Using Dedicated Outdoor Air System defies ASHRAE 62.1

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FYI : When using "OA-FROM-SYSTEM" in to model a Dedicated Outdoor Air
System (DOAS), you must specify "Sum of Zone OA" both for the DOAS and
the AHU that it is serving. The "OA-FROM-SYSTEM" does not correctly
update the DOAS Flow Rate to meet the AHU Outdoor Air Design Ventilation
if you specify "Set by Critical Zone".

This causes a problem as "Set by Critical Zone" is the correct way to
Design the Maximum Ventilation Rate according to ASHRAE 62-2004. Based
on my 200,000 ft2 School model, the "Sum of Zone OA" undersizes the
amount of Ventilation Air by 25%. Obviously significant.

Note that this is using Equest 3.6. Can someone tell me whether this
has been fixed in Equest 3.61.


Ian Doebber

Doebber, Ian's picture
Joined: 2011-09-30
Reputation: 0


The DOAS System is preconditioning the Outside Air before it is sent to
the Main AHU. The Ratio of Fresh Air to Return Air is based on the how
you are controlling your system.

By specifying "Sum of Zone OA" within Equest, the system is simply
summing up the necessary Outside Air to meet the cfm/occ and cfm/ft2.
Yet this strategy is not the best way to determine Ventilation
Requirements. For example, a fully occupied classroom may require an OA
Ratio of 40%. Yet other spaces served by the same AHU need an OA Ratio
of 15%. As a result, the final OA Ratio supplied will be somewhere in
between, probably around 20%. Therefore the fully occupied classroom is
not being provided a sufficient level of Ventilation. This control
strategy does not meet ASHRAE Standard 62-2004.

The other option of "Set by Critical Zone" is based on the Multi-Zone
Ventilation Strategy specified by ASHRAE Standard 62-2004. If you are
conducting an Energy Model to meet LEED, then you must use this control

If your model just has AHUs, then Equest works with the "Set by Critical
Zone" strategy.

Yet, if you have connected a DOAS to an AHU, then everything gets messed
up. The AHU provides Ventilation Air based on the "Set by Critical
Zone" strategy. Yet the DOAS unit provides Ventilation Air based on the
"Sum of Zone OA" strategy. To be correct EQUEST needs to have the DOAS
provide the same Ventilation Air as the attached AHU based on the "Set
by Critical Zone" strategy.

I hope that I explained it properly.


Doebber, Ian's picture
Joined: 2011-09-30
Reputation: 0