Although PID loop control has been discussed in Chapter 4, a modified version is used in most BEMSs to control the heating. This modified PID control is a compensator loop controller. It enables the heating system output to be varied in relation to the outside temperature. A whole building’s heating, or its zones, can be controlled by compensator loops, without the need for a myriad of room thermostats, wiring and valves. If windows are opened, the heating output is not increased and wasted. Hence compensation control is widely used, but there are complications and this chapter will explore them.