Damper actuators: torque, spring return and the things that go wrong

Damper actuators are easy to overlook. They’re small, they sit in a duct, and nobody thinks about them until a damper sticks open on a frosty night and freezes a coil. Choosing them properly and fitting them well saves a lot of grief.

Open/close or modulating?

Open/close (two-position) actuators are fine for isolation dampers and shut-off duty, such as an AHU’s fresh air damper that is either open when the unit’s running or closed when it’s off. They’re usually controlled by a relay output, and sometimes come as floating (three-point) types driven by two outputs.

Modulating actuators take a proportional signal, usually 0–10 V or 2–10 V, and position the damper anywhere in its range. Use them for mixing dampers, face and bypass dampers on heat recovery, and VAV boxes.

Check the control signal matches what the controller provides, and whether the strategy needs position feedback.

Sizing torque

An actuator has to be strong enough to move the damper and hold it shut against airflow and pressure. Damper manufacturers usually give a torque requirement, often as a figure per square metre of damper area, which varies with damper type, seals and air velocity.

• Work out the damper area and multiply by the manufacturer’s figure.
• Add a margin for wear, dirt and stiff bearings.
• Choose an actuator with at least that torque, or split large dampers into sections with an actuator each.

An undersized actuator might move a new damper on commissioning day and then stall a year later when the bearings stiffen up.

Spring return and fail-safe position

Spring-return actuators wind up a spring as they drive. When they lose power, the spring drives the damper to a set position. That’s essential wherever the damper’s position on power failure matters:

• Fresh air and exhaust dampers on AHUs usually need to fail closed, so freezing air can’t drift through the unit and freeze the coils. See AHU frost protection.
• Fire and smoke dampers have their own specific requirements and are a different discipline altogether. Never treat them as ordinary BMS dampers.

Non-spring-return actuators stay where they are when power is lost. That’s fine for many applications, but check it’s acceptable before choosing one.

Fitting and linkages

A lot of damper problems are mechanical rather than electrical:

• Direct-coupled actuators clamp straight onto the damper shaft. They’re simple and reliable, provided the clamp is tight and the anti-rotation bracket is fitted properly.
• Linkages between the actuator and damper blades can slip, bend or wear, so the actuator shows 100% while the damper is half shut.
• Damper blades can be seized, warped or blocked by debris.

During installation and commissioning, physically check that the damper actually moves through its full travel when the actuator does, not just that the actuator is turning.

Position feedback and end switches

Feedback tells the BMS where the damper really is:

• Modulating feedback (for example a 0–10 V position signal) lets the BMS compare command with actual position.
• End switches confirm fully open or fully closed. They’re often used to prove dampers are open before the fans start.

Feedback from the actuator only tells you the actuator moved. If the linkage has slipped, the damper might not have. For critical dampers, consider a switch on the damper itself or an airflow check.

Common faults on site

• Actuator moving, damper not. A slipped clamp, a broken linkage or a seized damper.
• Damper not closing fully. Wrong stroke set, obstruction, or worn seals.
• Wrong direction. The rotation direction is set on the actuator, and it’s easy to get backwards.
• Hunting. A modulating damper on a poorly tuned loop, or an oversized damper with poor authority (the same idea as with control valves).
• Springs that no longer return. Test spring return by removing power. Springs don’t last forever.

A quick commissioning checklist

• Drive the actuator from the BMS at 0%, 50% and 100%, and watch the damper itself.
• Check the direction of rotation and that the full stroke is used.
• Remove power and check the spring-return position.
• Check end switches and feedback signals against the actual position.
• Record actuator models, signals and fail-safe positions in the O&M documentation.