Spring-Return vs Non-Spring-Return Actuators: Which Do You Need?

The choice between spring-return and non-spring-return actuators is one of the most consequential specification decisions in HVAC and building automation. These two types behave entirely differently when power is cut or a control signal is lost — and specifying the wrong type can result in freeze-damaged coils, failed fire control sequences, or unnecessary capital expenditure across a large installation.

Selection should be driven by the fail-safe requirement of the specific application — not by habit, default specifications, or cost alone. To make the correct choice, you must understand what happens to the connected damper or valve when power is lost, and whether that outcome is acceptable given the system, its occupants, and the applicable Australian Standards. This guide sets out the technical differences, code obligations, and a practical decision framework for the most common HVAC applications.

How Spring-Return Actuators Work

A spring-return (SR) actuator uses an electric motor to drive a gear train that rotates the output shaft — and simultaneously compresses an internal coil spring. When the motor is de-energised (power removed, or control signal lost), the compressed spring releases and drives the shaft back to its starting, or fail-safe, position.

Spring-return actuators are available in two fail-safe configurations, which must be specified at time of order:

• Normally-closed (NC): the spring drives the shaft to the closed position (0°). The motor is energised to open the actuator. On power loss, the actuator closes.
• Normally-open (NO): the spring drives the shaft to the open position (90°). The motor is energised to close the actuator. On power loss, the actuator opens.

The fail-safe position is set at manufacture and cannot be reversed in the field. Always specify NC or NO on the purchase order. An important operational characteristic of spring-return actuators is that they draw continuous electrical power at any intermediate modulating position — the motor must continuously counteract the spring force to hold the shaft at positions other than the fail-safe position.

How Non-Spring-Return Actuators Work

A non-spring-return (NSR) actuator uses an electric motor to drive a self-locking worm gear that positions the output shaft. When the motor stops, the worm gear holds the shaft in position without any motor power — the worm gear geometry prevents back-driving, so air pressure, valve pressure, and gravity cannot move the shaft. On power loss, the result is hold-in-last-position: the shaft remains exactly where it was when power was cut.

Electronic fail-safe (capacitor-return) actuators exist as a variant that offers some of the size and power efficiency advantages of NSR actuators while providing a defined fail-safe position. These actuators store energy in an internal capacitor and, on power loss, use that stored energy to drive to a pre-programmed position. However, this approach relies entirely on the capacitor being functional — periodic testing and capacitor replacement are required to maintain the fail-safe behaviour, and capacitor-return actuators are not to be used in life-safety applications where spring-return is mandated by Australian Standards. For life-safety applications, mechanical spring-return remains the only compliant option.

The power draw of an NSR actuator holding position is less than 0.5 W, compared to 2–5 W continuously for a spring-return actuator holding an intermediate modulating position. In large installations with many actuators, this energy difference is measurable over a year of operation.

Comparing the Two Types

Feature	Spring-Return	Non-Spring-Return
Fail-safe position	Defined at manufacture (open or closed)	Holds last position (or programmable with capacitor — see note)
Code requirement for life-safety	Mandatory (fire/smoke dampers, OA dampers)	Not acceptable for life-safety applications
Operating power draw	Higher — motor continuously counteracts spring	Lower — worm gear holds position without motor power
Physical size for same torque	Larger — spring housing adds bulk and mass	More compact
Capital cost	Higher (typically 30–60% above equivalent NSR)	Lower
Mechanical life	Spring fatigue over time; check manufacturer cycle rating	Longer mechanical life in most applications
Manual override	Most include pushbutton or hex key override	Most include a manual override

When Spring-Return Is Required

Spring-return actuators are not an option in applications where a defined fail-safe position is required by Australian Standards or by the consequences of the application. Understanding these requirements before specifying is essential.

Fire and smoke dampers (AS 1668.1, AS 1851): Spring-return is mandatory for all fire and smoke damper actuators. The actuator must drive to the closed position on loss of electrical power or on a trip signal from the fire control panel. Non-spring-return actuators — including capacitor-return variants — are not compliant for this application. Auxiliary switch outputs providing proof-of-closure contacts are also required; confirm the switch configuration (normally-open or normally-closed contact, and the voltage and current rating) against the fire control panel specification before ordering.

Outdoor air dampers in most Australian applications: AS 1668.2 cold-air protection requirements and standard engineering practice require OA dampers to close on power loss in virtually all Australian commercial HVAC applications. This includes temperate climate zones such as Adelaide, Melbourne, and Canberra, where winter temperatures create freeze risk for chilled water coils and heating coils. Spring-return NC is the correct specification for OA damper actuators across the overwhelming majority of Australian commercial projects.

Beyond code obligations, spring-return should also be specified in any application where the consequence of the actuator failing to reach a defined position is operationally significant — including applications where the BMS cannot guarantee a prompt command to the actuator following power restoration, valves on high-pressure steam or high-temperature hot water circuits where an open valve during power failure would be hazardous, and applications in buildings where the BMS has a history of losing control of actuators during power disturbances.

