Opentherm library for Arduino Sucessfully tested on Ferolli Domiproject F24. Setting CH/DHW Setpont. Resetting faults, getting status/modulation level/temperatures, etc. http://habrahabr.ru/post/251539
For sending simple request just call communicate(MSG-TYPE,DATA-ID,DATA); and after 1s take answer complete(&type,&id,&data):
uint8_t type,id; uint16_t data;
communicate(1,56,10240); //set DHW setpoint to 40.0 deg
delay(1000);
complete(&type,&id,&data); // if accepted, type=5,id=56, data=10240;