Theben KNX Series Handbook page 94

Appendix
P controller as temperature controller
Just as in the previous example, the P controller behaves in a heating control.
The set temperature (21 °C) can never be completely reached.
The permanent control deviation is increased the higher the heat losses, i.e. the colder the
outdoor temperatures.
RAMSES 718 P / RAMSES 718 S 94