VRF Indoor Unit

ZoneHVAC:TerminalUnit:VariableRefrigerantFlow

Used in:   HVAC Zones

VRF Indoor units with variable refrigerant flow DX coils are used exclusively with variable refrigerant flow (VRF) systems. Indoor units operate to satisfy a heating or cooling load in a zone based on a zone thermostat temperature set point. Direct-expansion (DX) cooling and DX heating coils are specified and used depending on the operating mode required. Although a DX cooling and a DX heating coil are included in the terminal unit, only one may be used at any one time.

Outdoor ventilation air is modelled with the use of an optional outside air mixer object. Outside air can be provided to the zone in one of two ways: either only when the coil is operating, or continuously even when the coil is not operating. A supply air fan is also included and it can be modelled as either draw through or blow through.

The zone terminal unit will typically be the first equipment operating for both cooling and heating (i.e., Sequence = 1 on the Heating and Cooling Calculation Sequence tab of the HVAC zone group dialog). Other Zone HVAC equipment may be used in the same zone and should be sequenced to operate after the zone terminal units (i.e., sequence = 2 or higher).

General

Name

The name of the VRF indoor unit is defined internally and cannot be changed.

Operation

Availability schedule

Select the schedule that defines when the terminal unit is able to operate. A schedule value equal to 0 means that the unit is off at that time and a value greater than 0 denotes that it is available to operate. This schedule may be used to completely disable the unit as required.

Supply Air Flow Rates

Supply air flow rate during cooling operation

This numeric field defines the terminal unit’s operating volumetric air flow rate (in m3/s or ft3/min). This volumetric air flow rate is used when the terminal unit is operating in cooling mode.

Supply air flow rate during heating operation

This numeric field defines the terminal unit’s operating volumetric air flow rate (in m3/s or ft3/min). This volumetric air flow rate is used when the terminal unit is operating in heating mode.

Supply air flow rate when no cooling is needed

This numeric field defines the terminal unit’s operating volumetric air flow rate (in m3/s or ft3/min). This volumetric air flow rate is used when the terminal unit’s cooling coil is not operating and the previous mode was cooling.

Supply air flow rate when no heating is needed

This numeric field defines the terminal unit’s operating volumetric air flow rate (in m3/s or ft3/min). This volumetric air flow rate is used when the terminal unit’s heating coil is not operating and the previous mode was heating.

Outdoor Air Supply

Outdoor air supply

Check this checkbox if the VRF indoor unit has its own supply of fresh air. Then enter the 3 flow rates described below.

Outdoor air flow rate during cooling operation

This autosizable field defines the outdoor air volumetric air flow rate (in m3/s or ft3/min). This volumetric air flow rate is used when the terminal unit is operating in cooling mode.

Outdoor air flow rate during heating operation

This autosizable field defines the outdoor air volumetric air flow rate i(in m3/s or ft3/min). This volumetric air flow rate is used when the terminal unit is operating in heating mode.

Outdoor air flow rate when no cooling or heating is needed

This autosizable field defines the outdoor air volumetric air flow rate (in m3/s or ft3/min). This volumetric air flow rate is used when the terminal unit is not operating in cooling or heating mode.

Parasitic Electric Energy

Zone terminal unit on parasitic electric energy use

This numeric field defines the parasitic electrical energy use of the zone terminal unit when either terminal unit coil is operating. When in cooling mode, this electric energy use is reported in a zone terminal unit cooling electric consumption output variable. When in heating mode, this electric energy use is reported in a zone terminal unit heating electric consumption output variable. Units are W or Btu/hr.

Zone terminal unit off parasitic electric energy use

This numeric field defines the parasitic electrical energy use of the zone terminal unit when the terminal unit coil(s) is not operating. When the previous mode was cooling, this electric energy use is reported in a zone terminal unit cooling electric consumption output variable. When the previous mode was heating, this electric energy use is reported in a zone terminal unit heating electric consumption output variable. Units are W or Btu/hr.

Supply Fan

Fan type

This choice field contains the identifying type of supply air fan specified for the indoor unit. Fan type must be one of:

• 1-On/off fan used when the fan cycles on and off with the cooling or heating coil (i.e. Supply air fan operating mode schedule values are at times 0).
• 2-Constant volume fan used when the Supply air fan operating mode schedule values are never 0 and the fan operates continuously.

Supply air fan placement

This option has two choices:

• 1-Blow through which means that the unit consists of a fan followed by the DX coils. The fan “blows through” the cooling and heating coils. If an outside air mixer is used, the fan inlet connects to the outside air mixer’s mixed air node. If an outside air mixer is not used, the fan inlet connects to the zone exhaust node. For this configuration, the fan outlet always connects to the DX cooling coil inlet node (or if a DX cooling coil is not used, the DX heating coil inlet node).
• 2-Draw through. which means that the unit consists of the DX coil(s) followed by a fan. The fan “draws air through” the DX coil(s). In this case the fan inlet always connects to the DX heating coil outlet node (or if a DX heating coil is not use, the DX cooling coil outlet node) and the fan outlet node always connects to the zone inlet node.

Supply air fan operating mode schedule

Schedule values equal to 0 denote cycling fan/cycling coil operation. All other schedule values denote constant fan/cycling coil operation.

Heating Capacity

Override outdoor unit rated total heating capacity sizing ratio

Check this option if you would like to override the Rated total heating capacity sizing ratio used for the Outdoor unit.

Rated total heating capacity sizing ratio

This field defines the ratio of the heating coil to cooling coil size when DX heating coil capacity is autosized. It must be greater than or equal to 1. The default value is 1.