Modeling DOAS with a dummy zone

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Thanks for your replays to my email, it was very much appreciated. I tried
modeling the doas system with a dummy zone and OA from system. While looking
at my Air side reports I found that the DOAS system OSA equaled the sum OSA
from each zone in each system (fine that's logical). But each system has its
own OSA total which is larger than the sum of OSA requirements of its zones
(not sure why). Whether I model a doas dummy or not the individual system
still require the same amount of OSA. Therefore with the dummy air totals
added to the system air totals my building needs more OSA and consumes more
BTU's. Not sure what all this means but It would seem bad? Any insights,
does this sound right? This makes me think if I could eliminate this Dummy
Doas system and zones, and work in the components of DOAS into the other
systems I would at least find a different result, but right?


Brian Smith, Mechanical Design Engineer

Brian Smith's picture
Joined: 2011-09-30
Reputation: 0


First: here's what the help file says about how the system comes up with
OSA flows:


Takes the U-name of the system that supplies dehumidified make-up air
either directly, or through the corridor, to apartments or hotel rooms.
Make sure that minimum outside air of a make-up air system assigned to
the corridor matches the sum of the minimum outside air flow rate of the
apartments or hotel rooms. In the input, define the make-up air system
prior to the apartment or hotel room system. Several Error and Warning
messages are present to prevent incorrect simulations and only simple
sizing calculations are available for OA-FROM-SYSTEM referenced SYSTEMs.
In previous versions of DOE-2.2 (before 44c) and 2.1E, if a SYSTEM had
an OA-FROM-SYSTEM specified, the referencing SYSTEM value for
MIN-OUTSIDE-AIR was reset to the MIN-OUTSIDE-AIR of the referenced
OA-FROM-SYSTEM; this has been removed and replaced with the action as
described next. The sum of the OA requirements for all SYSTEMs that
reference an OA-FROM-SYSTEM is used to set the referenced OA-FROM-SYSTEM
value for MIN-OUTSIDE-AIR, SUPPLY-FLOW as well as adjust the referenced
OA-FROM-SYSTEM directly served ZONE maximum and minimum flows for both
heating and cooling. Additionally the referenced OA-FROM-SYSTEM has its
calculated/specified cooling (total and sensible), heating, and preheat
capacities adjusted if its SUPPLY-FLOW was increased. If the referenced
OA-FROM-SYSTEM and its ZONEs have sufficient minimum OA specified
(greater than that required by the referencing SYSTEMs) no adjustments
are made. If any adjustments are made a warning is issued that informs
the user of the adjustments and recommends the project be re-run with
the values in the input corrected. Also, errors are issued (and the
simulation terminated) if the referenced OA-FROM-SYSTEM is either an
incorrect type (a zonal system or a system that cannot have outside air)
or the referenced OA-FROM-SYSTEM is not placed into the input file
before all referencing SYSTEMs.

There could also be a couple of places causing the system OSA to exceed
the sum of the zone OSA for each system, depending on how you set the
model up:

- If your project has a specified elevation greater than 0 then
eQuest will be correcting airflows for elevation. ASHRAE Fundamentals
directs that if you're less than 1,000 ft above sea level you can
usually ignore elevation effects on CFM values, therefore if that
applies you can zero out that value (see below)

- If you're looking at the proposed model, make sure you don't
have any sizing factors in place for the system (unless you intend to
run the calculations that way).

Jeremy R. Poling, PE, LEED AP+BDC

Jeremy Poling2's picture
Joined: 2011-09-30
Reputation: 0

I would also add here that you can sign the OA for the system either
based on "Sum of Zone OA" or "Set by Critical Zone".

When you size using "Set by Critical Zone" the system OA will be greater
than the sum of the zone OA requirements to compensate for the diversity
in the OA requirements of the zones.

Kamel Haddad

Haddad, Kamel's picture
Joined: 2011-09-30
Reputation: 0

I would also check the outside air per person under the zones.

Charles Land's picture
Joined: 2011-09-30
Reputation: 0