Optimizing Comfort and Energy Use in Reheat Systems

This project investigated the relationship between energy use and human thermal comfort in a small commercial single zone VAV system with reheat. The reheat configuration is not optimal in terms of energy efficiency, but there are a large number of these systems installed; thus there is a need for improved operating strategies to reduce waste. This project was conducted in the Midwest U.S.; where cooling and dehumidification are needed during the summer. Data on electricity use by the reheat system was collected on days that met or exceeded the 2% summer design condition. Additional data on environmental conditions inside the building zone was also collected and put into a model for predicting occupant comfort. To extend the range of the project, a building energy simulation for the reheat system was developed and calibrated. The calibrated model was used to estimate energy/comfort trends over a broader range of indoor operating conditions. This experiment shows that electricity consumption for a single zone VAV system with reheat can be significantly reduced while satisfying the thermal comfort needs of over 90% of the building’s occupants by offsetting a lower dry-bulb temperature set point with correspondingly higher zone humidity. In other words, reheat systems can optimize energy and occupant comfort by operating at set points in the upper middle segment of the comfort zone from ASHRAE Standard 55.