VAV Reheat

Stencil: Zone HVAC Stencil: Grp-VAV w/reheat

Type: Air Terminal

Sub Type: VAV_Reheat

Note: Requires a connection to a hot water loop

EnergyPlus IDF Objects

EnergyPlus Object - AirTerminal:SingleDuct:VAV:Reheat

Variable air volume (VAV) systems control the dry-bulb temperature inside a zone by varying the supply air volume instead of the air temperature. At full cooling the VAV damper is fully open supplying the specified maximum air flow rate. As the cooling load decreases, the damper closes until it reaches the minimum stop specified by the zone minimum air flow fraction.

VAV systems can be used for interior or perimeter zones with a common fan system, air temperature control, and reheating devices. The VAV concept may vary according to the VAV box locations, air temperature controls and types of heating elements. Heating can usually be provided by use of reheat coils or thermostatic baseboard.

Component Properties

The table shows the properties that are displayed when the component is selected while in diagram mode. The second column shows the selection options available that are dictated by EnergyPlus or it shows the source for the library entries that are displayed in the drop down list.

Property Name	Value Sources/Options
Name	Simergy provides unique name (editable)
Availability Schedule Name
Maximum Air Flow Rate	0-Min; Autosize
Zone Minimum Air Flow Method	Scheduled, Fixed Flow Rate, Constant
Constant Minimum Air Flow Fraction	0 to 1; fraction of max air flow rate
Fixed Minimum Air Flow Rate	0 to 1
Minimum Air Flow Fraction Schedule Name
Reheat Coil Name	Libraries/Plant and Equipment/Air Terminals
Maximum Hot Water or Steam Flow Rate	0-Min; Autosize
Minimum Hot Water or Steam Flow Rate
Convergence Tolerance	0-Min; 0.001 Default
Damper Heating Action	Reverse, Normal
Maximum Flow Fraction During Reheat	value; autocalculate
Hot Water Equipment Load 1	value; autocalculate
Maximum Reheat Air Temperature	value; 32C minimum
Design Specification Outdoor Air Object Name	SizingParams/HVACDesign/OutdoorAirDesignSpec

Name

Simergy automatically defines a unique name for each component. This can be changed by the user if desired.

Field: Availability Schedule Name

Schedule that this component will operate or is available to operate.

Field: Maximum Air Flow Rate

The design maximum volume flow rate specified for VAV ADU.

Field: Zone Minimum Air Flow Method

This field is used to select how the program will determine the minimum flow rate to the zone while the system is operating. There are three choices for selecting how the minimum flow rate is specified: Constant, FixedFlowRate, and Scheduled. If Constant is entered, then the program will use the value for the constant minimum air flow fraction entered in the following field. If FixedFlowRate is entered, then the program will use the value entered in the field below called Fixed Minimum Air Flow Rate. If Scheduled is entered, then the program will obtain the value for minimum flow fraction from the schedule named in the field below called Minimum Air Flow Fraction Schedule Name.

Field: Constant Minimum Air Flow Fraction

The minimum flow rate to the zone while the system is operating, specified as a fraction of the maximum air flow rate. The minimum zone fraction is normally specified to meet the minimum ventilation requirement for the occupants. The reheat coil operates only when the damper is at this minimum flow rate when Damper Heating Action is set to Normal (the default). This field is used if the previous field is set to Constant. If the previous field is set to Scheduled (and the field Maximum Hot Water or Steam Flow Rate is set to autosize), then this field is optional and can be used to separately control the air flow rate used for sizing normal-action reheat coils. If this field and the following field have values, the greater of the two is used for sizing.

Field: Fixed Minimum Air Flow Rate

The minimum flow rate to the zone while the system is operating, specified as a fixed minimum air flow rate. The minimum air flow rate is normally specified to meet the minimum ventilation requirement for the occupants. The reheat coil operates only when the damper is at this minimum flow rate when Damper Heating Action is set to Normal (the default). This field is used if the Zone Minimum Air Flow Method field is set to FixedFlowRate. If the Zone Minimum Air Flow Method field is set to Scheduled (and the field Maximum Hot Water or Steam Flow Rate is set to autosize), then this field is optional and can be used to separately control the air flow rate used for sizing normal-action reheat coils. Only one of these two minimum air flow fields (i.e., this field and the previous field) should be used at any time. If this field and the previous field have values, the greater of the two is used for sizing.

Field: Minimum Air Flow Fraction Schedule Name

The name of a schedule that determines the value of the minimum air flow fraction. The schedule should contain fractions from 0.0 to 1.0. These values will define the minimum flow rate to the zone while the system is operating, specified as a fraction of the maximum air flow rate. The reheat coil operates only when the damper is at this minimum flow rate when Damper Heating Action is set to Normal (the default). This field is used if the previous field is set to Scheduled. If the previous field is left blank (and the field Maximum Hot Water or Steam Flow Rate is set to autosize), then the air flow rate used for sizing normal-action reheat coils is the average of the minimum and maximum values in this schedule. The air flow rate used for reheat coil sizing is reported with other component sizing information as “Reheat Coil Sizing Air Volume Flow Rate.”

