A Practical Guide to Actuator Selection for HVAC Applications

Every damper and control valve in a building automation system depends on an actuator to translate a control signal into physical movement. Get the selection wrong and the symptoms show up everywhere else first — a VAV box that hunts, a valve that won't fully close, a fire damper that fails its annual test — long before anyone traces the fault back to the actuator itself. This guide walks through the five decisions that determine correct actuator selection: load type, torque, fail-safe behaviour, control signal, and supply voltage.

Controls Traders supplies actuators across the Belimo and Siemens ranges to mechanical contractors and BAS integrators throughout Australia. If you'd rather skip straight to brand-specific comparisons, see our guides on Belimo vs. Siemens actuators and the different types of actuators and their applications.

Step 1 — Determine the Load Type

Actuator selection starts with what the actuator is driving, because damper actuators and valve actuators are not interchangeable even when their electrical specifications look similar. Damper actuators are rated in newton-metres (Nm) of rotational torque and typically produce a 90° stroke. Valve actuators are rated either in newton-metres for rotary valves (ball, butterfly) or in newtons of linear force for globe and 2-way/3-way valves with a linear stem, and the stroke length or rotation angle must match the specific valve body.

Step 2 — Calculate Required Torque or Force

Undersizing torque is the single most common actuator selection error, and it rarely fails outright — instead the actuator strains against the load, runs hot, and either stalls short of full travel or wears out its gear train early. Oversizing wastes money and, on smaller valve bodies, can accelerate wear on the stem packing or ball seats through excess closing force.

For dampers, manufacturers publish torque-per-square-metre figures that vary by blade type and design velocity — opposed-blade dampers typically need less torque than parallel-blade dampers of the same size, and higher face velocities increase the torque required to overcome air pressure across the blades. Multiply the published torque-per-square-metre figure by the damper's face area, then apply a safety margin (commonly 20%) to account for linkage friction, seal drag, and any bearing wear expected over the actuator's service life.

For valves, the manufacturer's valve datasheet states the maximum torque or force required to operate and close off against the system's differential pressure — always size the actuator to meet or exceed this figure, and confirm the actuator's close-off rating covers your system's maximum expected differential pressure, not just its nominal operating pressure.

Typical Damper Size Indicative Torque Class Common Application
Up to ~0.5 m² 2–5 Nm Small VAV terminal dampers, zone dampers
~0.5–1.5 m² 5–10 Nm Standard AHU outside air / return air dampers
~1.5–3 m² 10–20 Nm Large AHU mixing dampers, plant room isolation dampers
Above ~3 m² 20–40 Nm or multiple actuators Large multi-section dampers, often paired with a jackshaft or linked actuators

These bands are indicative starting points only — always confirm against the specific damper manufacturer's torque table for blade type, seal type, and design velocity before finalising a selection.

Step 3 — Choose Fail-Safe Behaviour

Fail-safe behaviour determines what the actuator does when it loses electrical power, and getting this wrong on a life-safety application is a compliance failure, not just an inconvenience.

Actuator Type Behaviour on Power Loss Typical Use
Spring return Internal spring drives to fail-safe position (open or closed, as configured) Fire/smoke dampers, outside air dampers, heating/cooling valves where fail-safe position matters
Non-spring return Holds last commanded position via motor gearing General VAV, exhaust dampers, modulating loops where fail-in-place is acceptable
Electronic fail-safe (capacitor-driven) Onboard capacitor drives actuator to fail-safe position over several seconds, then holds Applications needing fail-safe behaviour without the added torque draw of a mechanical spring

Fire and smoke dampers must use spring return actuators certified for that application — this is a code requirement, not a design preference, and substituting a non-spring return actuator on a life-safety damper is not compliant regardless of torque rating.

Step 4 — Match the Control Signal Type

The actuator's control signal must match what the BMS controller or field bus is capable of outputting — there is no universal adapter between signal types, so this decision needs to be locked in before ordering.

Step 5 — Confirm Supply Voltage and Mounting

Supply voltage (typically 24 VAC or 230 VAC, occasionally 110 VAC) must match the transformer or circuit already available at the actuator location — this is usually dictated by the wider BMS design rather than chosen independently for each actuator. Most modern BAS installations standardise on 24 VAC because it simplifies co-locating control and power cabling and is consistent across the analogue and digital actuator ranges of most manufacturers.

