Unit Heater

Stencil: Zone HVAC Stencil: Unit Heater

Type: Unitary Zone Equipment

Sub Type: Air Conditioner Window

Note: Requires a connection to a hot water loop

EnergyPlus IDF Objects

Unit heaters are zone equipment units which are assembled from other components and are a simplification of unit ventilators. They contain only a fan and a heating coil.

While the control of the heating coil is similar to the fan coil units and the unit ventilator, the overall control of the unit heater is much different. There are four different modes in which a unit heat can operate based on the user input:

Off: Unit is schedule off. All flow rates are set to zero and the temperatures are set to zone conditions.
NoLoad OR Cooling/OnOff Fan Control: Unit is available, but there is no heating load. All flow rates are set to zero and the temperatures are set to zone conditions.
NoLoad OR Cooling/Continuous Fan Control: Unit is available and the fan is running (if it is scheduled to be available also). No heating is provided, only circulation via the fan running.
Heating: The unit is on/available and there is a heating load. The heating coil is modulated (constant fan speed) to meet the heating load.
Control of the heating coil and its flow rate is identical to the fan coil unit and unit ventilator

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
Supply Air Fan Name	Libraries/Fans&Pumps/SupplyFan/ConstantVolume
Maximum Supply Air Flow Rate	0-Min, Autosize
Fan Control Type	Continuous On, Continuous Off
Heating Coil Name	Libraries/Coils & HX Eqmt/HeatingCoilDX/CoilAirHeatingElectric
Maximum Hot Water or Steam Flow Rate	0-Min, Autosize
Minimum Hot Water or Steam Flow Rate	0-Min, Autosize
Heating Convergence Tolerance	0-Min; 0.001 Default
Availability Manager List Name	Libraries/ControlSchemeList/SchemeList/AirLoopAvailabilityMgrScheme

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: Supply Air Fan Name

This field is the name of a fan (ref: Fan:ConstantVolume, Fan:VariableVolume) that is part of the unit heater system. This name links the unit heater to particular fan data entered elsewhere in the input data file. A fan name is required since it is the prime mover of air in the unit heater system.

Field: Maximum Supply Air Flow Rate

This field allows the user to enter the maximum volumetric flow rate of air through the unit heater system in m³/s. This parameter should be some real number greater than zero.

Field: Fan Control Type

This field allows the user to define how the unit heater will operate under “no load” or cooling conditions. The user may select from two options. If the “OnOff” control is selected, then the fan will not run unless there is a heating load. If the fan does not run, this effectively shuts the unit heater system off (there is no option to simply run the heating coil and allow natural convection to transfer heat from the unit to the zone). If the “Continuous” control is selected, then the fan will always run if the unit heater is available. This will produce air movement in the zone but also add some amount of heat to the energy balance since the fan will generate some heat. For more information on controlling the operation of the unit heater, see the general description given above.

Field: Heating Coil Name

This field is the name of the heating coil that is part of the unit heater system. It is assumed that there is always some sort of heating coil associated with a unit heater system. This name links the unit ventilator to particular heating coil data entered elsewhere in the input data file.

Field: Maximum Hot Water or Steam Flow Rate

This field allows the user to enter a maximum volumetric flow rate of water through a water heating coil in m³/sec. This number should be some number greater than zero and greater than the minimum hot water volumetric flow rate (see next field). Note that this parameter has no meaning for either an electric or a gas heating coil.

Field: Minimum Hot Water or Steam Flow Rate

This field allows the user to enter a minimum volumetric flow rate of water through a water heating coil in m³/sec. This number should be some number greater than zero and less than the maximum hot water volumetric flow rate (see previous field). Note that this parameter has no meaning for either an electric or a gas heating coil.

Field: Heating Convergence Tolerance

The convergence tolerance for the unit heating output. This field allows the user some control over how closely the heating coil will control the air side conditions. The relative size of this parameter relates directly to the closeness of the control. A very small value in this field will result in tight control and will probably result in larger numbers of iterations. A large value in this field will result in looser controls and could result in unsatisfactory fluctuations in supply conditions (that could in turn result in excessive iterations). Initial experience with this parameter lends to the recommendation of using 0.001 as a starting point. This field is ignored for gas and electric heating coils.

The unit is controlled by matching the unit output to the zone demand. For units with water coils, the model must be numerically inverted to obtain a specified output. The heating convergence tolerance is the error tolerance used to terminate the numerical inversion procedure. Basically this is the fraction:

Field: Availability Manager List Name

This optional input field is the name of an AvailabilityManagerAssignmentList object. An Availability Manager Assignment List is a list of Availability Managers giving both Availability Manager type and name. The availability managers in the list apply to this window air conditioner or object’s fan. If the window air conditioner is available (per the Availability Schedule Name input field above) and this input field has a valid availability manager assignment list name, then the availability managers in the list determine when and if the fan of this window air conditioner object should be on or off.

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