Center for the Built Environment

The thermostat setpoint range (deadband) in office buildings impacts both occupant thermal comfort and energy consumption. Zones operating within the deadband require no heating or cooling, and the terminal unit airflow volume rate may be reduced to its design minimum. Wider deadbands allow energy savings as well as lower total airflows through the terminal. The extent of such savings has not been systematically quantified. Reference models representing standard HVAC and building design practice were used to simulate the impact of thermostat setpoint ranges on annual HVAC energy consumption.  Heating and cooling setpoints were varied parametrically in seven ASHRAE climate zones and in six distinct medium-sized office buildings, each representing either a new building design or a building controls retrofit. The minimum airflow volume rates through the VAV terminal units were also varied to represent both standard and best practices. The simulations are compared to empirical data from monitored buildings. Without reducing satisfaction levels, by increasing the cooling setpoint of 22.2°C (72°F) to 25°C (77°F), an average of 29% of cooling energy and 27% total HVAC energy savings are achieved. Reducing the heating setpoint of 21.1°C (70°F) to 20°C (68°F) saves an average of 34% of terminal heating energy. Further widened temperature bands achieved with fans or personal controls can result in HVAC savings in the range of 32%–73% depending on the climate. It is demonstrated that in order to fully realize energy savings from widening thermostat temperature setpoints, today’s typical VAV minimum volume flow rates should be reduced.