Mounting also needs confirmation before ordering: damper actuators need a shaft adapter kit matched to the damper's shaft diameter and shape (round or square), and valve actuators need the correct mounting bracket or direct-coupled adapter for the specific valve body — cross-brand valve and actuator combinations are common but always require checking the manufacturer's compatibility documentation first.

Common Actuator Selection Mistakes

Need help selecting the right actuator for your project?

Controls Traders supplies the full Belimo and Siemens actuator ranges, including spring return, non-spring return, modulating, and BACnet/Modbus variants for damper and valve applications across Australia.

Contact Controls Traders — our technical team can confirm torque, fail-safe, and control signal requirements for your specific application.

Frequently Asked Questions

How do I know what torque rating my actuator needs?

For dampers, torque requirement scales with damper area, blade type, and air velocity — manufacturers publish torque-per-square-metre tables for opposed-blade and parallel-blade dampers that you multiply by your damper's face area, then add a safety margin of around 20% to cover linkage friction and seal drag. For valves, required torque or force depends on valve type, size, and the differential pressure the actuator must close against — ball and butterfly valves list a maximum required torque in the manufacturer's datasheet, and the actuator must meet or exceed that figure, including any close-off pressure rating. Always size to the datasheet figures rather than estimating; undersized actuators stall or fail to fully open and close, and oversized actuators waste money and can shorten linkage life through excess mechanical stress.

What's the difference between spring return and non-spring return actuators?

A spring return actuator stores mechanical energy in an internal spring while driving open under motor power; if electrical power is lost, the spring drives the damper or valve back to its fail-safe position — typically fully closed for fire and smoke dampers, or fully open or closed for valves depending on the failure mode required by the application. A non-spring return actuator has no internal spring and simply holds its last commanded position on power loss, using the motor's own gearing for mechanical holding torque. Spring return actuators are mandatory for life-safety applications such as fire/smoke dampers and are commonly specified for outside air dampers and heating/cooling valves where an uncontrolled failure position could cause coil freezing or energy waste. Non-spring return actuators are typically specified for general VAV, general exhaust, or modulating control loops where a fail-in-place behaviour is acceptable or preferred.

Can I use a 24V actuator on a 230V system, or vice versa?

No — supply voltage must match exactly, and 24V and 230V (or 110V) actuators are not interchangeable or field-convertible in almost all cases. Applying line voltage to a 24V-rated actuator will destroy the internal transformer or electronics immediately. The choice of supply voltage is usually driven by the BMS transformer already installed for the zone or plant room — most modern DDC and BAS installations standardise on 24 VAC for actuators and sensors because it is safer to wire, easier to run alongside low-voltage control cabling, and consistent across most manufacturers' analogue and digital control ranges. Line-voltage actuators are more common in retrofit situations where a 230V supply is already at the damper or valve location and running a separate 24V transformer circuit is impractical.

What's the difference between floating point and modulating control signals?

A floating point (also called tri-state or three-point) actuator has no internal positioning electronics — the BMS controller sends short open or close pulses on two separate digital outputs, and the actuator moves incrementally in that direction for as long as the pulse is held, with the controller inferring position from elapsed run time rather than reading actual position feedback. A modulating actuator accepts a continuous analogue signal — typically 0-10 V or 4-20 mA — and drives to a proportional position corresponding to that signal value, using an internal potentiometer or digital position sensor for closed-loop position control. Modulating actuators give more accurate and repeatable positioning and are the standard choice for critical control loops such as AHU cooling and heating valves, while floating point actuators are a lower-cost option still widely used for VAV damper control where BMS controllers are commonly built around tri-state damper outputs.

Do I need a BACnet or Modbus actuator, or is analogue fine?

For a single damper or valve on a conventional analogue or digital output from a local DDC controller, an analogue (0-10 V, 4-20 mA) or floating point actuator is simpler to wire and commission and is entirely fine. BACnet MS/TP or Modbus RTU actuators become worthwhile when you have many actuators on a shared communication trunk, when you need position feedback and diagnostic data (running current, stall detection, hours run) reported back to the BMS without dedicating separate analogue input points, or when the controller itself is a BACnet/Modbus device with no spare analogue outputs. Communicating actuators cost more per unit but reduce field wiring significantly on multi-actuator installations such as VAV risers or large AHU plant rooms, since a single two-wire RS-485 trunk can address dozens of devices rather than requiring an individual home run per actuator.

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