[BLDG-SIM] Rooftops with gas fired furnaces

Molinini, Louis - ETA Molinini.Louis at dol.gov
Wed Sep 1 12:37:18 PDT 2004


We are building a small two story dormitory in Albuquerque, NM with packaged
rooftop units for each floor and exposure.
 
I am simulating per the ECB methodology and the baseline system is a PTAC
system.  However, I went ahead and created two ECB models; one with PTAC and
the other with packaged rooftops.  
 
The PTAC is modeled with hydronic heating with a natural gas fired boiler.
 
The rooftops have gas fired furnaces.
 
The PTAC's space heating energy is 5 to 6 times that of the rooftops.  Even
when simulating the PTAC with gas furnace heating, the space heating energy
is still 5 to 6 times that of the rooftops.  Space cooling usage amongst all
three models is very similar.
 
The peak heating loads are all the same for the various zones.

Has anyone had any issues modeling space heating with a gas furnaces in a
rooftops?  Basically, the space heating component is coming back
consistently with the rooftops at less than 3kbtu/sf/yr.  I believe this to
be too low.
 
Also, I can't seem to find any reports which show the Heating Input Ratio
for the gas furnaces in the rooftops...I look at the SV-A report and I see
Heating EIR which comes back as Zero all the time.  When I vary the HIR for
the rooftops, the space heating load varies slightly.
 
 
 
 

Louis Molinini

Energy Engineer

LEEDTM Accredited Professional

 

PBDewberry

2101 Wilson Blvd.

Suite 200

Arlington, VA 22201

 

703-875-7642

molinini.louis at dol.gov <mailto:molinini.louis at dol.gov> 

-----Original Message-----
From: postman at gard.com [mailto:postman at gard.com] On Behalf Of Jeff Hirsch
Sent: Tuesday, August 31, 2004 11:19 AM
To: BLDG-SIM at gard.com
Subject: [BLDG-SIM] Modeling 100% OA VAV systems in eQuest (and new
EQUEST/DOE-2 versions)


The 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
<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/
<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
       http://doe2.com/download/DOE-22/DOE22Vol6-NewFeatures.pdf
<http://doe2.com/download/DOE-22/DOE22Vol6-NewFeatures.pdf> 
 
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 <mailto:Jeff.Hirsch at DOE2.com> 
web: http://DOE2.com <http://DOE2.com> 
 
 

----- Original Message ----- 
From: Keir Howell <mailto:keirh at bluestoneenergy.com>  
To: BLDG-SIM at gard.com <mailto: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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