Connecting Tado thermostat using OpenTherm to Baxi Boiler

[Hammy193]

So here is the current situation. The Tado thermostat is currently connected to the Baxi Assure 36 Combi Boiler using Relay as you can see from the pic below. The Brown wire is in the NO terminal and the Black wire is in the COM terminal in Tado smart thermostat. There is no need for a Live and Neutral as the Tado thermostat is battery powered. See also the Tado connection instructions below.

I've also taken a picture of the tado removed from the wall and the available wires at my disposal for the Thermostat is the Black, Brown, Grey and Yellow/Green.

Now I've taken pics of the current wiring within the Baxi boiler G block and I can see the Black wire is connected to terminal 1, yellow/green to terminal 3, Blue to terminal 4, Brown to terminal 5 and Grey isn't even used. No idea why Blue is there since its not in use or available from the wires where the thermostat is....

The OpenTherm connections in block H are available in terminal 1 and 2. Below I've also included pics of the pages from the Baxi installation manual which shows how it should be wired and the terminals.

I understand for the Tado I need to remove the two wires currently is NO and COM and put them in the (-) and (+) terminals for digital connection. I then need to connect those two wires into the OpenTherm terminals 1 and 2 in the Baxi boiler.

What are you recommendations as to how you would set this connection up for OpenTherm based on the current wiring in the Baxi Boiler and available wires?

 

[Hammy193]

Perhaps the Blue neutral wire is connected into the plug socket. There is definitely no Blue wire where the thermostat is. Just 1 Black, 1 Brown, 1 Grey, 1 Yellow/Green.

This is tricky.... seems it is baffling others as well since no engineers have responded

 

[RandomGrinch]

This is tricky.... seems it is baffling others as well since no engineers have responded

Have you looked at the wiring behind your FCU?

It seems as if extra connections have been made somewhere, and this is a likely place (especially as it is flex connected to the boiler, and 3 core and earth connected at the stat!).

However, given the destructive possibilities of mixing mains voltage and OT's extra low voltage, I would considering running a new, separate, two core cable to the Tado.