

 

[stem]

You are making very hard work of this. Assuming the system was working as it should before you started, there was no need to alter anything at the boiler or do anything to the orange and grey wires from the motorised valves. They should all remain exactly as they were, because it's the motorised valves that control the boiler, not the ST9400c, and that won't change when the Nest is installed, the motorised valves will still be responsible for controlling the boiler.

I would suggest you put it all back exactly as it was before you started, then when you are back to the 'status quo' and everything is working, then:

1. Swap the ST9400c. for the Nest Heat link

2. Remove the analogue thermostat and insert the link to complete the circuit.

That's all that needs to be done.