Lesson 5 – Ventilation Rate Procedure for Multiple-Zone Systems
Multi-zone systems represent the point where ventilation calculations start to feel complicated for many engineers.
Unlike single-zone systems, multiple spaces with different occupancy patterns and ventilation needs are served by the same air-handling system.
This lesson explains why the Ventilation Rate Procedure becomes more complex in multi-zone systems and introduces the concept of system ventilation efficiency without diving into equations yet.
The goal is to understand the logic before dealing with numbers.
What Is a Multiple-Zone System?
A multiple-zone system is an HVAC system that serves more than one space or thermal zone using a common air-handling unit.
In this configuration, the supplied air is distributed to different zones, and each zone may have different ventilation requirements based on occupancy, floor area, or usage.
Unlike the single-zone case, outdoor air supplied at the system level is shared among multiple zones.
Why Ventilation Becomes More Complex in Multi-Zone Systems
In a multi-zone system, not all zones require the same amount of outdoor air relative to their supply airflow.
Some zones may be densely occupied and ventilation-critical, while others may require relatively little outdoor air.
However, all zones receive air from the same system, which creates an imbalance in how outdoor air is distributed.
As a result, simply summing zone ventilation requirements is not sufficient to ensure acceptable indoor air quality in all zones.
The Core Problem: Uneven Outdoor Air Distribution
The fundamental challenge in multi-zone ventilation is that outdoor air does not automatically reach all zones in the required proportion.
Zones with higher supply airflow but lower ventilation demand may receive more outdoor air than needed, while critical zones may receive less than required.
This uneven distribution is the main reason why multi-zone ventilation calculations require additional correction factors.
Introduction to System Ventilation Efficiency
To address the issue of uneven outdoor air distribution, the Ventilation Rate Procedure introduces the concept of system ventilation efficiency.
System ventilation efficiency represents how effectively the outdoor air supplied at the system level is delivered to the zones that need it most.
In an ideal system, all zones would receive exactly the outdoor air they require.
In reality, system-level inefficiencies reduce this effectiveness.
Why System Ventilation Efficiency Is Always Less Than One
In multi-zone systems, system ventilation efficiency is almost always less than one.
This does not indicate a design error, but rather reflects the inherent limitations of serving multiple zones with different ventilation needs using a single air-handling system.
To compensate for this inefficiency, the required outdoor air intake at the system level must be increased.
This adjustment is often the point where outdoor air flow rates increase significantly compared to single-zone calculations.
Where Oversizing Accelerates in Multi-Zone Systems
Many engineers first encounter unexpectedly high outdoor air requirements when performing multi-zone ventilation calculations.
This often leads to the assumption that the standard is overly conservative or impractical.
In reality, the increase is usually driven by a combination of conservative assumptions made earlier, such as maximum occupancy, peak operating conditions, and uniform schedules applied across all zones.
When these assumptions are combined with reduced system ventilation efficiency, the resulting outdoor air requirements can grow rapidly.
Understanding the Intent Behind the Calculation
The purpose of the multi-zone Ventilation Rate Procedure is not to penalize system design, but to ensure that the most ventilation-critical zones receive sufficient outdoor air.
The calculation forces the designer to acknowledge that shared systems introduce compromises in air distribution.
Understanding this intent is essential before attempting to optimize or challenge the results.
Why This Lesson Matters Before Any Equation
Many engineers attempt to memorize the multi-zone ventilation equations without fully understanding why they exist.
This often results in mechanical application of formulas, frustration with large airflow values, and a tendency to oversize systems as a precaution.
By understanding the conceptual problem first, the equations become logical tools rather than obstacles.
Key Takeaway
Multi-zone ventilation is not complex because of mathematics, but because of airflow distribution realities.
System ventilation efficiency exists to correct for uneven outdoor air delivery, and ignoring its meaning leads directly to oversizing and energy penalties.
Reflection Question
In your experience, how often are large outdoor air requirements in multi-zone systems accepted without questioning the assumptions that led to them?