[Bldg-sim] Minimum OA Ratio for VAV System

David S Eldridge DSE at grummanbutkus.com
Tue Sep 2 11:59:49 PDT 2008


Remember also that just because it is cold outside, doesn't mean all zones will be in heating mode.

Any core areas, or other areas with lots of internal gains from occupancy or equipment, or high exhaust requirements, might not be at minimum VAV.

>From your question you may be specifically looking at some perimeter zones which might be at minimum airflow even when fully occupied.

David

> -----Original Message-----
> From: bldg-sim-bounces at lists.onebuilding.org [mailto:bldg-sim-
> bounces at lists.onebuilding.org] On Behalf Of Paul Riemer
> Sent: Tuesday, September 02, 2008 10:47 AM
> To: 'Doebber, Ian'; Matutinovic, Luka; bldg-sim at lists.onebuilding.org
> Subject: Re: [Bldg-sim] Minimum OA Ratio for VAV System
>
> I suggest rethinking the 30% and 90-95 F condition.
> It seems you are arguing that not enough air will reach the breathing zone.
> If so, it will probably not reach the thermostat either and thus the box
> would open wider if allowed to do so.  Coincidently, I read the DCV
> appendix of Std 62 to suggest the first thing to do with an under
> ventilated zone is to increase its supply air volume (not increase the
> system OA %).
>
> 62.1 + 90.1 + 8760 = tons of fun!
>
> Paul Riemer
>
> -----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 10:11 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
> _______________________________________________
> 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
> _______________________________________________
> 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