Applied Example 2 – Multi-Zone Example and Why Ev Changes Everything

This applied example builds directly on the single-zone VRP calculation from Applied Example 1, but now with a simple two-zone system served by one AHU. The purpose is to show, with small numbers, why multi-zone ventilation can “jump” upward and why system ventilation efficiency (Ev) is not a random penalty—it is a correction for real distribution effects.

We will keep the math light, focus on the logic, and use a clear sensitivity check so you can see where oversizing starts and how to control it.

Scenario Description (Two Zones, One AHU)

Assume one AHU serving two zones:

Zone A (dense occupancy): a meeting/training room
Zone B (light occupancy): a small office area

Given (example learning values):

Zone A area (AzA) = 50 m²
Zone A design people (PzA) = 20 people
Zone B area (AzB) = 150 m²
Zone B design people (PzB) = 5 people

Assume standard ventilation rates (representative for learning):

R_p = 10 L/s per person
R_a = 0.3 L/s per m²

Also assume the AHU supply airflow distribution (this is the part many engineers ignore):

Supply air to Zone A (SaA) = 1,000 L/s
Supply air to Zone B (SaB) = 2,000 L/s

Total AHU supply air:

Vps = SaA + SaB = 3,000 L/s

Important note: Rp/Ra values and supply air flows depend on the real project and the standard table. Here we use simple numbers to understand the behavior.

Step 1: Calculate Each Zone Outdoor Air Requirement (Voz)

For each zone in VRP (single-zone logic per zone), we compute:

Zone outdoor air = (Rp × Pz) + (Ra × Az)

Zone A:

People component = 10 × 20 = 200 L/s
Area component = 0.3 × 50 = 15 L/s
VozA = 200 + 15 = 215 L/s

Zone B:

People component = 10 × 5 = 50 L/s
Area component = 0.3 × 150 = 45 L/s
VozB = 50 + 45 = 95 L/s

At this point, many people stop and think the system outdoor air should be:
215 + 95 = 310 L/s

But in multi-zone systems, that is often not enough, because outdoor air is shared and must be delivered in the right proportion.

Step 2: Compare Each Zone “Need” to Its Supply Air

This is the key multi-zone idea: each zone receives a certain supply air flow, but it requires a certain outdoor air flow. The ratio of outdoor air required to supply air is what matters.

Define a simple ratio for each zone:
Zone outdoor air fraction = Voz / Supply air to that zone

Zone A:

ZpA = 215 / 1,000 = 0.215 (21.5%)

Zone B:

ZpB = 95 / 2,000 = 0.0475 (4.75%)

Interpretation:
Zone A is ventilation-critical. It needs a high outdoor air fraction compared to its supply air.
Zone B needs relatively little outdoor air compared to its supply air.

This imbalance is why system ventilation efficiency becomes important.

Step 3: Why Ev Appears (Conceptual)

In a shared system, outdoor air is mixed and distributed through the supply air.
If the system outdoor air fraction is not high enough, the critical zone (Zone A) will not receive its required outdoor air.

Because zones have different needs, the system must “work harder” so that even the critical zone is satisfied. Ev captures that correction.

A practical way to think about it:

The more uneven the zone fractions are, the more likely Ev will drop below 1.0
When Ev drops, required system outdoor air increases

Step 4: Apply a Simple Ev to See the Impact

To keep this example simple, we will not derive Ev using the full standard’s multiple-zone equations. Instead, we will apply a realistic learning assumption.

Assume system ventilation efficiency:

Ev = 0.80

Then system outdoor air required is:
System outdoor air = (Sum of zone outdoor air requirements) / Ev

Sum of zone outdoor air requirements:

ΣVoz = 215 + 95 = 310 L/s

System outdoor air:

Vou = 310 / 0.80 = 387.5 L/s
Round:
Vou ≈ 388 L/s

So you can see the “jump”:

Without Ev correction: 310 L/s
With Ev = 0.80: 388 L/s
Increase:
78 L/s (about 25%)

This increase is not arbitrary—it exists to ensure the ventilation-critical zone is actually satisfied when air is shared.

Convert to m³/h (Optional)

1 L/s = 3.6 m³/h

So:
388 L/s × 3.6 = 1,396.8 m³/h

Minimum system outdoor air ≈ 1,397 m³/h

What This Example Is Teaching You

The “problem” is not that the standard is harsh.
The problem is that Zone A needs much more outdoor air relative to its supply air than Zone B.

In many projects, this situation becomes worse because:

The critical zone is assigned small supply air for temperature reasons, but needs high ventilation
Occupancy assumptions are inflated
Zoning is not aligned with occupancy patterns
Air distribution is shared across dissimilar spaces

Ev is essentially saying:
If your system serves very different zones together, you pay a ventilation efficiency cost.

Quick Sensitivity Check (The Oversizing Trigger)

Let’s reduce one assumption: Zone A design occupancy.

If Zone A is planned at 15 people instead of 20:

People component becomes 10 × 15 = 150 L/s
Area component remains 15 L/s
VozA becomes 165 L/s

Now:

ΣVoz = 165 + 95 = 260 L/s

If Ev stays 0.80:

Vou = 260 / 0.80 = 325 L/s

Compare:

Original system OA ≈ 388 L/s
Revised system OA ≈ 325 L/s

A single assumption (occupancy) changes system outdoor air significantly.
This is why multi-zone results often look “too big”: they amplify upstream assumptions.

Practical Interpretation (What an Engineer Should Do)

When you see large system outdoor air in multi-zone calculations, don’t panic—and don’t accept it blindly.

Instead, check these items in order:

Are occupancies realistic and supported by schedules?
Are zones grouped logically by similar occupancy type and density?
Is the ventilation-critical zone receiving enough supply air relative to its need?
Would a different system concept reduce the efficiency penalty (for example, better zoning, or dedicated outdoor air strategies)?

These are engineering decisions, not just calculation steps.

Key Takeaway

In multi-zone systems, the required system outdoor air is not simply the sum of zone outdoor air values.
System ventilation efficiency (Ev) increases system outdoor air to compensate for uneven outdoor air distribution among zones.

The more uneven your zones are (high-density zones mixed with low-density zones), the more Ev can drop—and the more outdoor air you will need.

Reflection Question

When you get a “high outdoor air” result in a multi-zone system, do you typically revisit zoning and assumptions, or do you increase equipment size and move on?

Pause here and reflect before continuing. Think about one real project where this happened.

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