The Belimo FSTF120-S US is a fail-safe fire & smoke damper actuator delivering 18 in-lb of torque on 120VAC with on/off control. It drives UL555/UL555S listed fire and smoke dampers and uses a thermally-triggered, spring-return fail-safe that drives the damper to its safe position on power loss or at its thermal limit (350°F [177°C] for 30 minutes). Two built-in SPST auxiliary switches provide position indication.
Specifications per the Belimo FSTF120-S US product datasheet.
Overview
Belimo FS-Series actuators are UL555S-listed fail-safe drives for life-safety fire and smoke dampers in HVAC systems. The FSTF120-S US produces 18 in-lb of torque, mounts directly to the damper shaft, and on loss of power — or when its thermal sensor reaches the rated limit — springs the damper to its fail-safe position within <15 s. A UL Type 2 housing and on/off control make it a direct, code-compliant replacement actuator.
Best for
- Life-safety fire and smoke damper control in commercial HVAC (18 in-lb damper torque)
- UL555 / UL555S listed assemblies requiring a thermally-triggered fail-safe actuator
- 120VAC systems using on/off control
- Direct, code-compliant replacement of an existing fire/smoke damper actuator
Key Features
Thermal fail-safe
Spring-return mechanism drives the damper to its safe position on power loss, and a thermal sensor trips the fail-safe at the rated temperature limit (350°F [177°C] for 30 min).
18 in-lb torque
Sized for fire and smoke dampers requiring up to 18 in-lb, with a 15 s motor running time and <15 s fail-safe time.
UL555 / UL555S listed
Listed for use on UL-classified fire dampers (UL555) and smoke / combination fire-smoke dampers (UL555S).
On/Off control on 120VAC
On/Off control signal with a 120VAC supply and a UL Type 2 housing for standard duct and plenum mounting.
Built-in aux switches
Two SPST auxiliary switches (3 A resistive @ 250 V) for damper position indication and interlock.
Technical Specifications
| Parameter | Specification |
|---|---|
| Manufacturer | Belimo |
| Part Number | FSTF120-S US |
| Base Model | FSTF120-S US |
| Actuator Type | Fail-Safe Fire & Smoke Damper Actuator |
| Torque | 18 in-lb |
| Control Signal | On/Off |
| Power Supply | 120VAC |
| Fail-Safe | Spring return (mechanical) |
| Thermal Rating | 350°F [177°C] for 30 min |
| Motor Running Time | 15 s |
| Fail-Safe Time | <15 s |
| Auxiliary Switch | 2x SPST (3 A resistive @ 250 V) |
| Conduit Connector | Standard |
| Housing | UL Type 2 |
| Listings | UL555, UL555S |
| Warranty | 5-Year Belimo Warranty |
| Approx. Weight | 1.2 lb |
Documentation
Belimo FSTF120-S US DatasheetRelated Products
Frequently Asked Questions
Is this a fail-safe actuator?
Yes. It uses a mechanical spring return that drives the damper to its safe position on loss of power, and a thermal sensor that trips the fail-safe at the rated temperature limit.
What dampers is it used on?
UL555-classified fire dampers and UL555S-classified smoke and combination fire/smoke dampers in commercial HVAC systems.
What torque does it provide?
18 in-lb, suitable for fire/smoke dampers rated up to that torque.
What does the thermal rating mean?
The actuator holds its rating at 350°F [177°C] for 30 minutes, then trips to fail-safe — meeting life-safety damper requirements.
What do the -S auxiliary switches do?
The -S models add two SPST switches (3 A @ 250 V) for position feedback or interlock wiring.
- Actuator Type:
- Fire & Smoke Actuator
- Torque:
- 18 in-lb
- Voltage:
- 120VAC
- Control Signal:
- On/Off
- Aux Switch:
- 2x SPST
- Connector:
- Standard
- Housing:
- UL Type 2
- Pack Size:
- Single (1 pc)
