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I wondered where to exactly install the OTGW, given the following warning in the user manual:

Connecting between boiler and thermostat
- Verify the thermostat connection is OpenTherm not mains voltage!
- Decide where to install the gateway between boiler and thermostat
- Disconnect the boiler from the mains supply
- Cut the 2 core wire that connects the thermostat to the boiler

First of all, how do I verify the first point? I have a two-wire going from the boiler to a small connector with power (see below). And there is also a normal 220V cord and plug. I assume the former wire is the one I need. (I mean it seems too obvious .

Secondly, I have this connector that adds 24V to that two-wire line, see attachment.

Should I connect the OTGW between this connector and the boiler or this connector and the thermostat?

Thanks!

The OTGW goes between the boiler and thermostat. If the adapter in the picture is part of the thermostat, the OTGW must be placed between the boiler and the adapter. If on the other hand it belongs to the boiler, the OTGW goes between the adapter and the thermostat. You didn't indicate which thermostat and boiler you have, so I can't say for sure which one it is. If the purpose is to add 24V on the line to function as additional power for the thermostat, then the adapter must be considered part of the thermostat.

The trouble shooting instructions explain how to determine if the boiler is providing voltage levels that are compatible with the Opentherm specifications.

Thanks.

It's a Remeha Avanta ACE and a Toon (v2 I think). The adapater is indeed part of the Toon, so I'll place it inbetween it and the boiler.

BTW the Remeha showed ~7V on the wires (which was much less than I expected, given the troubleshooting instructions say "less than 30V".

But I'll give it a shot once I have a shorter USB cable, the current 2m one only produced 3.75V between pins 5 & 14...

The troubleshooting instructions refer to open boiler connections. You must have been measuring with the Toon still connected. In that case the voltage should normally be below 8V, with periodic short bursts to 15 to 18V. But those burst will be too short for a regular multimeter to register.