When Non-Spring-Return Is Acceptable

VAV box primary air dampers in most commercial buildings: Hold-in-last-position is acceptable for the primary air damper in a standard VAV terminal unit in commercial office, retail, or education applications. The majority of VAV actuator installations in Australian commercial buildings use NSR actuators. Belimo LM series and Siemens SAS series both offer NSR variants specifically suited to VAV terminal mounting. Where the project specification or brief explicitly requires spring-return on VAV boxes — for example, for specific critical-environment zones — spring-return must be used; but it is not the default requirement for commercial VAV applications.

Modulating mixing box dampers in mild climates: Where freeze protection of coils is not a concern and the system includes independent safeguards (such as a low-limit temperature controller), an NSR actuator on a mixing box return-air or relief-air damper may be acceptable. This requires a documented fail-safe analysis and sign-off from the mechanical engineer responsible for the project — it is not a blanket permission.

Cost-constrained applications with appropriate fail-safe analysis: Where capital cost is a significant project constraint and the application is not life-safety critical, a structured fail-safe analysis demonstrating that hold-in-last-position is an acceptable outcome can support the use of NSR actuators. The analysis must be documented, reviewed by the responsible engineer, and retained as part of the project record.

Interior supply and return air dampers with no life-safety function and no freeze risk: Balancing dampers, zone isolation dampers, and similar interior air-distribution components where the fail-safe position is genuinely immaterial — where neither fully open nor fully closed creates a safety or significant comfort concern — are appropriate candidates for NSR actuators.

Energy and Cost Considerations

Spring-return actuators draw 2–5 W continuously when holding at intermediate modulating positions. For a large installation, this adds up: a building with 200 modulating spring-return actuators averaging 4 W continuous power draw over 8,760 hours per year consumes approximately 7,008 kWh annually. At a commercial electricity rate of $0.25/kWh, this represents around $1,750 per year in electricity costs attributable to actuator holding power — before factoring in cooling load on the air conditioning system.

NSR actuators draw less than 0.5 W holding position. The same 200 actuators consume approximately 875 kWh per year — a reduction of more than 85%. For large projects where NSR actuators are technically appropriate, this energy difference is a meaningful lifecycle cost consideration.

Capital cost differences are equally real. Spring-return actuators typically cost 30–60% more than equivalent NSR models for the same torque output and control type. On a project with many actuators, the capital premium for spring-return over NSR can be substantial — and is only justified where the fail-safe requirement genuinely demands it. Blanket specification of spring-return for applications where NSR is technically acceptable increases project cost without improving system safety or performance.

Spring fatigue over the actuator's service life is a consideration for lifecycle planning. Manufacturers typically rate spring-return actuators for 60,000–100,000 full cycles. An OA damper cycling once per occupied hour accumulates approximately 3,000 cycles per year — implying a spring life of 20–33 years under normal occupied-building conditions. Applications with higher cycle rates, such as demand-controlled ventilation with frequent CO₂-driven resets, will experience spring fatigue sooner. Include planned spring and actuator replacement in the lifecycle maintenance plan for all critical and life-safety applications, as required by AS 1851.

Belimo and Siemens Options from Controls Traders

Belimo spring-return variants are identified by the "SR" suffix in the model number (for example, LM24A-SR is the spring-return variant of the LM24A NSR model). This convention applies consistently across the LM, NM, AM, and GM series for damper actuators, and across the valve actuator ranges. NSR models incorporate the mechanical self-locking worm gear that holds position without motor power and cannot be back-driven by air pressure or fluid pressure. Browse the full Belimo product range or see the Belimo model numbers guide for a complete breakdown of the model number structure.

Siemens offers both SR and NSR variants across the SSA series (3–45 Nm, AHU and general HVAC damper applications) and the SAS series (purpose-designed for VAV terminal unit mounting). As with Belimo, the spring-return variants are distinguished by specific model number suffixes — refer to the Siemens actuator series guide for model selection guidance, or browse the Siemens product range on the Controls Traders website.

Contact the Controls Traders team for specification support, fail-safe analysis guidance, or to confirm stock availability for project programmes.

Practical Decision Guide

Use this guide as a starting point for each application on a project. In all cases, the responsible mechanical or controls engineer must confirm the fail-safe analysis for the specific application before ordering.

• OA damper, AHU application: spring-return NC. In virtually all Australian applications, without exception.
• Fire/smoke damper: spring-return NC. Mandatory per AS 1668.1 and AS 1851. No alternative.
• Relief/exhaust damper: spring-return NO or NSR depending on system design and fail-safe analysis — confirm with mechanical engineer.
• VAV box primary air damper: NSR acceptable in most commercial applications. Specify spring-return NC where the project brief or specification requires it, or where the application makes holding-last-position unacceptable.
• Chilled water coil valve (2-way): spring-return fail-closed (NC) is standard; for heating coils and some process applications, confirm the required fail-safe direction — fail-open (NO) is specified in some heating coil applications to prevent overheating.
• Low-risk interior supply/return air damper: NSR acceptable where hold-in-last-position is not a safety or comfort concern.
• Any life-safety or code-driven application: spring-return required. Confirm fail-safe direction (NC or NO) and auxiliary switch requirements against the applicable Australian Standard before ordering.