A standard control loop on almost all BEMSs is the optimizer loop. This switches on the heating plant at the latest possible time to get the building warm by the start of occupancy. It can also be used with air-conditioning systems with both heating and cooling. In terms of the notation in Fig. 9.1, the optimizer determines the shortest preheat time, T3-T2, to get the occupied space up to the design temperature, td. Temperature tp is the inside temperature at which the optimizer switches on the heating and starts the preheat, which occurs at the time denoted by T2. The outside temperature is tao.