How do I size an actuator for a large outdoor air damper?

Beyond Standard Actuator Sizing

Large outdoor air dampers — those exceeding 2 m² face area — present actuator sizing challenges that do not arise with smaller indoor dampers. Wind loading, seal breakaway torque, and thermal effects compound, and the consequences of undersizing are severe: a damper that fails to close in freezing conditions can destroy a heating coil within hours.

### Wind Loading

Outdoor air dampers face wind pressure that indoor dampers never experience. A 50 km/h wind gust against the louvre face can add 50–125 Pa (0.2–0.5 in. w.g.) of differential pressure across the damper — on top of the fan-induced pressure drop. In coastal or high-rise locations where wind speeds regularly exceed 80 km/h, this loading doubles. The actuator torque calculation must include the worst-case combined pressure: fan static pressure at the damper location plus the wind pressure corresponding to the site's 99.6% wind speed per ASHRAE climatic design data.

### Seal Breakaway Torque

Dampers that remain in one position for extended periods — and outdoor air dampers sit closed for 12–16 hours daily in many climates — develop seal compression set. EPDM and silicone blade edge seals slowly conform to the closed position, and the force required to break them free ('breakaway torque') can be 30–50% higher than the running torque needed to move the damper once it is in motion. The actuator must be sized for breakaway, not running torque. Belimo's selection methodology explicitly adds a breakaway factor for dampers with edge seals that are expected to remain stationary for more than 8 hours at a time.

### Jackshafting and Dual Actuators

For dampers between 2–4 m², a single 20 N·m actuator with a properly designed jackshaft (linkage system that transmits torque from the actuator to the damper blade axles) is typically sufficient — provided the jackshaft is sized to transmit the full actuator torque without torsional deflection exceeding 1° per metre of damper width. For dampers exceeding 4 m², dual actuators driving opposite ends of the same jackshaft are recommended. This distributes torque more evenly, reduces torsional wind-up in the jackshaft, and provides a measure of redundancy.

### Code Requirements

Per NFPA 90A and the BCA/NCC, outdoor air dampers in smoke control systems require spring-return actuators with NEMA 4 (IP66) weatherproof enclosures. Even when not part of a smoke control system, ASHRAE 90.1 effectively requires spring-return for outdoor air dampers in cold climates (Climate Zones 4–8) to ensure fail-closed operation that protects downstream coils from freeze damage.