[BLDG-SIM] Modeling 100% OA VAV systems in eQuest (and new EQUEST/DOE-2 versions)

Jeff Hirsch Jeff.Hirsch at DOE2.com
Tue Aug 31 08:18:54 PDT 2004

MessageThe problem being asked about arises due to the different uses of the
alternative methods available in eQUEST freeware (a MS Windows implementation
of DOE-2.2) to describe both the minimum outside air (OA) damper position
as well as the minimum flow fraction setting of the VAV boxes in each zone.  

The min-flow-ratio for a VAVS system (DOE-2 keyword found on the SYSTEM
properties sheet, fan tab, flow parameters sub tab in the eQUEST details mode
interface) is the minimum VAV box flow fraction for all boxes supplied by the "parent"
air handler for the zone; this value can be specified for each individual box or for
a system and thus "inherited" by all zone boxes supplied by the system. If not given
a value (min flow fraction control on the mentioned subtab) defaults to the larger of
the minimum ventilation requirement, the flow required to satisfy the zone peak heating
load heating, or other available minimum flow specifications (such as cfm/sqft, etc.)
In dual duct or dual fan dual duct VAV systems the boxes have many more controls
including separate heat/cool and overall box minimum flow; consult the DOE-2.2
dictionary (http://doe2.com/download/DOE-22/DOE22Vol2-Dictionary.pdf) or eQUEST
online help (although this is less complete and not as up to date) for more detail.

When the outside air fraction is set to 1 this is interpreted as a minimum ventilation
requirement and thus is found to be the maximum value of the various defaults for
the VAV box minimum flow value.  If the minimum cfm fraction is specified (on the
subtab mentioned above) rather than letting it default, the specified value will be used
for all ZONE VAV boxes in that SYSTEM.

For example, on the system properties sheet the fan tab and flow parameters sub tab,
you can place a value for min flow ratio, and that will become the min flow for all VAV
boxes on the SV-A report  ...
the hourly OA fraction will then be governed by the fraction of design vs. hourly values
choice on the system properties sheet outdoor air tab, outside air and economizer subtab.
If (on the same properties sheet fans /flow subtab) the min flow cfm/ft2 control is set instead,
you would still get OA=1 all hours because this is one of the values compared to the ventilation
to choose the maximum needed minimum box flow setting.  (I know the online help is a bit
dated and poorly worded ... we are trying to get funding to improve it ... but this information
is all there, just requires a bit of a careful read.)

By the way, the SV-A, and other DOE-2 reports are now easily examined: the eQUEST
version 3.51 freeware pre-release posted on http://DOE2.com/eQUEST/ has a new simulation
results details viewer that allows viewing of multiple .sim files with direct access to reports through
an index. This viewer can be used to view .sim results file produced by any version of DOE-2.2
freeware, but is best used with versions 43 and beyond due to fully uniform report headers (in those
versions) that the viewer uses to locate reports and error messages within a .sim file.

Version 3.51 of eQUEST has many other improvements to both the simulation engine and interface
including: a 5-10 times speed increase of project open and compliance simulation times, an first version
of a BDL error manager to view an error tree to assist in navigation to and fixing of model errors, and new DOE-2
simulation features that can be found documented (with other post version 40 additions) in Volume 6
of the DOE-2.2 documentation at

This is a pre-release to allow extensive public comment and review (followed by our fixing issues
identified) prior to the retirement of older versions; this version has been in wide use for some time
now (starting with version 3.50) and appears to be close to "ready for prime time."

Happy simulating ...

Jeff Hirsch
James J. Hirsch & Associates
Building Performance Analysis Software & Consulting
12185 Presilla Road
Camarillo, CA 93012-9243 USA
phone: (805) 553-9000
fax: (805) 532-2401
email: Jeff.Hirsch at DOE2.com
web: http://DOE2.com

  ----- Original Message ----- 
  From: Keir Howell 
  To: BLDG-SIM at gard.com 
  Sent: Monday, August 30, 2004 8:25 AM
  Subject: [BLDG-SIM] Modeling 100% OA VAV systems in eQuest

  Hello. Whenever I try to model a VAV system with 100% OA in eQuest, I end up with a constant volume system. It seems to be multiplying the Minimum OA Ratio (which I have set at 1.00) by the design air flow, rather than the hourly air flow, even though I have set the Minimum OA Method to "Fraction of Hourly Flow". As far as I know, I have not set any other variables (such as zonal settings) that might override the system settings. Please help. Thanks,

  Keir Howell
  Bluestone Energy Services, Inc.
  phone:    781-356-8865
  fax:    781-356-8864

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