I am having some difficulty setting up multiple HVAC systems serving one zone.
The situation is that I am modeling a building with packaged rooftop units as
well as exhaust fans. However, I can't model the exhaust fans using the exhaust
fan object because the exhaust fans are controlled based on enthalpy
differential with outdoor air (as I understand it, exhaust fan availability
managers only allow temperature differential).

So I thought I would set up another HVAC system in addition to the RTUs that
would function like the exhaust fans (with a supply fan, rather than exhaust, no
heating or cooling coils). I could then use an outside air controller to
specify the enthapy differential control. The issue I am having is setting up
both systems to serve one zone. Do I connect the second system (exhaust
system)to the zone inlet node, or upstream of the zone splitter associated with
the RTUs? Where do I connect downstream of the zone?

Any help would be much appreciated. Of course, if anyone has a way of getting
the exhaust fan object to run on differential enthalpy, I would take suggestions
there too!!

Andrew