In AHU projects, customers often complain that the indoor air feels stuffy after the system has operated for some time. When maintenance personnel inspect the system, they may find that the CO₂ reading has already reached 1,800 ppm, the alarm light has turned on, the fresh air damper has reached 100% opening, and the fan still runs normally. However, the CO₂ concentration does not decrease.
After further investigation, we found that dust had completely blocked the fresh air filter. The system may have supplied only half of the designed fresh air volume. Even with the damper fully open, very little outdoor air could enter because the hardware had already developed a problem.
Therefore, before we discuss which control method to use, we must first determine whether the system has a reasonable airflow design and whether the design considers equipment losses under actual operating conditions.
Assuming the system design is reasonable, should the fresh air system use a fixed minimum damper opening or CO₂ PID control?
The answer depends on the actual application. We must keep the control solution flexible.
1. The Essential Difference Between the Two Control Methods
Minimum Fresh Air Damper Position
The minimum fresh air damper position means that the control system keeps the fresh air damper at a preset minimum opening, such as 30%, while the AHU operates normally, even when the indoor CO₂ concentration remains relatively low.
Regardless of how many people stay indoors or how the CO₂ concentration changes, the system basically maintains a fixed fresh air level.
For example, engineers can configure the control program as follows:
· After the AHU starts and the PLC confirms the supply fan status, the controller keeps the fresh air damper at a minimum opening of 20%.
· When the system has no additional control demand, the controller prevents the fresh air damper from closing below 20%.
This control method uses simple logic, does not depend on a CO₂ sensor, requires less commissioning work, and reduces maintenance costs.
It suits places with stable occupancy density, such as offices.
However, it also has obvious disadvantages. In places where occupancy changes significantly, insufficient fresh air may cause the CO₂ concentration to rise rapidly. When fewer people occupy the space, the system may introduce unnecessary outdoor air and increase energy consumption.
CO₂ PID Control
CO₂ PID control uses CO₂ sensors inside the room, in the return air duct, or in another representative area to continuously measure the indoor CO₂ concentration.
The controller automatically adjusts the fresh air damper opening or the fan frequency according to the setpoint. It keeps the CO₂ concentration in the occupied area within a specified range, usually around 800–1,000 ppm.
When the number of occupants increases, the CO₂ concentration gradually rises, and the PID controller increases the fresh air damper opening or motor frequency.
When the number of occupants decreases and the CO₂ concentration falls, the controller gradually reduces the damper opening or motor frequency.
In theory, this method provides better energy efficiency and control accuracy. However, engineers must pay attention to several issues during on-site commissioning.
It is important to note that engineers normally use the CO₂ control loop only as an adjustment signal on top of the minimum fresh air backup logic. They do not use it as a completely independent closed-loop control system.
The minimum fresh air logic always maintains a basic fresh air volume. When the CO₂ concentration rises, the controller increases the fresh air volume. When the CO₂ concentration falls, the system returns to the minimum value.
The controller should not completely close the fresh air damper simply because the CO₂ concentration remains low.
A pure CO₂ closed-loop system without minimum fresh air backup creates a high risk in actual projects. If the sensor drifts, the system may stop supplying fresh air for an extended period.
In actual projects:
Final fresh air damper position = the greater value between the minimum fresh air damper position and the CO₂ PID output.
For example:
· Minimum fresh air damper position: 20%
· CO₂ PID output range: 0% to 100%
· When the CO₂ concentration remains low, the PID output reaches 10%, so the controller keeps the final fresh air damper position at 20%.
· When the CO₂ concentration rises, the PID output increases to 45%, so the controller opens the fresh air damper to 45%.
· When the CO₂ concentration decreases and the PID output falls to 15%, the controller still keeps the fresh air damper at 20%.
This control method provides two functions:
1. It ensures that the AHU always maintains the minimum required fresh air volume.
2. It increases the fresh air volume according to the actual indoor occupancy load.
Engineers can summarize the PLC logic inside an HVAC control panel as follows:
If supply fan is running:
Fresh air damper command =
MAX(minimum damper position, CO₂ PID output)
Else:
Fresh air damper command = 0%
2. Common Reasons Why CO₂ Control Fails in Actual Projects
1) Incorrect Sensor Installation Position
The conclusion is simple: install the CO₂ sensor where the CO₂ exhaled by occupants can mix sufficiently with the indoor air. Engineers normally place it near the return air outlet.
