Why Does Higher Zp in ASHRAE 62.1-2007 Indicate Lower Ventilation Efficiency?

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SUMMARY

The discussion centers on the relationship between the primary outdoor air fraction (Zp) and ventilation efficiency (Ev) as defined in ASHRAE 62.1-2007. According to equation 6-5, Zp is calculated as the ratio of outside airflow (Voz) to primary airflow (Vpz). Table 6-3 indicates that a higher Zp correlates with lower ventilation efficiency, which contradicts the intuitive assumption that more outside air would enhance efficiency. The conclusion drawn is that systems with varying outdoor air needs across zones lead to inefficiencies, necessitating a uniform outdoor air fraction for optimal performance.

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  • Understanding of ASHRAE 62.1-2007 standards
  • Knowledge of ventilation efficiency metrics
  • Familiarity with airflow calculations (Voz and Vpz)
  • Concept of zone-specific outdoor air requirements
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Great Smoky
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There is an equation in this standard for a primary outdoor air fraction: Zp=Voz/Vpz (eqn 6-5) which is pretty much a percentage of outside air where Voz is the outside airflow and Vpz is the primary airflow.

But then it shows in table 6-3 in the same standard that the higher the Zp value, the lower the ventilation efficiency (Ev). I would guess that the higher Zp (percentage outside air), the higher efficiency it should be.

This is a big mystery for me. Am I seeing things or interpreting it wrong? Anybody who has experience with this standard and can help would be greatly appreciated.
 
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I don't have a copy of this at home (I'll check tomorrow at work to verify...), but ventilation efficiency is the efficiency at which ventilation is delivered to the occupants, not the quantity delievered. So a system with a lower ventilation efficiency requires more outside air to achieve the same effective ventilation.
 
So I checked the standard and my answer was sort of correct, but incomplete:

Since outside air is mixed with return air at the unit, the only way to ensure a zone with an unusually high OA need gets what it needs is to increase the OA fraction of the unit. This results in some zones being overventilated. So the most efficient system is one where each zone needs the same OA fraction.
 

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