In a COBP Zoning System, the rooftop unit (RTU) uses a constant volume fan. Air bypasses from the RTU supply duct back to the RTU return to air duct to achieve air volume control. A static pressure sensor controls the bypass air. The sensor is located in the supply air duct, downstream of the unity supply air discharge. An RTU with variable fan speed can be used instead of a bypass damper to control duct static pressure, however you must ensure that you maintain the minimum airflow required for proper heating and cooling at all times.
The bypass damper opens and closes based on the static pressure in the duct. The discharge air temperature at the RTU varies depending on the demand from the zones. Figure 1 provides an overview of the COBP Zoning System.
The COBP Zoning System RTUs have heating and cooling capabilities. The fan supplies a constant volume of cold or hot air to the duct system. Modulating zone dampers then feed the air to the individual zones. Each zone controller sends its heating or cooling needs to the Zone Coordinator. The Zone Coordinator selects the RTU mode of operation of heating, cooling, or fan only, based on what the zones need. The Zone Coordinator uses a voting system to select the correct mode of operation (see Zone Voting Logic).
The heating and cooling setpoints of the zone controller determine if it uses the air that the RTU supplies. For example, if one of the zones calls for cooling when the temperature in the duct is above the zone’s cooling setpoint, then the zone moves to its minimum cooling position to minimize the amount of warm air that enters the space. The zone dampers within the COBP Zoning System are usually pressure dependent. However, you can also use pressure independent zone boxes.