Installing the sensor near the fresh air inlet will make the measured value lower than the actual indoor concentration.
Installing it in an indoor dead zone will make the measured value higher than the actual indoor concentration.
Recommended installation position:
· Near the return air outlet
· Approximately 0.3–1 metre away from the return air outlet
Do not install the sensor:
· Directly opposite a supply air outlet
· Near a doorway
· Directly above an area where people remain for long periods
2) Sensor Accuracy Drift
CO₂ sensors act as consumable components, and their measurement accuracy decreases over time.
A new sensor normally provides a measurement accuracy of approximately ±50 ppm. However, after two or three years, the measurement deviation may increase to approximately ±200 ppm.
Based on our experience, sensor drift usually moves in one direction. The measured value either remains consistently higher or consistently lower than the actual value.
When this type of drift occurs, no amount of PID tuning can correct the CO₂ reading. Maintenance personnel therefore need to replace the sensor periodically.
3) The CO₂ Setpoint Is Too Low
I once encountered a project where the customer set the CO₂ target at 600 ppm and continuously complained that the indoor CO₂ concentration could not reach the target.
I explained that 600 ppm already sits very close to the CO₂ concentration of fresh outdoor air.
Even if the system opens the fresh air damper to 100% and runs the fresh air fan at full load, it may still struggle to reduce the indoor concentration to this level.
The system essentially exchanges indoor air with outdoor air. If the source air already has a CO₂ concentration close to the indoor target, how can the system reduce the indoor concentration any further?
If the customer keeps this setpoint throughout the year, the CO₂ control function becomes almost meaningless. It will not save energy and will significantly increase wear on motors and other equipment.
A reasonable reference value is:
· Indoor CO₂ concentration: ≤1,000 ppm
· Typical control target: 800–1,000 ppm
This range gives the system sufficient adjustment space.
3. How Should We Select the Two Control Methods?
Places with Stable Occupancy Loads
For production areas, equipment rooms, or some industrial areas where the number of occupants changes very little and the operating schedule remains fixed, engineers can prioritize minimum fresh air damper position control.
This option requires a clearly defined fresh air volume and completed on-site airflow balancing.
In this type of project, a fixed minimum damper position can normally meet the basic ventilation requirement while reducing control system complexity.
Places with Large Occupancy Changes
In meeting rooms, classrooms, shopping centres, offices, restaurants, exhibition halls, and multifunction rooms, the number of occupants may change significantly throughout the day.
Using only a fixed fresh air damper position can easily create two problems:
· When fewer people occupy the space, excessive fresh air increases air-conditioning energy consumption.
· When more people occupy the space, insufficient fresh air causes the CO₂ concentration to rise.
CO₂ PID control suits these applications better.
However, engineers should not let CO₂ PID control completely replace minimum fresh air control. The PID loop should dynamically adjust the fresh air volume above the minimum ventilation level.
4. Which Control Method Saves More Energy?
From an energy-saving perspective, CO₂ PID control normally suits buildings with significant occupancy changes better.
The AHU must filter, cool, heat, dehumidify, or humidify outdoor air before supplying it indoors.
Too much outdoor air directly increases the load on the air-conditioning system, especially in hot and humid climates or cold regions.
However, engineers should not reduce the fresh air volume without limit simply to save energy.
Energy-saving control should not aim to close the fresh air damper as much as possible. Instead, it should adjust the fresh air volume according to actual demand while meeting indoor air quality and minimum ventilation requirements.
Therefore, the most reasonable control strategy normally does not require a simple choice between fixed damper position control and CO₂ PID control.
Instead:
The minimum fresh air volume provides basic ventilation and safety, while CO₂ PID control dynamically increases the fresh air volume according to the indoor occupancy load.
Conclusion
Minimum fresh air damper position control and CO₂ PID control solve two different problems.
The minimum damper position ensures basic ventilation, while CO₂ PID control dynamically adjusts the fresh air volume according to occupancy.
For systems with stable occupancy and relatively simple control requirements, engineers can use a fixed minimum damper position after completing on-site airflow balancing.
For AHU systems with large occupancy changes and higher energy-saving requirements, we recommend the following control method:
Use the minimum fresh air damper position as the lower control limit and use CO₂ PID as the dynamic adjustment signal.
If the project requires higher control accuracy, use a closed-loop fresh air volume control system instead of estimating the actual fresh air volume only from the damper position.