I am trying to model exhaust fans for a building and have had a couple
of issues regarding the EXHAUST-SOURCE input under the zone Outside Air
and Exhaust section. I have two RTU's, one is a 100% OA constant volume
system with constant volume fume hood exhausts. The other RTU is a VAV
system with a couple exhaust fans.

Starting with the VAV system, when I input the EXHAUST-SOURCE as Air
Handler, my OSA Ventilation rate for the RTU is quite a bit more than
the sum of the individual OSA rates. I cannot figure this out. When I
change the EXHAUST-SOURCE to Balanced Infiltration my RTU ventilation
rate matches the sum of the zones, but my energy use goes up. Is there
something I am missing here?

For the 100% OA CV system, if I model the EXHAUST-SOURCE as Balanced
Infiltration or Infiltration vs Air Handler my space heating energy rate
over doubles. This does not seem correct and makes me nervous to use
the Balanced Infiltration input for my VAV fans.

Has anyone run into this issue before? Is this a bug or would these
results be expected?

Ryan Del Balso