To model a constant volume system when would I choose a cav system such as that in 5ZoneCAV_MaxTemp.idf example file vs using a AirLoopHVAC:UnitaryHeatCool object. For my test model, I see a significant difference in the heating energy use in the two cases.
The system in 5ZoneCAV_MaxTemp uses AirTerminal:SingleDuct:ConstantVolume:Reheat with hot water reheat coils. Supply air temperature is controlled using SetpointManager:WarmestTemperatureFlowwith the TemperatureFirst strategy with a maximum setpoint temperature of 16C (60.8F). With those settings, even if applied to a single zone, there will be times when 16C supply air will overcool the space and the reheat will kick in, providing some dehumidification in mild weather. If the max setpoint were unlimited (99C), then the system should just meet the zone load with no simultaneous heating and cooling. This system type is intended to model a built-up air handler serving multiple zones with continuous fan operation during occupied hours, and a night-cycle availability manager. This system can be modeled with any type of cooling coil (chilled water or DX).
AirLoopHVAC:UnitaryHeatCool is intended to represent a packaged unit with a single thermostat control zone. A key difference from the above system is the option to use cycling fan or continuous fan (Supply Air Fan Operating Mode Schedule Name, if blank the system defaults to cycling fan). AirLoopHVAC:UnitaryHeatCool only supports DX cooling coils, but the similar AirLoopHVAC:UnitarySystem can model any type of cooling coil.
To get similar results from these two system types in a single-zone application, raise the max setpoint temperature in the 5ZoneCAV_MaxTemp model, and use continuous fan in the AirLoopHVAC:UnitaryHeatCool model.
Asked: 2018-08-10 11:16:46 -0500
Seen: 181 times
Last updated: Aug 21 '18
