Minimum OA Ratio for VAV System

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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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Regards,
Simon Rees

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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

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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.

James V. Dirkes II, P.E., LEED AP

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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

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

Brenda V. Morawa, PE LEED AP QCxP

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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

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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...

James V. Dirkes II, P.E., LEED AP

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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

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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.

David

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Paul

You make a good point about the Air Not Reaching the Thermostat. Based on this effect, the VAV system would provide more airflow which might improve the ventilation in the space but then the zone would be over heated. The problem has then switched from a poorly ventilated space to a thermally uncomfortable space. Again, there would be negative energy implications.

Regarding your second point, specifying a maximum OA Ratio would maintain a higher supply air volume. For a VAV System with Demand Controlled Ventilation (DCV), the Supply Air Flow is controlled to meet two conditions (Temperature Setpoint & CO2 PPM). When the temperature is satisfied then the VAV Box controls the Damper based on CO2 PPM.

For example, the Space is at the correct temperature but needs 200 cfm of Outdoor Air. If the VAV System is allowed to provide a 100% OA Ratio, then only 200 cfm of conditioned air is supplied. If the VAV System is limited to 50% OA Ratio, then 400 cfm of conditioned air is supplied. The later scenario ensures that the Fresh Air is thrown into the occupied space.

Ian

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I may be missing something in this thread, but it occurred to me that none
of the mixing being discussed here is known by an energy analysis program
unless there's something out there which incorporates a CFD capability.

Lacking CFD, you're back to good engineering design practices for the air
distribution system.

James V. Dirkes II, P.E., LEED AP

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