A low-cost technique to retrofit a constant volume multizone system to a more energy efficient variable volume system was demonstrated on five systems at Fort Bragg, NC, and the U.S. Army Engineer Research and Development Center-Construction Engineering Research Laboratory – (ERDC-CERL) in Champaign, IL. When starting with a constant volume multizone air handler with modern Direct Digital Controls (DDCs), the conversion required programming changes to the control strategy executed by the control system as well as the installation of an Air Flow Measurement Array (AFMA) and Variable Frequency Drives (VFD)s for the supply and return fans (if so equipped). A key feature of this approach was that the physical system was only minimally affected and except for the location at which the AFMA is installed, the ductwork was not modified.
The updated control strategy varied the fan speeds based on the zone demand as determined by zone damper position, minimizing the fan energy used as well as the cooling and heating energy required to maintain occupant comfort by reducing the amount of simultaneous heating and cooling that occurs in a zone. Heating and cooling energy savings were most pronounced in traditional multizone systems with a hot deck and cold deck that operated simultaneously, but were also realized in systems with a neutral deck.
The five demonstration systems were retrofitted and operated for a period of approximately one year, alternating between three test modes. Test Mode 0 simulated the pre-retrofit condition and operated the system as a constant volume multizone with a fixed outside air damper position. Test Modes 1 and 2 employed variable volume control strategies. Test Mode 1 operated with a fixed outside air flow setpoint, and test Mode 2 introduced demand controlled ventilation schemes for determining the outside air flow setpoint. Additional instrumentation including British Thermal Unit (BTU) and electric meters was installed on the demonstration system at the time of retrofit to provide data for analysis of system performance. The existing Utility Monitoring and Control Systems (UMCS1) were used to log data from the system throughout the demonstration period.
The five systems were analyzed for energy savings, life cycle cost, occupant thermal comfort, and maintainability, where each of these factors were compared to the baseline constant-volume system.
Energy Savings: All systems easily met the energy savings goals of 10% energy use reduction, with energy reduction ranging from 24%-60%.
Life Cycle Cost: One of the five systems met the life cycle cost goals of a 3-year payback period assuming the conversion is added to an existing DDC system or planned renovation (“incremental retrofit”) and 10-year payback for the complete renovation of a system from non-DDC to DDC with variable volume control. Three other systems had longer payback periods less than the system life for the incremental retrofit; however, demonstrating that the addition of variable volume control to a DDC retrofit is still economical in those cases. Since retrofit costs are relatively static across system sizes, the long payback periods for smaller systems can be expected and demonstrates that some care should be exercised in selecting appropriate systems on which to apply this technique.
Thermal Comfort: The two systems at CERL performed nearly the same across all three operating modes. The difference across modes was more significant at Fort Bragg, where two had worse comfort performance in the variable volume modes, where one system spent 29-33% of time within the thermal comfort range in Modes 1 and 2 versus 39% in Mode 0. The other system spent 55% of the time in the comfort range in Modes 1 and 2 and 61% in Mode 0. The third system at Fort Bragg performed significantly better in Modes 1 and 2, however, with 53-54% of the time in the comfort range versus 34% for Mode 0. In all systems, however, the average deviation from zone setpoint did not increase more than 0.5 °F in Modes 1 and 2 versus Mode 0, which is well within the normal variation of space temperatures in a building indicating that the occupants were highly unlikely to notice a difference between modes. Although individual system results were mixed, the variable volume modes did not perform significantly worse overall than the constant volume system and comfort performance was considered acceptable.
System Maintenance: System maintenance was acceptable as neither demonstration site reported any maintenance concerns with the retrofitted systems.
Overall, the demonstration of the conversion of a constant volume multizone to variable volume was successful as the results demonstrated the potential for the conversion to meet energy savings, comfort, cost, and maintenance requirements. Selection of systems for application of this technique should consider multizone type and size, with preference given to larger multizone, and traditional 2-deck multizone systems.