What settings does the central heating unit expect for parity and stop bits ? Does it need the serial data to be terminated with a carriage return/line feed/either or both ?
When I, while the UNO is busy sending the data to the heating unit, I simultanously start the serial capture programm Realterm the sniffer Device Monitor says "serial port opened by Realterm" and shows the data coming from the UNO.
This sounds like you are trying to read the Arduino Serial output with 2 devices. This won't work.