Good news, problem solved. Bad news is I've kept you busy with a problem in node red that didn't actually exist, sorry about that & shame on me. On the other hand, I have learned a lot thanks to your support, so again many many big thanks!!! I hope I can return the favor in the future!!
Actualy as I suspected. Even when I filter the sended setpoint of the temperature in NodeRed so it only pass 'on change', it keeps comming in to the PLC's function block. So I used the same function as the rbe node in Node red, just before the function block in the PLC's program that controls the room and that did the job. So the filternode whas the exact solution, only the position had to be in the plc program and not in node red.