[Bldg-sim] Minimum OA Ratio for VAV System

[James V. Dirkes II P.E.]

Mixing effectiveness should not be an issue if the air is delivered via
common distribution system (the two airstreams will mix en route).  If
you're using two delivery systems, then you're probably right; good mixing
may not occur in the zone.  It seems that the mixing is not an outdoor air
problem, but rather an air distribution problem...


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 Brenda V.
Morawa
Sent: Tuesday, September 02, 2008 11:25 AM
To: 'Doebber, Ian'; 'Matutinovic, Luka'; bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] Minimum OA Ratio for VAV System

I'm having a hard time understanding how limiting the OAR will change the
ventilation effectiveness.

Brenda V. Morawa, PE LEED AP QCxP
Co-Vice-Chair, USGBC LEED Certification Committee

404.806.2018  EXT 101
404.210.6593 (cell)


-----Original Message-----
From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Doebber, Ian
Sent: Tuesday, September 02, 2008 11:11 AM
To: Matutinovic, Luka; bldg-sim at lists.onebuilding.org
Cc: Doebber, Ian
Subject: Re: [Bldg-sim] Minimum OA Ratio for VAV System

Luka

Thanks for you response.  The sole reason that I'm concerned about 100%
Outside Air is that the Ventilation Air is not reaching the occupied space.
The 100% OA condition typically occurs when the VAV system is in heating
mode which means that the Supply Air is most likely between 90°F-95°F.  

Therefore the Supply Air has a positive buoyancy relative to the space air
temperature of 70°F.  Also, the VAV damper is at its minimum position, ~30%,
such that the Supply Air is not being thrown from the diffuser to counteract
the positive buoyancy.   

As a result, the VAV system may be supplying 400 cfm of 100% Outside Air
into the space yet possibly only 50% ever reaches the occupied space while
the rest of it clings to the ceiling and exits the Return Grill.  Therefore,
you have only provided 1/2 of the necessary fresh air to the space.

The two remedies are either setting a limit to the Outdoor Air Ratio or
providing Fan Powered Boxes.  These strategies ensures that the Supply Air
is sufficiently being thrown into the Occupied Zone.  But both have a
significant energy penalty.

Does anyone have any thoughts on how to control VAV systems to make sure the
fresh air reaches the occupied space while not taking a hit on energy.  

Regards,
Ian Doebber

-----Original Message-----
From: Matutinovic, Luka [mailto:LMatutinovic at halsall.com] 
Sent: Tuesday, September 02, 2008 6:16 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
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