[Equest-users] Modeling DOAS with a dummy zone

[Jeremy Poling]

Brian,

 

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

 

OA-FROM-SYSTEM

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



 

From: equest-users-bounces at lists.onebuilding.org
[mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Brian
Smith
Sent: Wednesday, March 30, 2011 11:13 AM
To: equest-users at lists.onebuilding.org
Subject: [Equest-users] Modeling DOAS with a dummy zone

 

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?

 

 

Regards,

Brian Smith, Mechanical Design Engineer

Murray & Associates, P.C.

T/907.780.6151 F/907.780.6182

www.murraypc.com

 

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