This post from Alberto Ventura is a strong example of what good BAS graphics are supposed to do: make the sequence of operations visible. The graphic may not be perfectly to scale, but it accurately reflects logic, intent, and control relationships—and that is what matters most.
The question raised in the comments is a very good one:
Why are there both a supply air flow setpoint and a supply air static pressure setpoint? How is the supply fan actually being controlled?
This is not a graphics question.
It is a controls architecture question, and the graphic is doing its job by surfacing it.
1. What the graphic is telling us at a glance
From the AHU graphic, we can immediately see:
- Supply fan and return fan are both on VFDs
- The AHU is serving downstream VAV zones
- The system displays:
- Supply air flow setpoint (CFM)
- Measured supply air flow (CFM)
- Supply air static pressure (in. w.c.)
- Supply air static pressure setpoint (in. w.c.)
- Zone demand is aggregated from VAV boxes
- Supply air temperature is being reset based on heating/cooling requests
This tells us the AHU is operating in a VAV-based control strategy, not a simple constant-volume system.
2. Why both airflow and static pressure exist
In a modern VAV AHU, these two values serve different roles.
Supply air static pressure setpoint (in. w.c.)
- This is the primary control variable for the supply fan VFD
- The fan modulates speed to maintain duct pressure
- Static pressure setpoint is typically:
- Reset based on the “most-open” VAV damper
- Or based on zone demand logic
- This minimizes fan energy while ensuring airflow reaches all zones
In most VAV systems, the fan is controlled by static pressure — not airflow.
Supply air flow setpoint (CFM)
This is not always a direct fan control signal. Instead, it is often used for:
- Minimum ventilation enforcement
- ASHRAE 62.1 compliance
- Economizer logic
- OA/RA/EA damper coordination
- Diagnostics and verification
- Transitional modes (warm-up, cool-down, smoke control)
In other words:
- Static pressure controls fan speed
- Airflow validates system performance and ventilation intent
Both are necessary in a fully instrumented AHU.
3. Common control architectures that explain this graphic
There are several legitimate sequences where both setpoints appear.
Case 1: Static-pressure–controlled VAV system (most common)
- Supply fan VFD maintains duct static pressure
- Static pressure setpoint resets based on VAV demand
- Airflow is:
- Measured
- Displayed
- Compared against minimum OA and design targets
- Fan is not directly controlled by airflow
This is the most likely case shown in the graphic.
Case 2: Hybrid control (advanced sequences)
- Static pressure is primary
- Airflow becomes a secondary limiter
- Prevents exceeding design CFM
- Protects coils and filters
- During certain modes:
- Fan may temporarily target airflow (CFM)
- Then revert to static pressure control
This is sometimes seen in:
- Labs
- Healthcare
- High-performance office buildings
Case 3: Graphics exposing intent, not command
The graphic may intentionally show both values to:
- Make commissioning transparent
- Allow operators to understand:
- What the fan is doing
- Why it is doing it
- Avoid “black box” behavior
This aligns perfectly with Alberto’s point: graphics should reflect the ruleset, not hide it.
4. Why this question matters to Controls Technicians
This is exactly the kind of question that separates:
- Screen readers
from - System thinkers
If we cannot answer:
- What is the controlling variable?
- What is the limiting variable?
- What resets what?
Then:
- Graphics become misleading
- Operators lose trust
- Energy optimization stalls
Good graphics do not eliminate questions—they invite the right ones.
5. The real lesson from this exchange
This LinkedIn discussion highlights something important:
- Graphics are not just visual aids
- They are living documentation of control logic
- When built from the sequence of operations:
- They expose ambiguity
- They force clarity
- They improve design quality
The comment questioning airflow vs. static pressure is not criticism—it is validation that the graphic is doing its job.
Principal Insight
Seeing both supply airflow and static pressure setpoints on an AHU graphic is not a mistake—it is often a sign of a well-instrumented, well-documented VAV system. Static pressure typically controls the fan, while airflow supports ventilation, diagnostics, and compliance. When BAS graphics are derived directly from the sequence of operations, they transform from pictures into engineering conversations—and that is exactly where better buildings begin.
