Ryan,

When you input the EXHAUST-SOURCE as Air Handler, the EXHAUST-FLOW is made up via outdoor air supplied by the air handler. In this mode, The minimum outside air requirement of the air handler will be forced to be at least the exhaust flow. In addition, the default minimum airflow requirement of VAV terminals will be at least the exhaust requirement. This may be why OSA Ventilation rate for the RTU is more than the sum of the individual OSA rates you set.

When you input the EXHAUST-SOURCE as INFILTRATION, your heating or cooling energy use go up because outdoor air infiltration increases and the exhaust fan operates independently of the space conditioning system, and may be working longer than the RTU.

Min-Hwan Yang, CEM, LEED AP