Lesson 6 – Understanding System Ventilation Efficiency (Ev) with a Simple Example
System Ventilation Efficiency, commonly referred to as Ev, is one of the most misunderstood concepts in ventilation design.
Many engineers encounter Ev for the first time during multi-zone calculations and immediately see it as a mathematical penalty rather than a physical reality.
This lesson explains what Ev represents, why it appears in the Ventilation Rate Procedure, and how it affects outdoor air requirements, using a simple conceptual example rather than equations.
What Is System Ventilation Efficiency (Ev)?
System Ventilation Efficiency represents how effectively outdoor air supplied at the system level reaches the zones that require it most.
In other words, Ev answers a simple question:
How much of the outdoor air introduced into the system is actually useful for maintaining acceptable indoor air quality in all zones?
If all zones received exactly the outdoor air they needed, Ev would be equal to one.
In real multi-zone systems, this condition is rarely achieved.
Why Ev Does Not Appear in Single-Zone Systems
In a single-zone system, all supplied air is delivered to the same space.
Because there is no sharing of air between multiple zones, all outdoor air supplied reaches the occupied space directly.
For this reason, ventilation effectiveness is assumed to be ideal, and Ev is equal to one.
This is why Ev is not introduced until multi-zone systems are considered.
When Ev is less than one, it indicates that outdoor air is not distributed evenly or efficiently across all zones.
Some zones receive more outdoor air than they need, while others receive less than required.
To compensate for this imbalance, the system-level outdoor air intake must be increased.
This adjustment ensures that even the most ventilation-critical zone receives sufficient outdoor air.
A Simple Conceptual Example
Consider an air-handling unit serving two zones:
Zone A is densely occupied and requires a relatively high outdoor air rate per unit of supply air.
Zone B has low occupancy and requires much less outdoor air.
Both zones receive air from the same system.
Even if the total outdoor air supplied matches the combined minimum requirement of both zones, the distribution of that outdoor air may not align with the actual needs of each zone.
Zone B may receive more outdoor air than necessary simply because it has higher supply airflow, while Zone A may receive less than required.
This imbalance is the reason Ev is introduced.
Why Outdoor Air Increases When Ev Is Applied
Once system ventilation efficiency is considered, the required outdoor air at the system level is divided by Ev.
Because Ev is less than one, the required outdoor air intake increases.
This increase is not arbitrary.
It reflects the need to overcome distribution inefficiencies inherent in shared systems.
Many engineers interpret this increase as excessive or conservative, without realizing that it corrects for assumptions made earlier in the design process.
Where Engineers Often Get Frustrated
Frustration with Ev often arises when large outdoor air flow rates appear suddenly in calculations.
At this point, the tendency is to question the standard or treat the result as overly conservative.
In reality, the calculation is revealing the cumulative effect of earlier assumptions such as peak occupancy, uniform schedules, and limited zone diversity.
Ev does not create oversizing by itself; it exposes it.
How to Think About Ev Correctly
Ev should be viewed as a diagnostic indicator rather than a penalty factor.
A low Ev suggests that the system configuration or zoning strategy may be inefficient from a ventilation perspective.
Understanding this opens the door to design improvements, such as better zoning, dedicated outdoor air strategies, or alternative ventilation approaches discussed later in the course.
Key Takeaway
System Ventilation Efficiency exists to correct for uneven outdoor air distribution in multi-zone systems.
It does not make ventilation requirements conservative by default, but it highlights inefficiencies that already exist in system design assumptions.
Understanding Ev conceptually makes the associated calculations logical rather than frustrating.
Reflection Question
When Ev significantly increases outdoor air requirements in your projects, do you usually accept the result as fixed, or do you revisit the assumptions and system configuration that led to it?
Pause here and reflect before continuing.
Consider how zoning decisions and occupancy assumptions influenced Ev in past designs.