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The answer depends on the type of rooftop that is being used. Most rooftops fall into one of three categories –Constant Volume, Variable Volume, and Constant Volume Fan with Variable Volume Terminal Devices.

For a constant volume RTU, there are three airside system types that are typically used – Single Zone, Variable Temperature Constant Volume, and Terminal Reheat.

The single zone and VTCV system types are very similar. They should be used to model RTU’s that have one heating coil, one cooling coil, and one constant volume fan that serve one or multiple spaces. For the single zone and VTCV system types to serve multiple rooms in TRACE, a zone must be created in Assign Rooms to Systems and all rooms served by the RTU must be assigned to that one zone.

Each zone assigned to this system type or each room directly assigned to the system type will be modeled as a separate RTU. Please refer to the document Zoning.ppt for a more details on zoning with SZ and VTCV systems.

The terminal reheat system type has one system level cooling coil, one system level heating coil, and multiple zone level main/reheat heating coils (the heating coil in the terminal device). The terminal reheat system is very similar to a typical VAV with reheat system, except that the airflow for the rooms and the supply fan is at a constant volume. Zones should be created for this system if the same terminal device will serve multiple rooms. All rooms and zones assigned to this system type will be served by the same supply fan and cooling coil.

For a variable volume (at the supply fan and at the terminal devices) RTU, there are several different airside system types that are typically used – Variable Volume Reheat, VAV with Baseboard Heating, Series Fan-Powered VAV, etc. The schematic drawing of the systems in Create Airside Systems can be used to help find the correct system type based on the location of the heating coils, what type of terminal devices will be used, etc. For these system types, zones should be created if the same terminal device will serve multiple rooms. The same supply fan and cooling coil will serve all rooms and zones assigned to these system types.

For a system with a constant volume fan and variable volume terminal devices (also know as a VariTrac system), there are three airside system types that are typically used – Changeover-Bypass VAV, Changeover-Bypass VAV with Local Heat, and Changeover-Bypass VAV with Reheat. The schematic drawing of the systems in Create Airside Systems can be used to help find the correct system type based on the heating coil location/s. For these system types, zones should be created if the same terminal device will serve multiple rooms. The same supply fan and cooling coil will serve all rooms and zones assigned to these system types.

For the compressors and whatever type of heating will be used, the user will need go to Create Plants. The Air-Cooled Unitary equipment category contains all of the rooftop compressor models. The specific type of rooftop can be selected using the Equipment Type pull down menu on the Cooling Equipment tab. For the heating side a gas-fired heat exchanger, boiler, and/or electric resistance equipment category can be used depending on what is appropriate for the model. The specific type of heating equipment can be selected using the Equipment Type pull down menu on the Heating Equipment tab. If two different heating sources will be used in the same system (gas-fired heat exchanger and electric resistance reheat), then two separate heating plants will need to be created. The appropriate coils should be assigned to the heating plant/s in Assign Systems to Plants.