Lesson 4 – Outdoor Air Requirements for Single-Zone Systems
Single-zone systems represent the simplest case for applying the Ventilation Rate Procedure.
Understanding this case clearly is essential, because it forms the foundation for all multi-zone ventilation calculations discussed later.
In this lesson, we focus on concept and logic first, before introducing any numerical examples.
What Is a Single-Zone System?
A single-zone system is an HVAC system that serves one space or one thermal zone only.
In this configuration, all supplied air is delivered to the same zone, and all return air comes back from that zone.
- One zone served by one air-handling system
- No air sharing between multiple spaces
- No zone diversity effects
- Simplified ventilation behavior
Why Single-Zone Systems Are the Starting Point
ASHRAE ventilation procedures always begin with the single-zone case.
This is because single-zone systems do not suffer from air distribution inefficiencies caused by multiple zones with different ventilation needs.
As a result, ventilation effectiveness is assumed to be ideal for this case.
Ventilation Logic for a Single-Zone System
For a single-zone system, the required outdoor air is directly related to the needs of that zone alone.
There is no need to correct for system ventilation efficiency, because all outdoor air supplied reaches the occupied space.
This makes the single-zone case conceptually simple, but also easy to misuse if assumptions are ignored.
Key Inputs Affecting Outdoor Air Requirements
Even in a single-zone system, several inputs directly influence the required outdoor air rate.
- Occupancy level and density
- Type of space and activity
- Floor area of the zone
- Ventilation rate per person
- Ventilation rate per unit area
Misunderstanding any of these inputs can result in higher-than-necessary outdoor air flow.
Where Oversizing Often Begins
Single-zone systems are often assumed to be immune to ventilation oversizing.
In practice, oversizing commonly starts here, especially when conservative assumptions are applied simultaneously.
- Using maximum occupancy at all times
- Ignoring realistic schedules
- Selecting higher-than-necessary ventilation rates
- Treating minimum values as design targets
Because the calculation appears simple, these assumptions are rarely questioned.
Single-Zone Systems as a Reference Case
The single-zone ventilation calculation should be treated as a reference case.
It helps engineers understand the magnitude of outdoor air required before introducing system-level corrections in multi-zone systems.
Key Takeaway
Single-zone systems provide the clearest and most direct application of the Ventilation Rate Procedure.
Errors made at this stage propagate and amplify when moving to multi-zone systems.
Reflection Question
In your experience, how often are single-zone ventilation calculations treated as a formality rather than a critical design decision?
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