Changeover-Bypass
Changeover-Bypass VAV
This system has a central air handler with integral heating and cooling coils that can supply either heated or cooled supply air to one or more rooms. The volume supplied to each zone is individually regulated by a zone VAV damper. Each VAV damper is linked to a zone thermostat sensor, thus enabling the occupants to choose their own local space temperature. Treated air is supplied to the zone VAV dampers via a network of low velocity ductwork supplied from a built-up air handling unit or a packaged rooftop unit. This central unit delivers a constant volume of air but is sized according to the block (coincident) cooling load. Any air that is not needed to heat or cool the rooms is simply returned back to the air handling unit via a system-level bypass loop. Each room VAV damper communicates with the main AHU as to whether the room requires hot or cold air to satisfy the room setpoint temperature.
Priority Control Mode
The operation of the VAV dampers in conjunction with the primary cooling and heating coils is determined by the Operating Mode in the changeover-bypass VAV equipment library. If Cooling Priority is selected, the system operates to meet the cooling load if any zone served by this system (air loop) requires cooling. If no zones require cooling, then the system operates in heating mode if needed. If Heating Priority is selected, the system operates to meet the heating load if any zone requires heating. If no zones require heating, then the system operates in cooling mode if needed. If Highest Mode Priority is selected, the system operates based on the maximum number of zones requiring either heating or cooling. If the number of zones requiring cooling is greater than the number of zones requiring heating, then the system operates in cooling mode. If the number of zones requiring heating is greater than the number of zones requiring cooling, then the system operates in heating mode. If the number of zones requiring cooling equals the number of zones requiring heating, then the largest combined load (i.e., the sum of the cooling loads for zones requiring cooling compared to the sum of the heating loads for zones that require heating) sets the cooling or heating operating mode for the system during that simulation time step. The operation of the individual room VAV dampers depends on the Air Terminal device chosen.
Deadband Mode
If none of the rooms are in either cooling or heating mode, then both the cooling and heating coils are deactivated and the fan runs per the field “System Air Flow Rate When No Cooling or Heating is Needed” This is written behind the scenes and is autosized based on the system loads. This field is only used when the unitary system’s supply air fan cycling mode is specified as continuous fan operation. If the system’s supply air fan cycling mode is specified as continuous fan operation and this value is set to zero or the field is left blank, then the model assumes that the system air flow rate when no heating/cooling is needed is equal to the system air flow rate when the coils were last operating (for cooling operation or heating operation).
Real-life Application Considerations
This system is best suited to applications that require either heating or cooling but not both since at any given time the air handling unit must supply either warm or cool air. For isolated rooms that may occasionally require heating when the rest of the system needs cooling, electric or hot water reheat coils or baseboards can be provided to maintain comfort conditions. To satisfy minimum ventilation requirements, a room VAV minimum supply air setting can be defined to guarantee a minimum supply airflow into the room even if there is no demand for heating or cooling.
Changeover-Bypass VAV with Local Heat
Notice that in this configuration, the VAV boxes have no reheat coil, just a damper. The user could easily switch out the baseboard with a floor or ceiling radiant panel in the zone equipment tab of the systems section in the project.
Cooling Mode Operation
If the AHU is in cooling mode, the VAV damper opens as needed in each room to provide sensible cooling to those room(s); however, with the AHU in cooling mode, the room(s) needing heat or in the deadband will receive cold supply air equal to the VAV minimum setting. See figure below. If this cold minimum supply airflow causes the space temperature to fall below a room’s heating thermostat, that room’s baseboard unit will activate to meet this “reheat” load.
Heating Mode Operation
If the majority of rooms call for heating, the rooms are first served by their VAV box which will increase the heated supply air flow rate as needed to meet higher space heating loads. See figure below. If however, this heated supply air is inadequate to meet an individual room’s heating thermostat setpoint, the baseboard is activated to meet this remaining load. Any rooms that do not require heating will still receive heated supply airflow equal to their VAV minimum airflow setting and could temporarily lose control of their space temperature.
Changeover-Bypass VAV with Reheat
Notice that in the above configuration, the VAV boxes include a reheat coil as the only source of heating, and the damper heating action is set to Dual Maximum.
Cooling Mode Operation
If the AHU is in cooling mode, the VAV damper opens as needed in each zone to provide sensible cooling to those zone(s); however, with the AHU in cooling mode, the zone(s) needing heat or in the deadband will receive cold supply air equal to the VAV minimum setting. See figure below. If this cold minimum supply airflow causes the space temperature to fall below a room’s heating thermostat, that room’s reheat coil will activate to meet this “reheat” load.
Heating Mode Operation
If the majority of rooms call for heating, the rooms are first served by their VAV box which will increase the heated supply air flow rate as needed to meet higher space heating loads. See figure below. If additional heat is required (beyond what the terminal unit can provide with its damper fully open), then the reheat coil is modulated as needed to meet the additional heating load. Any rooms that do not require heating will still receive heated supply airflow equal to their VAV minimum airflow setting and could temporarily lose control of their space temperature.
