How to map PID output to actuator control?
I don't think you would need to use a PID library (or any control loop for that matter) to control a radiator. You could use a simple threshold code, like this:

Pseudocode:

Check temperature
If radiator is off and temperature &gt; (ideal temp - threshold)
  turn radiator on
If radiator is on and temperature &gt; (ideal temp + threshold)
  turn radiator off