Properly setting the minimum outside air position on an economizer is essential for comfort, energy efficiency, and code compliance. While controls explain how to adjust a minimum position, they don’t tell you where to physically set the damper blades to deliver the correct airflow. Here’s why—and how to approach it correctly.
Why Minimum Position Matters
ASHRAE Standard 62.1 requires a fixed amount of fresh outdoor air during occupied hours. During unoccupied periods, economizer dampers may close completely, unless free cooling is active.
Setting the minimum position correctly ensures:
- Adequate ventilation for occupant health
- Balanced humidity levels
- Reduced risk of short-cycling
- Lower energy consumption
Too much outdoor air = higher heating/cooling load.
Too little = poor indoor air quality and comfort complaints.
A Real-World Example
MicroMetl technicians recently visited a large automotive showroom experiencing persistent humidity issues. All economizers were set with their outside-air dampers fully closed, preventing any ventilation.
Because the units were equipped with single enthalpy sensors instead of differential enthalpy, humidity was never sensed or corrected. The RTUs short-cycled—reaching temperature setpoints without running long enough to remove humidity.
After properly setting minimum outside air positions, humidity complaints disappeared.
Why “Eyeballing” Minimum Position Fails
A common misconception is to assume damper blade position equals CFM proportionally. For example:
“If 90° blade rotation = fully open, 20% outside air must equal ~18° open.”
This is not accurate.
Due to damper design, duct static pressure, and RTU fan characteristics:
- Airflow does not increase linearly with blade angle
- Often, airflow at 50% open and 100% open is nearly identical, with turbulence being the only difference
- Even closed dampers on older economizers may leak 5–10% of outside air
This makes visual estimation highly unreliable.
The Challenge With Older Economizers
Older designs can leak significant outside air even when “closed.” This adds:
- Unnecessary load during unoccupied hours
- Difficulty achieving accurate minimum airflow
- Potential humidity or comfort issues
When leakage is unpredictable, precise ventilation control becomes impossible.
Title 24 Low-Leak Requirements
California’s Title 24 addressed widespread economizer failures by requiring:
- Low-leak or ultra-low-leak dampers
- Fault detection diagnostics
- Clear service indicators when economizers fail or disconnect
Low-leak designs allow technicians to confidently set “closed” to near-zero airflow—restoring the accuracy of minimum position calculations.
How to Determine Minimum Open Position (Properly)
Because every installation differs in:
- CFM requirements
- Static pressure
- Duct configuration
- Sensor strategy
- RTU fan characteristics
The only reliable method is measurement.
Technicians should use airflow measuring tools such as:
- Flow hoods
- Pitot tubes
- Differential pressure sensors
- CFM measurement equipment built into advanced controls
This ensures the minimum position delivers exactly the required CFM.
Final Thoughts
Economizers are designed to improve efficiency and deliver a measurable return on investment. But they can only do so when minimum position is set correctly. Estimating by visual blade angle introduces large errors that increase operating costs and compromise indoor air quality.
Take the time to measure.
Verify airflow.
Document your settings.
It’s the most important step in ensuring your economizer delivers the performance and savings it was designed to achieve.
