RTU Retrofit Technologies Assessment (RTU Supply Fan Cycling Control)

Single zone, constant volume packaged rooftop units (RTUs) are the most common air conditioning configuration for small-to-medium commercial facilities like offices or retail buildings. RTUs typically provide cooling, heating, and ventilation to a space in a building. In the past, building codes generally required a minimum amount of ventilation air to be supplied to a space during all times that the space is normally occupied. Recent modifications to codes allow for the RTU’s supply fan to be cycled off for up to thirty minutes at a time as long as the average ventilation rate is maintained. This modification provides the opportunity to reduce the fan energy required for ventilation through the application of retrofit controls. This technology evaluation tests a package of controls that cycles the supply fan off for up to thirty minutes while using an algorithm to ensure the average ventilation delivered meets the ventilation requirements of the space. The controls also incorporate Demand Control Ventilation (DCV) controls that set the ventilation requirement based on CO2 concentration in the space. In addition to fan cycling and DCV control, occupancy sensors are used to increase the cooling setpoint and decrease the heating setpoint when there are no occupants in the space. In combination, the controls have the potential to reduce supply fan energy as well as heating and cooling energy. The Fan Cycling Controls as described are evaluated against the baseline operation, defined as a constant volume unit operating the supply fan continuously during occupied hours without DCV or occupancy sensors. The test is conducted on two (2) RTU heat pumps, AC10 (5 tons nominal capacity) and AC-M (4 tons nominal capacity) serving an office building in Irwindale, CA. Energy savings are assessed by installing the controls and monitoring their performance for 6 months. Analysis is conducted on data for a 6 week period to determine electrical energy savings, impact on indoor air quality, and assessment of economizer performance. It was found that the Fan Cycling Controls on one of the units, AC-10, did not perform as designed, potentially due to a wiring or hardware malfunction which prevented the controller from initiating mechanical cooling operation. Therefore the energy savings results for AC-10 are invalid. The Fan Cycling Controls achieved significant energy savings on the other unit in the test, AC-M. The energy savings results for AC-M were determined to be valid. Table-ES 1 provides a summary of energy consumption and savings during the test for unit AC-M.