Controlling the outside-air ratio of an air handling unit is the main lever for balancing indoor air quality against heating and cooling energy, and three control strategies dominate: fixed, enthalpy, and CO2-based.
The Fresh-Air Trade-Off
Every cubic metre of outside air must be heated, cooled, humidified, or dehumidified to room condition, so over-ventilating wastes energy while under-ventilating risks CO2 climb and stuffiness. Codes set a minimum outside-air fraction, often expressed per person (about 8โ10 L/s per occupant in many standards) and per floor area, and the controller's job is to meet that minimum without exceeding it when conditions allow free cooling. The two failure modes are opposite: a stuck-100% damper heats the whole winter, while a starved space fails air quality audits and risks complaints.
Good control sits between, and the right strategy depends on how variable the occupancy and the climate are. A steady occupied space gains little from demand control, while a gym or auditorium with swinging occupancy gains a great deal, because the ventilation follows the bodies rather than the clock.
Establishing the minimum is harder than controlling it. A ventilation rate written as litres per second per person only becomes a damper position once someone decides how many people are actually present, and design occupancy is usually higher than reality. Two rooms served by the same unit can also have very different needs, which is why the critical-zone calculation exists: a multi-zone unit has to supply enough outdoor air that the worst-served zone still meets its rate, and that can push the unit-level fraction well above the simple average. Getting this wrong at design stage means no control strategy can rescue it later, because a damper cannot deliver air the unit was never sized to draw in.
Three Control Strategies
A fixed strategy holds a set minimum outdoor-air damper position regardless of conditions, simple but wasteful. An enthalpy economizer compares outside and return air enthalpy and opens the damper when outside air is "cheaper" to condition, capturing free cooling. A CO2-based strategy modulates toward the minimum as occupancy falls, using space CO2 as a proxy for people load, which is why it works best where people are the dominant source of the ventilation demand.
| Item | Specification | Note |
|---|---|---|
| Fixed minimum | 10โ15% | Always-on base ventilation |
| Enthalpy economizer | Free cooling | Opens when OA enthalpy lower |
| CO2 setpoint | 800โ1,000 ppm | Demand-controlled ventilation |
| Occupancy deadband | ยฑ 100 ppm | Avoids damper hunting |
| Min OA interlock | Never below code | Safety floor preserved |
| Ventilation basis | ≈ 8–10 L/s per person | Plus a floor-area component |
| Damper leakage | Low-leakage class preferred | A closed blade still passes some air |
In practice a hybrid is common: a CO2-trimmed minimum during occupied hours plus an enthalpy economizer during swing seasons, with the code minimum always enforced as a hard floor that demand control can never erode. The economizer and the CO2 loop share the same dampers but serve different goals, so the sequence must define which wins when both want to move.
Hardware decides whether the sequence works. A CO2 sensor in the return duct reads a mixed average and responds slowly, while a wall sensor at breathing height reads what occupants actually experience; the duct location is cheaper but should not be used where one zone dominates. Enthalpy changeover needs outdoor temperature and humidity measured in a shaded, ventilated enclosure, because a sensor baking in the sun will lock out free cooling on exactly the mild days it should be used. Freeze protection is the third piece: in cold climates the outdoor damper must close and preheat must enable before the coil approaches 0°C, and that interlock overrides both economizer and demand control.
Control Selection Checklist
- Know the occupancy: Variable-density spaces (auditoriums, offices) gain most from CO2 control.
- Check climate: Enthalpy economizers pay back fastest in mixed humid climates with clear swing seasons.
- Keep the code floor: Never let demand control drop below the regulatory minimum outside air.
- Tune the deadband: A 100 ppm band stops the damper from chattering and wearing actuators.
- Calibrate sensors: CO2 drift is real; schedule recalibration so the loop trusts its own signal.
- Site the sensors properly: Read CO2 in the occupied zone where one area dominates, not only in the return duct.
- Fix the override order: Freeze protection outranks both the economizer and demand control in the sequence.
Why Ratio Control Matters
The right strategy typically trims ventilation energy by 10โ30% while keeping air quality auditable, a rare win-win. Keruite designs its AHU mixing boxes and damper drives to accept these control strategies so the building operator can choose fixed, enthalpy, or CO2 logic on one platform, and the same hardware serves all three without rework.
Fresh-air control is where the mechanical unit and the control system either cooperate or blame each other, so damper hardware has to be specified with the sequence in mind. Mixing sections built in an 80,000 m² plant, by a manufacturer with more than 30 years of unit fabrication behind it, are supplied with defined damper leakage class and actuator torque data, which is what a controls contractor needs to write a sequence that holds position. ISO 9001:2015 and CCC certification cover the build and electrical side, and with units in more than 50 countries the same section is configured for both per-person and per-area ventilation codes.
Our engineers respond to selection enquiries within 24 hours, including mixing-box and damper control options for AHU fresh-air management.