Applied Example 1 – Simple VRP Calculation (Single-Zone)
This applied example demonstrates a simple Ventilation Rate Procedure (VRP) outdoor air calculation for a single-zone space.
The purpose is not to memorize equations, but to understand what drives outdoor air requirements and how assumptions can inflate results.
We will use a realistic office space example and keep the math minimal and clear.
Scenario Description (Single-Zone)
Assume a small office space served by one single-zone air system (one zone only).
We will determine the minimum outdoor air required using the VRP logic.
Given data for the space:
- Floor area (Az) = 100 m²
- Design occupancy (Pz) = 10 people
- Ventilation rates from the standard (example values used for learning):
- Outdoor air rate per person (Rp) = 10 L/s per person
- Outdoor air rate per unit area (Ra) = 0.3 L/s per m²
Important note:
The exact Rp and Ra values depend on the space type table in the standard.
In this training example, we use simple representative numbers to focus on the method and the impact of assumptions.
Step 1: Understand the Two Components of Outdoor Air
In VRP, outdoor air for a zone is typically built from two parts:
- A people component (based on occupants)
- An area component (based on floor area)
This reflects two realities:
- People generate contaminants and bioeffluents.
- Buildings and materials also contribute contaminants, and area is used as a proxy.
Step 2: Calculate the People Component
People component = Rp × Pz
Substitute the values:
Rp × Pz = 10 L/s-person × 10 people = 100 L/s
So, outdoor air required due to occupants is 100 L/s.
Step 3: Calculate the Area Component
Area component = Ra × Az
Substitute the values:
Ra × Az = 0.3 L/s-m² × 100 m² = 30 L/s
So, outdoor air required due to the space area is 30 L/s.
Step 4: Calculate the Total Zone Outdoor Air (Single-Zone)
Total zone outdoor air = (Rp × Pz) + (Ra × Az)
Substitute the results:
Total outdoor air = 100 L/s + 30 L/s = 130 L/s
This is the required outdoor air intake for this single-zone space in our simplified example.
Convert to a More Familiar Unit (Optional)
Some designers prefer m³/h.
To convert L/s to m³/h:
1 L/s = 3.6 m³/h
So:
130 L/s × 3.6 = 468 m³/h
Result:
Minimum outdoor air ≈ 468 m³/h
What This Example Is Teaching You
The equation is simple, but the design impact can be significant.
Notice what controls the result:
- Occupancy (Pz) directly drives the largest part of outdoor air
- Floor area (Az) adds a base ventilation requirement even if occupancy is low
- The chosen Rp and Ra values strongly influence final airflow
In many real projects, the “oversizing” begins here when occupancy is assumed too high or unrealistic.
A Quick Sensitivity Check (Why Assumptions Matter)
Let’s keep everything the same, but assume the space is typically occupied by only 6 people instead of 10.
People component becomes:
Rp × Pz = 10 × 6 = 60 L/s
Area component remains:
Ra × Az = 30 L/s
Total outdoor air becomes:
60 + 30 = 90 L/s
Compare the two results:
- With 10 people: 130 L/s
- With 6 people: 90 L/s
This is a major difference caused by one assumption only.
This is why occupancy schedules and realistic design occupancy matter, even before multi-zone complexity and Ev appear.
Practical Interpretation (In Hot and Humid Climates)
In hot and humid climates, outdoor air is not “free.”
Higher outdoor air means:
- Higher cooling load
- Higher dehumidification load
- Larger coils or more chilled water capacity
- Higher fan energy and possibly larger ductwork
- Higher operational cost if controls are not optimized
This is why ventilation decisions are also energy decisions.
Key Takeaway
In a single-zone VRP calculation, outdoor air is driven mainly by two inputs: people and area.
The method is simple, but conservative assumptions—especially design occupancy—can inflate outdoor air significantly and trigger oversizing.
Reflection Question
When you estimate occupancy in your projects, do you use a realistic value supported by schedules, or do you default to maximum occupancy as a “safe” assumption?
Pause here and reflect before moving to the next applied example.
Think about how this single-zone logic scales up in multi-zone systems.