The Belimo NFB24-S is a robust spring-return (fail-safe) damper actuator delivering 90 in-lb [10 Nm] of torque with two-position on/off control on AC/DC 24 V power. It direct-couples to round or square damper shafts up to 1.05 in. with a universal clamp and is a member of the NFB platform for commercial HVAC dampers that must fail to a safe position. On power loss the internal spring drives the damper to its fail-safe position in under 20 seconds. Two built-in SPDT auxiliary switches signal damper position or interlock equipment.
Specifications sourced from the official Belimo NFB24-S technical data sheet (rev 2025-12-23).
Overview
Belimo EF-Series actuators are spring-return drives for commercial HVAC control dampers requiring fail-safe operation. The NFB24-S produces 90 in-lb [10 Nm] and mounts directly to round or square damper shafts up to 1.05 in. with its universal clamp (a crank arm and brackets are available for jackshaft and linkage applications). The fail-safe direction is reversible by CW/CCW mounting, and a 5 mm hex crank allows manual positioning. The spring returns the damper to its fail-safe position in under 20 seconds when power is removed. Two-position on/off control drives the damper open and the spring returns it on signal or power loss.
Best for
- Commercial HVAC dampers needing fail-safe (spring-return) operation
- Medium-torque damper and louver control (90 in-lb)
- Outdoor-air, mixing-box and economizer dampers that must fail safe
- End-position signaling or equipment interlock (2x SPDT)
Key Features
Spring-Return Fail-Safe
A mechanical spring drives the damper to a safe position on power or signal loss (under 20 seconds), with no external power required to fail safe.
90 in-lb Torque
Sized for mid-size commercial dampers; direct-couples to round or square damper shafts up to 1.05 in. with a universal clamp.
On/Off Control
Accepts two-position on/off control from a controller, relay or switch.
Reversible Fail-Safe Direction
The spring-return (fail-safe) direction is selectable clockwise or counterclockwise simply by mounting orientation.
Manual Override Crank
A supplied 5 mm hex crank (3/16" Allen) lets you position the damper by hand for setup or service.
Two SPDT Aux Switches
Two built-in SPDT auxiliary switches (1 mA-3 A) signal damper end position or interlock other equipment.
Technical Specifications
| Parameter | Specification |
|---|---|
| Manufacturer | Belimo |
| Part Number | NFB24-S |
| Actuator Type | Spring-Return (Fail-Safe) Damper Actuator |
| Nominal Voltage | AC/DC 24 V |
| Nominal Frequency | 50/60 Hz |
| Power Consumption (Running) | 6 W |
| Torque (Motor & Spring) | 90 in-lb [10 Nm] (minimum, full rotation) |
| Control Signal | On/Off (two-position, 1 or 3-wire) |
| Shaft Range | Up to 1.05 in. [26.7 mm] via universal clamp; crank arm available |
| Angle of Rotation | 95° (adjustable mechanical stops) |
| Running Time (Motor) | 75 s / 90° |
| Spring Return Time | < 20 s (< 60 s below -22°F) for fail-safe |
| Behavior on Power Loss | Spring returns damper to fail-safe position |
| Fail-Safe Direction | Reversible (CW/CCW) by mounting orientation |
| Auxiliary Switch | 2x SPDT, 1 mA-3 A (0.5 A inductive), DC 5 V-AC 250 V |
| Manual Override | 5 mm hex crank (3/16" Allen), supplied |
| Electrical Connection | 18 AWG plenum cable, 3 ft [1 m], 1/2" NPT conduit connector |
| Overload Protection | Electronic, full 0-95° rotation |
| Electrical Protection | Double insulated (Class 2) |
| Enclosure Rating | NEMA 2 / UL Type 2 |
| Ambient Temperature | -22 to 122°F [-30 to 50°C] |
| Agency Listing | cULus (UL60730-1A/-2-14, CAN/CSA) |
| Quality Standard | ISO 9001 |
| Plenum Rated | UL 2043 (suitable for air plenums) |
| Weight | 5.4 lb [2.4 kg] |
| Warranty | 5 Years |
Documentation
Related Products
Accessories & Mounting
Common Belimo accessories for mounting, linkage, signaling and powering the NFB24-S, drawn from the actuator datasheet. All are stocked by ZOT Supply as an authorized Belimo distributor.
Mounting & Linkage
Electrical Add-Ons
Frequently Asked Questions
What is the Belimo NFB24-S?
The Belimo NFB24-S is a robust spring-return (fail-safe) damper actuator for commercial HVAC dampers. It produces 90 in-lb [10 Nm] of torque, uses two-position on/off control on AC/DC 24 V power, and on power loss its spring drives the damper to a fail-safe position. Its housing is rated NEMA 2.
Is the NFB24-S a spring-return (fail-safe) actuator?
Yes. The NFB24-S is spring-return: a mechanical spring drives the damper to its fail-safe position when power or the control signal is lost — no external power is needed to fail safe.
What happens to the NFB24-S on power loss?
Its internal spring returns the damper to the fail-safe position in < 20 s (< 60 s below -22°F). The fail-safe direction (CW or CCW) is set by the mounting orientation.
How much torque does the NFB24-S provide?
It delivers 90 in-lb [10 Nm] of minimum running and spring-return torque, sizing it for mid-size commercial dampers.
What shaft sizes does the NFB24-S fit?
It direct-couples to round or square damper shafts up to 1.05 in. using its universal clamp. For jackshafts or off-shaft mounting, a crank arm (KH-AFB) and brackets (ZG-100, ZG-118) are available.
What control signal does the NFB24-S use?
It uses two-position on/off control. On/Off control is driven from a controller, relay or switch.
Does the NFB24-S have auxiliary switches?
Yes. It has two built-in SPDT auxiliary switches (rated 1 mA to 3 A) for end-position signaling or interlocking other equipment.
How do you manually position the NFB24-S?
Use the supplied 5 mm hex crank (3/16" Allen) to wind the actuator by hand for setup or service.
Is the Belimo NFB24-S available from an authorized Belimo distributor?
Yes. ZOT Supply is an authorized Belimo distributor and carries the NFB24-S. Every unit ships factory new with Belimo's full 5-year warranty and application support.
- Actuator Type:
- Fail-Safe
- Torque:
- 90 in-lb
- Control Signal:
- On/Off
- Voltage:
- 24 VAC/DC
- Auxiliary Switch:
- 2x SPDT
- Enclosure:
- NEMA 2
- Series:
- NF