Field: Reheat Coil Name

Reheat Coil Object name being simulated with this ADU. Applicable for all coils. If there is no reheat coil,use object AirTerminal:SingleDuct:VAV:NoReheat instead of this object.

Field: Maximum Hot Water or Steam Flow Rate

This field is 0 for gas and electric coils. Set to the maximum design water flow ) for the hot water coil. This field is autosizable. If there is no reheat coil, this is left blank.

Field: Minimum Hot Water or Steam Flow Rate

This field is zero for gas and electric coils. Set to the minimum design water flow (m³/sec) for the hot water coil, normally set to be a shut off valve that is set to zero. If there is no reheat coil, this is left blank.

Field: Convergence Tolerance

The coil is controlled by knowing the zone demand determined by the zone thermostat and setting the outlet conditions to meet this demand. For the electric and gas coils, this is set exactly since the coil model solution can be inverted. With the hot water coil that uses an effectiveness-NTU method, the solution cannot be inverted directly. Therefore, to determine the correct mass flow rate for the hot water the solution is solved for by iteration. The iterative solution uses an interval halving routine and needs a termination criteria that is set with the Convergence Tolerance parameter. This control offset is set to a decimal fraction of the zone demand as the criteria, i.e. 0.001. The default for the field is 0.001.

Field: Damper Heating Action

in the VAV terminal unit as the zone moves above or below the zone setpoint. With both control options, the damper is at the minimum air flow rate whenever the zone temperature is between the cooling and heating setpoints (deadband condition).

With Normal (the default) action, the damper will remain at the minimum air flow rate during heating operation. As the heating load increases, the water flow rate in the reheat coil will be increased to maintain temperature in the zone until the maximum water flow rate is reached or the user-specified maximum reheat air temperature is reached
With Reverse, as the heating load increases, the unit starts at minimum air flow and minimum hot water flow. The hot water flow is increased until it reaches maximum flow or the user-specified maximum reheat air temperature is reached, then the air damper starts to open to meet the load. This option is used if the minimum air flow rate is not adequate to serve the peak heating load. This is sometimes called the dual maximum control logic as illustrated in following figure. For heating coil types other than the hot-water coil, e.g. electric, steam, and gas, the reverse action works the same as the normal action – always keeping the air flow at the minimum during heating.

Control Fields for Maximum Flow During Reheat:

The following two fields are used only when Reheat Coil Object Type = Coil:Heating:Water and Damper Heating Action = Reverse. Maximum Flow per Zone Floor Area During Reheat and Maximum Flow Fraction During Reheat are two optional methods to calculate the maximum allowable air flow rate during reheat operation. If both are entered, the greater resulting flow rate is used. If Design Specification Outdoor Air Object Name is also specified, it may increase this limit to meet the outdoor air flow rate requirement. At no time will the maximum flow rate calculated here exceed the value for Maximum Air Flow Rate.

This limit is active only when the zone thermostat requests heating and the VAV box damper is reverse acting.

Field: Maximum Flow per Zone Floor Area During Reheat

This factor (m³/s-m²) is multiplied by the zone area, to determine the maximum volume flow rate (m³/s) allowed during reheat operation (see detailed explanation above). This field is autocalculatable. If autocalculate is selected, the value is set to 0.002032 m³/s-m² (0.4 cfm/ft²). If this field and the following field are entered, the greater of the two inputs is used. If this field and the following field are left blank, the maximum flow will not be limited.

Field: Maximum Flow Fraction During Reheat

This fraction is multiplied by the Maximum Air Flow Rate to determine the maximum volume flow rate (m³/s) allowed during reheat operation (see detailed explanation above). This field is autocalculatable. If autocalculate is selected, the value is set to 0.002032 m³/s-m² (0.4 cfm/ft²) multiplied by the zone floor area divided by the Maximum Air Flow Rate. If this field and the previous field are entered, the greater of the two inputs is used. If this field and the previous field are left blank, the maximum flow will not be limited.

Field: Maximum Reheat Air Temperature

This field specifies a maximum supply air temperature leaving the reheat coil in a VAV terminal unit during heating operation. If leaving blank, the temperature of the supply air to the space in heating operation may get unrealistic high.

Field: Design Specification Outdoor Air Object Name

This alpha field specifies the name of a DesignSpecification:OutdoorAir object. When this field is used, the terminal unit will increase flow as needed to meet this outdoor air requirement. If Outdoor Air Flow per Person is non-zero, then the outdoor air requirement will be computed based on the current number of occupants in the zone. At no time will the supply air flow rate exceed the value for Maximum Air Flow Rate. If this field is blank, then the terminal unit will not be controlled for outdoor air flow. See documentation for the zone HVAC outdoor air object for further information (Ref DesignSpecification:OutdoorAir).

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