Single Zone VAV
Single Zone Variable Air Volume
This system consists of a variable volume fan, a DX cooling coil and a heating coil (usually electric resistance heat) and serves a single zone. The coils and fan are therefore located at the zone level, meaning that a separate Single Zone VAV (SZVAV) unit will be created for each zone attached to that system type. If a given zone is comprised of several rooms, this system is typically controlled by a single thermostat located in one of the rooms. This means that all the rooms get the same supply airflow fraction and supply air temperature as the “control” room.
Each zone assigned to this system will be given its own set of coils. For example, if 10 zones are assigned to an individual Single Zone VAV system, 10 systems will be created, one per zone.
Supply Airflow/Temperature Control
For Single Zone VAV, the cooling supply air temperature each time step will default to the design cooling supply air dry bulb (SADBC), i.e., the program assumes that the hourly SADBC Setpoint = Design SADBC for the entire year. This default value can be altered by using Supply Air Reset controls on the controls tab in the system properties.
The minimum cooling airflow defaults to 30% and can be overridden by changing the VAV Minimum Cooling Airflow value on the terminal device tab of the system properties. In cooling mode, the supply air dry bulb is fixed while the supply airflow is varied to match the load. While the thermostat signal is in the deadband region (essentially, no load condition), the Single Zone VAV unit will continue to deliver minimum airflow at the SADBC setpoint. However, once the control thermostat senses that the room temperature has fallen below the heating thermostat setpoint, the cooling coil is deactivated and the heating coil becomes active. In heating mode, the supply airflow is maintained at its minimum setting and SADBH is varied to meet the space load. However, once the supply air dry bulb heating has reached its maximum (design) value, the airflow is increased to match the space load. The maximum heating-mode airflow is based on the VAV Minimum Heating Airflow value defined on the terminal device tab of the system properties. The control scheme over the range of space loads is shown in the diagram below.
Design Considerations
Since the Single Zone VAV air handler and coils are located at the zone level, the design cooling airflow is block at the zone level. This means that the total cooling design airflow across the system is the sum of the zone block airflows rather than the block airflow at the time of the system block. The system checksums will display the sum of the zone block airflows (collection of zone-level fans) rather than the system block airflow (which would only be correct if the fan were at the system level).
DesignClgSupplyAirflowzn = QBlockClgSpaceSensiblezn / [k * (DsRmdbc – SADBC)]
DesignClgSupplyAirflowsys = ∑DesignClgSupplyBlockAirflowzn
The zone design heating airflow will equal the sum of the design VAV heating airflows (the VAV maximum heating airflow setting as defined on the Create Room – Airflows screen). However, if the VAV maximum heating airflow value has not been defined, the VAV minimum cooling airflow is used instead. The maximum heating for a room occurs when the heating airflow and SADB are both at their maximums. If the AHU serves more than one room, the design heating supply airflow is the sum of the maximum heating supply airflows from the attached rooms:
DesignHtgSupplyAirflowzn = ∑MaxHtgSupplyAirflowrm
The system checksums represents the sum of the zone AHU’s and so will need to display the sum-of-the peak zone heating airflows (which also equals the sum-of-the-peak room heating design airflows).
DesignHtgSupplyAirflowsys = ∑ DesignHtgSupplyAirflowzn
The zone AHU’s design heating supply air dry bulb is based on the largest SADBH required after scanning all the rooms attached to a particular zone. First, the required SADBHrm is calculated for each room based on that room’s design supply heating airflow, design space sensible heating load, and DsRmdbhrm:
SADBHrm = DsRmdbhrm - QHtgSpaceSensRequiredrm / [k * DesignHtgSupplyAirflowrm]
Then the highest of these values is used to set the design value of SADBH for the zone-level AHU.
SADBHzn = Largest of [SADBHrm] for rooms attached to this zone
The design heating capacity is determined by first calculating the entering coil condition at the winter design condition:
Cedbhzn =[ (DesignHtgSupplyAirflowzn – DesignVentAirflowz) * Radbh + DesignVentAirflow*TVentDeckLvg] / DesignHtgSupplyAirflow
The zone-level AHU’s heating capacity is then given by:
QMainHtgCapacityzn = k * DesignHtgSupplyAirflowzn * (SADBHzn - Cedbhzn)
Because the SADBHzn is determined by finding the highest SADBHrm from all the rooms attached to that zone AHU, a net space sensible oversizing occurs for the rooms whose required SADBHrm was less than SADBHzn:
QHtgSpaceSensActualrm = k * DesignHtgSupplyAirflowrm * (SADBHzn – Cedbhrm)
QHtgSpaceSensOversizing = QHtgSpaceSensRequiredrm – QHtgSpaceSensActualrm = k * DesignHtgSupplyAirflowrm * (SADBHrm – SADBHzn)
Zoning Considerations
Since the Single Zone VAV system does not have reheat coils at the room terminal units and because the thermostat sensor is typically located in just one of the rooms attached to the zone-level AHU, all rooms will receive the same temperature and supply airflow fraction as the “control” room. Therefore, it is extremely important to assign rooms with similar thermal loading to their parent zone-level AHU, i.e., the attached rooms facades (if any) should face the same direction and have nearly identical internal load schedules and load/floor area. Wild room temperature swings can occur if the control room’s thermal profile is not similar to the other attached rooms. Of course, if an AHU is assigned to each room, i.e., each room is its own zone, this consideration becomes moot.
Simulation notes
The program simulation is modeled in hour increments which can cause “non-reality” incidents. For example, the cooling coil would normally deactivate when the thermostat falls below the heating thermostat; however, if the introduction of minimum cooling airflow is the cause of the room temperature falling below the heating thermostat, the cooling coil will be off for part of the hour which is modeled by calculating an average SADBC for the hour instead of forcing DsnSADBC the entire hour.