I have a large room which is connected to a heating system. I have created a custom circuit that can toggle the heater on or off. There is also a temperature sensor in the room at a reasonable location.

So all I need is something to turn the heater on/off depending on the room temperature. It's a very slow system. It takes about 1h to raise the temperature by 3 Kelvin in the room. The preheat time is not a problem, I can account for that. I set up SAT as PID controller with the room temperature sensor as sensor and the heating system switch as the PID switch.

However, it doesn't seem to do anything with the switch except turn it on when the mode is set to heat and the temperature is set to something like 23°C. I would expect it to turn the heater switch off and on as it approaches (or even overshoots) the set temperature, but it just does nothing.

Am I doing something wrong or is it not meant to be used like that?