Feasibility of retrofitting centralized HVAC systems for room-level zoning

Heating, ventilation, and cooling (HVAC) accounts for 38% of building energy usage, and over 15% of all US energy usage, making it one of the nation's largest energy consumers. Many attempts have been made to optimize the control of HVAC systems by minimizing the energy wasted in conditioning buildings that are unoccupied. Systems have been proposed that turn off HVAC systems when a house is unoccupied, or put the system into an energy saving deep-setback mode when the occupants are asleep. An area that has not been as well explored is the retrofitting of centralized HVAC systems to save energy when the residents are at home and awake. In this paper, we demonstrate how to use cheap, off-the-shelf sensors and actuators to retrofit a centralized HVAC system and enable rooms to be heated or cooled individually, in order to reduce waste caused by conditioning unoccupied rooms. We call this approach room-level zoning. Sensors are used to detect occupancy in rooms which allows the learning of occupancy patterns and prediction of room occupancy. Unoccupied rooms can be allowed to drift away from a user defined comfortable temperature if they are less likely to be used in the near future while occupied rooms are maintained at a comfortable temperature. We implement room-level zoning in a 1400 square foot house by retrofitting an existing centralized HVAC system with wireless temperature sensors to monitor room-level temperature, motion sensors to monitor occupancy, and wirelessly actuatable dampers to control the flow of conditioned air through the house. Initial analysis indicates that this method has a 20.5% energy savings over the existing single-zoned thermostat.