Browsing the internet I came across this: http://www.tclcode.com/opentherm/index.html#intro
This person built an interface to connect his HomeVision system to his Remeha Calenta.
This looks rather promising and the print looks rather easy to assemble.
These are the main functions
Forward OpenTherm messages between the thermostat and the boiler
Report about the messages on the serial interface
Change the thermostat setpoint
Set the clock of the thermostat
Send the outside temperature to the thermostat
Control the domestic hot water option of the boiler
Obtain additional information from the boiler