Algorithms for Modeling Multiple Air Handling Units Serving the Same Zone in EnergyPlus

A new zone air balance method has been developed for EnergyPlus to allow the simulation of more than one air handling unit (AHU) serving the same zone. The new method allows each zone to be served by one or more AHUs and one or more zone HVAC units. The zone air mass flow balance allocates return air flows to multiple AHUs while accounting for zone exhaust flows, infiltration, and interzone mixing. At the same time, each AHU must balance supply and return flow with available outdoor air. Infiltration and interzone mixing flows may be automatically adjusted. The method can model various applications which previously could not be modeled or required awkward workarounds including pressurized systems (hospital), transfer air (food service), dedicated outdoor air systems in parallel with multi-zone VAV systems (school), and systems with no return (warehouse). This paper documents new algorithms at the heart of the EnergyPlus HVAC simulation. Modeling multiple air handlers serving the same zone(s) is a significant new capability for a widely used building energy modeling tool.