[Bldg-sim] Minimum OA Ratio for VAV System

Dan Russell danr at engineeringinc.com
Tue Sep 2 08:25:42 PDT 2008


Though I have not used them in design before, I know there are available VAV diffusers which maintain adequate throw lengths at reduced velocities.  Using this type of diffuser should resolve the ventilation effectiveness issue at low-flow (VAV min. ~30%) conditions.  

Also, I've just recently seen one gril/reg manufacturer come out with a linear bar grille for use on the perimeter which directs the air horizontally during cooling mode, then re-directs to a downward discharge during heating mode.  These have to be placed along the perimeter to avoid discharging hot air directly on the occupants, but it seems like a great solution for counteracting the buoyancy and stratification issues.

Unfortunately I haven't had any direct experience with these technologies yet.  Has anyone out there found these to be helpful (or not) for the discussion topic?

Thanks,

 Dan Russell, EIT 



-----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 9:11 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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