Honeywell to Nest assistance

[ben mitchell]

Hi All,


I am after a bit of confirmation from the people in the know. The more I think about it and run through it in my head, the more I doubt my logic and start to ask more questions of whether my thoughts are indeed correct! I have a new Nest 3rd Gen wireless thermostat that the boss has bought. This is to replace a Salus wireless thermostat that is rubbish and has been since we moved in two years ago. It keeps losing connection with the receiver, no matter how close it is placed to the receiver and its getting very annoying. Well it will be when the heating is back on and I get calls at work from her saying the house is cold! Hence why she has bought the Nest.


Now, as I understand it, the nest will sort heating and hot water, so we can do away with the Honeywell ST9400 programmer, which again could do with some tlc. The clips for mounting on the back plate are either broken off or cracked so it is prone to being dislodged by towels etc if the airing cupboard is being used, another source of “I have no water or heat” calls!


Now I have looked at the wiring diagrams etc for the nest and other existing components. Every time I think I have sorted the replacement, I doubt myself and start over the thought process. Could someone out there confirm what should be fed into the Nest heat link box please? I have sketched out the current set up, it will be a good opportunity to tidy up the wiring that is so jumbled at present it took some deciphering to get the sketch!


I should add that the Nest is replacing the Salus wireless stat so will be wireless to the heat link box as there are no existing stat wires.

Thanks

Ben

 

[stem]

First of all Ben welcome to the forum and congratulations on an excellent first post that actually contains the necessary information. If only they were all like this

Step 1. The Nest Heatlink replaces the ST9400 programmer

From the diagram of the ST9400 shown in the thumbnail below we can ascertain that the wire currently in 3 is Hot Water Call for Heat, and the wire in 4 is Heating Call for Heat



They are moved to the Heatlink as follows:
1. The Hot Water Call for Heat wire then goes to '6' at the Heatlink
2. The Heating Call for Heat wire then goes to '3' at the Heatlink
3. The 'N' and 'L' supply is self explanatory and is just moved from the ST9400 to the corresponding terminals at the Heatlink
4. Heatlink terminal 'L' also needs linking to the two common terminals '2' and '5' (these connections are made internally with the ST9400)



Step 2. Removing the Salus Thermostat Receiver

Because the Nest carries out the functions of the room thermostat, the old wired thermostat receiver needs to be decommissioned to prevent it overriding the Nest. It can't simply be disconnected as this would leave the heating wiring 'open circuit' and it will not operate.
1. Disconnect and remove the N & L 230v supply to the Salus receiver
2. Connect the wires in 'com' and 'no' together, so that the end result is that the Heatlink terminal 3 (Heating Call for Heat) is connected to the wiring centre terminal 5 ie the live to the heating motorised valve.

Step 3. Power the Nest Thermostat

This can be done by wiring the Nest thermostat terminals T1 & T2 to the corresponding Heatlink terminals T1 & T2, or by using a separate plug in power supply.

 

[ben mitchell]

Hi Stem,

Thanks for your simple, clear reply. I think that all makes sense and was generally where i had got to. The clarification on the Salus receiver is the part that i really needed. Was worried that removing it would not allow correct functioning and there would have to be a little more fiddling around to get it back to how it should be.

I have already considered the power for the Thermostat. we have the power cord etc so it will be placed in the hall on the dresser.

Once again, thanks for the response and assistance.

Regards

Ben