System sizing for DOAS with FourPipeBeam
I am looking for some clarification on what type of system sizing we should use for the AHU serving the AirTerminal:SingleDuct:ConstantVolume:FourPipeBeam. The inputoutputreference pdf for the terminal states the following
" The four pipe beam air terminal sizing differs from other air terminals in that the primary supply air flow rate is sized using the entire performance model and the flow rate is not the direct result from the Sizing:Zone and Sizing:System calculations. The flow rates will be somewhere between what an air terminal would size out using VentilationRequirement or Sensible in the Sizing:System object (either setting can be used). "
I see that when I specify "VentilationRequirement" in Sizing:System for the AHU serving the four pipe beam, I get a lot of unmet cooling hours. However, when I specify "Sensible" as the type of load to size on the unmet cooling hours are gone. I think that the issue is the beam requires a certain amount of primary air to meet the load. When the Design outside air is below that requirement the beam is not able to fully cool. However with sensible as the type of load to size on, the primary airflow is sized appropriately. But this causes a significant portion of the cooling load to be taken up by the central cooling coils and also increases the central AHU fan power, making this not a true DOAS system. Am I missing any other settings to make this work. Or does the chilled beam with DOAS system just not work for spaces that have a low ventilation requirement.
