A PID controller is what is used on modern programmable thermostats. PID stands for proportional–integral–derivative controller. It uses a controller calculation or algorithm to make adjustments to a system.
For Honeywell, Hunter and Lux Products thermostats it is a PID controller that is used for the smart response function or adaptive intelligent recovery.
Basically the PID controller remembers the times needed to heat or cool a room previously and checks this information against the current room temperature. The PID controller will come up with a time when the thermostat needs to activate the heating or cooling so that the set point temperature is reached at the beginning of a programmed period.
The advantage of using a PID controller with smart response is that it ‘learns’ how long it takes to pre-heat or pre-cool a room. The optimum time needed to make a room comfortable changes throughout the year – in mild fall and spring weather less time is needed and in summer and winter more time is needed. Also hot water systems take longer to change room temperature than do central air systems. PID takes the guess work out of doing this. Without PID a user would either have to wake up to an uncomfortable house or guess when to set the thermostat to start prior to waking up. The issue of the purpose of adaptive intelligent recovery is covered in another post.
Finally, the PID controller on programmable thermostats is used to keep the temperature stable. As said before, it learns about the behavior of a room with regard to heating and cooling. PID prevents overshoots with heating or cooling. The PID controller tells the thermostat when to stop heating or cooling so the room temperature is nudged up to the desired level. If you want a room at 21 degrees Celsius then if the heating was on until this temperature was reached then the room would go higher than 21 degrees. It is PID that tells the thermostat to turn the heating off at 19 or 18 degrees and allow the residual heat to bring the temperature up.