A thermostat for a building space. The thermostat includes a communications interface and a processing circuit. The communications interface is configured to engage in bidirectional communications with heating, ventilation, or air conditioning (HVAC) equipment and to receive an indication of a current heating or cooling load from the HVAC equipment. The processing circuit is configured to determine an occupancy of the building space based on the indication of the current heating or cooling load received from the HVAC equipment. The processing circuit is further configured to operate the HVAC equipment based on the determined occupancy of the building space.