I bought a digital thermostat and I don't know how to connect it to my boiler...

Thread Starter

babaliaris

Joined Nov 19, 2019
160
Hello.

I bought this thermostat and I can't figure out how to connect it. This thermostat has a relay with 3 pins: com, nc, and no in order to control the boiler. On my wall, I have 3 conductors, a grey one that seems to be the phase (I checked it with an electrical screwdriver) and bleu one (neutral), and a green-yellowish one that is the PE (when this PE is floating the electrical screwdriver shows current which is weird, maybe a grounding fault somewhere?).

After hours of searching and using chat-gpt, I found that I have to bridge the com and no pins connect the phase to the com pin, and leave the neutral and PE floating. I can't understand how this works...

From what I can see, the boiler turns on when the phase and the neutral (of the boiler) touch each other. So if the neutral is not connected to the relay, even when the relay is closed how does the current flow from the phase back to the boiler's neutral to turn it on?

By the way, the video from the link I showed you, tells you to connect the phase into the com and the PE to the no pin (the NC pin is left floating the same with the neutral conductor). Maybe this green-yellowish conductor is not the PE after all? Can someone explain to me how this connection works?
 
Last edited:

Irving

Joined Jan 30, 2016
3,795
What was there before?

Typical wiring for a digital thermostat would be as below

Brown might look more grey on older wiring... these are EU/UK cable colours, other countries may vary. Your thermostat may be 'double insulated' and not have/require an earth connection. It would be unusual for it not to have a Neutral connection. On no account connect a 'Live' to 'Neutral'. If its still not clear, post pictures of thermostat and cables... If still in doubt pay a professional to fit it.

1697280897371.png
 

Thread Starter

babaliaris

Joined Nov 19, 2019
160
I had the exact same thoughts. I'm not sure how it works but no matter where I was searching, I was seeing "DO NOT CONNECT THE NEUTRAL" as this would burn your thermostat. This is a thermostat that works with batteries (My older one had the phase and the neutral directly connected to it as you show in your diagram, and did not have any batteries).

Anyway, I risked it and made the connection that the video was showing me to do:
1) Connect the phase to the com pin.
2) Connect the green-yellowish conductor to the NO pin.
3) Leave the neutral conductor and the NC pin floating.

And the thermostat works. The boiler turns on when the room temperature drops below the temperature I configured in my thermostat.
 

Pyrex

Joined Feb 16, 2022
237
Hi,
The heating unit is to be controlled with two cores cable, first core to be connected to the COM terminal, second- to the NO terminal, as described in the manual:

https://device.report/manual/9295395

I presume, the mains supply cable is allready terminated to L and N terminals of the controller. And, the heating unit is powered via it's own power cable
 

Irving

Joined Jan 30, 2016
3,795
Ah, makes more sense now I've seen the diagram, but this is very bad from a safety aspect as it assumes a heating/cooling unit that has its own mains supply and an isolated zero-volt command input. The video assumes the use of the brown & yellow/green cores of a T&E cable for the command input but that may not (IMHO should not ever) be the case. My boiler has a permanent 240V AC connection and requires a switched live to activate it, so my connection would be incoming live (brown) to COM and a wire from NO to the switched live input for which I would use a brown, red or grey cable but NEVER yellow/green which implies an earth connection.
 
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