[Bldg-sim] Minimum OA Ratio for VAV System

James V. Dirkes II P.E. jvd2pe at tds.net
Tue Sep 2 05:51:49 PDT 2008


 I agree!

Limiting outdoor air is only beneficial if you're not paying attention to
the enthalpy of the air or, in the case of a dry bulb economizer, the
changeover temperature.

Other reasons also exist for large amounts of outdoor air, such as
replacement air for exhaust systems.  When coupling even 100% outdoor air
with energy recovery, you can gain air quality and reduced energy.


The Building Performance Team
James V. Dirkes II, P.E., LEED AP
1631 Acacia Drive NW
Grand Rapids, MI 49504
616 450 8653
 

-----Original Message-----
From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Matutinovic,
Luka
Sent: Tuesday, September 02, 2008 8:16 AM
To: Doebber, Ian; bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] Minimum OA Ratio for VAV System

Why wouldn't you want the max to be 100% to take advantage of economizer
free-cooling if the climate and system configuration allowed for it.  I
agree that there are large energy implications of conditioning excessive
OA and there may be viable means to limit it when you don't need it, but
there are also large implications of cooling return air when you don't
have to.  Considering ASHRAE 90.1 mandates economizers under certain
climates, you would be penalizing yourself by capping OA.

Any thoughts?


Luka Matutinovic

-----Original Message-----
From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Doebber,
Ian
Sent: Monday, September 01, 2008 3:55 PM
To: bldg-sim at lists.onebuilding.org
Cc: Doebber, Ian
Subject: [Bldg-sim] Minimum OA Ratio for VAV System

ASHRAE 90.1-2004 does not specifically state anywhere a MAXIMUM OA RATIO
that a VAV system can not exceed.  Yet ASHRAE 62-2004 insinuates this
with its reference to Ventilation Effectiveness based on OA Ratio (Zp)
in Table 6-3.  

It would make perfect sense that a VAV system should be limited to never
have an OA Ratio exceed say 0.45.  This is applicable especially in
heating where the effects of Thermal Stratification will cause
Ventilation Short Circuiting.

VAV control systems have the capability of limiting the OA Ratio which
has extremely large Energy Implications.  For example, if the MAXIMUM OA
RATIO is 0.45, then the VAV system is limited to the how low the Total
Supply Air can reduce to.  Consequently a lot more fan energy will be
required to maintain that higher Flow Rate.

Has anyone ever incorporated a MAXIMUM OA RATIO in their VAV System
Design?

Regards,
Ian Doebber
_______________________________________________
Bldg-sim mailing list
http://lists.onebuilding.org/listinfo.cgi/bldg-sim-onebuilding.org
To unsubscribe from this mailing list send  a blank message to
BLDG-SIM-UNSUBSCRIBE at ONEBUILDING.ORG
_______________________________________________
Bldg-sim mailing list
http://lists.onebuilding.org/listinfo.cgi/bldg-sim-onebuilding.org
To unsubscribe from this mailing list send  a blank message to
BLDG-SIM-UNSUBSCRIBE at ONEBUILDING.ORG




More information about the Bldg-sim mailing list