CH-12-C080 -- Advanced Unitary HVAC Control Sequence PDF

Smaller rooftop unitary packaged HVAC systems (RTUs) have a high proportion of improperly set or failing controls that negatively impact indoor air quality and energy use. Improved advanced control sequences operating in a more reliable controller have been field tested with controllers from multiple manufacturers. Hourly simulation shows that an advanced control sequence upgrade can produce HVAC energy savings ranging from 30% to 48% across the US. These control savings are many times greater than the savings from a SEER 13 to SEER 15 upgrade. Achieving these savings requires a comprehensive package of control measures designed to improve the reliability of economizer control and optimum start, while improving ventilation control, and reducing the energy used for fans, heating, cooling and ventilation. This paper expands on earlier discussions of the need for improved ventilation and details a field tested advanced sequence of operation—providing a synergy of improved ventilation and energy savings for RTUs. The sequence is field tested with BACnet controllers from three manufacturers and is designed to be implemented in either a stand-alone controller or as part of a DDC system. A significant improvement over currently available programmable thermostats, the advanced controls include stand-alone direct digital controls, demand controlled ventilation with integrated fan cycling, premium economizer control, night flush, occupancy sensor standby, and a more reliable optimum start routine. The comprehensive control upgrade package was installed on test units and put through functional field testing and monitoring that confirmed the advanced control algorithms meet ASHRAE Standard 62.1 while either cycling the fan or modulating fan speed. Resolution of issues related to user override interface, economizer stability, and managing electric heat demand for heat pumps during optimum start was part of the testing process. A streamlined acceptance testing protocol was also developed and the contractor setup method for the demand controlled ventilation was improved. The result demonstrates that cost-effective energy savings and improved ventilation are possible when retrofitting RTUs with advanced unitary HVAC controls.