Thanks for your reply.
I have also found today a related topicon this forum:
https://community.hestiapi.com/t/advice-replacing-my-old-thermostat-with-hestiapi/548
The problem wasn’t solved in that case. Altought it was with a triphase installation, wich is not my case.
Installation and maintenance manual of my Furnace (Wiessmann Vitodens 100-W) shows 2 wires for control.
I doesn’t provide more information.
As I understand it, the thermostat should shortcut the 2 wires when requesting heating to the furnace.
And should unconnect them when not requesting heating.
So the furnace probably provides its mains Line on one of the two wires and check if it gets the same voltage level on the other wire.
My concern about connecting L (mains Line) to the furnace is that the furnace itself may not be connected to the same mains Line than the HestiaPi.