100% Outdoor Air Systems

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I have searched the archives and red all of the older opinions on this
topic but wanted to see if there are any new ideas on this topic. I
have a hospital that is served completely by 100% outdoor air units via
6 separate air handlers. Total supply cfm to the facility is 240,000
cfm. There are six large exhaust fans on the roof that remove
approximately 80% of that from the facility. Unfortunately there is no
heat recovery to date. They used to have a run around loop but it was

What is everyone's take on modeling this in equest? At first glance I
was planning on simply defining the amount of O/A to equal the supply
cfm but wanted some feedback before I continue.

Jason L. Moody, P.E.

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Joined: 2011-09-30
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The O/A flow rate IS equal to the supply flow rate, correct? So of
course you will want to model 240,000 CFM of O/A, to accurately model
the heating and cooling energy. I'm guessing you are really asking about
the exhaust. If you will be modeling heat recovery as an ECM, then the
exhaust flow rate and pathways are important. If not, you might get away
with modeling the exhaust rate identical to the supply flow. Either way,
I would try to figure out if the other 20% is being exhausted by other
fans, or exfiltrated or both. You could potentially zero out your
infiltration in some or all zones. Model the fan energy as best you can,
since it is likely to be high and a potential area for energy
improvement. You will probably need actual pressure measurements, rather
than design values, since the runaround loop (and associated pressure
drops) were removed.


William Bishop, PE, BEMP, LEED(r) AP

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Joined: 2011-09-30
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