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FIRE ALARMHVAC INTERLOCKNFPA 90A

Fan Shutdown Relay
HVAC Control via the Fire Alarm

Duct smoke detectors detect smoke entering the air-handling system, and a fire alarm relay tells the fan to stop. NFPA 90A + §6.4 + the mechanical code set the rules. Here is how the relay is wired, why it interrupts control voltage instead of main power, and how BMS integration can undo the safety if coordination is not explicit.

By Samektra · 10 min read · Last updated April 2026

Why HVAC Shutdown Matters

A running air-handling unit will distribute smoke from a developing fire throughout the building in minutes — sometimes faster than the fire spreads by conduction, radiation, or convection through structure. The fan shutdown interlock cuts off that distribution path the instant the fire alarm system detects smoke in the duct. NFPA 90A §6.4 and the International Mechanical Code §606 both require this interlock for any HVAC system moving more than 2,000 CFM.

The mechanical device that makes it happen is the fan shutdown relay — a listed fire alarm output that interrupts the motor control signal when commanded. It is not glamorous and it is not complex, but its correct installation, supervision, and periodic testing are non-negotiable.

How the Relay Is Wired

The relay sits in the control circuit of the HVAC fan motor — not in the line-voltage circuit. Three common wiring topologies:

  • Starter hold-in circuit. The relay is wired in series with the 120 VAC hold-in coil of a magnetic motor starter. De-energizing the relay drops out the starter, which opens the motor contactor. Simple, robust, still common on standalone fans.
  • VFD safe-torque-off or run-command input. Modern VFDs have a hard-wired safety input (Safe Torque Off, STO) or a run-command digital input. The relay interrupts that signal; the VFD coasts the motor to a stop without regeneration.
  • BMS relay with hard-wired override. The BMS normally commands the fan. A parallel relay from the fire alarm panel opens a separate input on the motor starter that takes priority over the BMS command — so the BMS cannot override or reset the shutdown.

The relay is typically a listed addressable relay module on the SLC (e.g. Notifier FRM-1, System Sensor MR-100/MR-200) or a fixed-address output from a conventional FACP. Listing is via UL 864 as a fire alarm accessory; the output contacts are commonly rated 1 A at 30 VDC or 24 VAC.

Fail-Safe Direction

The relay should be wired fail-safe in the shutdown direction: loss of power to the relay should shut the fan down, not keep it running. Practically this means the relay's normally-closed (NC) contact carries the run-command signal, and any break in the command path (intentional by alarm, or unintentional by wiring fault) drops the fan.

This arrangement is slightly inconvenient during routine maintenance — a technician has to jumper out the relay to keep the fan running while troubleshooting — but it ensures that a wiring failure always fails to the safe side.

Supervision Requirements

NFPA 72 requires the interconnection between the fire alarm panel and the HVAC control circuit to be supervised such that an open or short in the wiring annunciates at the FACP as a trouble signal. On an addressable system, the relay module itself reports its status through the SLC; on a conventional system, a supervised Style 4 or Style 7 circuit with appropriate end-of-line resistor serves the same purpose.

ITM — The Piece Most Commonly Skipped

NFPA 72 Table 14.4.5 requires an annual functional test of every relay output. Unlike most FA devices, fan shutdown testing is frequently cut short because:

  • Testing requires actually running the HVAC fan, then activating an alarm to verify shutdown — this disturbs building operations.
  • Facility teams often prefer to skip the live fan test and verify only the relay contact operation at the panel.

That shortcut is not a complete test. The code requires verification that the fan actually stops when the relay operates — not just that the contacts move. On BMS-integrated systems this is especially important because a BMS programmed to auto-restart or override the FA command can silently defeat the interlock. Schedule the test during off-hours, coordinate with operations, and verify the fan stops and stays stopped until reset.

Common Field Issues

  • BMS auto-restart. BMS re-commands the fan within seconds of FA shutdown. Solution: hard-wired interlock upstream of the BMS command path.
  • VFD auto-restart after fault clear. VFD is programmed to auto-restart after a fault or momentary power loss. Solution: configure the VFD to require manual reset or FA-alarm-cleared handshake before restart.
  • Duct detector in wrong location. Detector installed in a dead-air section of ductwork, or downstream of the fan so it never sees smoke that is being pulled into the system. Solution: verify installation per NFPA 72 §17.7.4 — supply-air detector upstream of any filter, return-air detector in a location representative of the full return.
  • Unsupervised interconnect. A contractor ran the relay output on unsupervised low-voltage wire. A broken wire goes undetected for months. Solution: supervised Style circuit with EOL resistor.

Frequently Asked Questions

When is a fan shutdown relay required?
NFPA 90A §6.4 requires automatic air-handler shutdown on activation of a duct smoke detector in the supply or return air stream of any HVAC system moving more than 2,000 CFM. The shutdown is accomplished by a fire alarm relay that interrupts the fan motor control circuit.
Does the relay cut the motor power directly?
No — in almost all cases the relay interrupts a low-voltage control signal to the motor starter or VFD, not the main line power. Cutting 480 V motor power with a fire alarm relay would require a contactor far larger than what the FA system supplies. The motor starter or VFD sees the loss of command and de-energizes the fan.
What about smoke control or stair pressurization fans?
Those are different. Smoke-control fans are typically commanded to run, not to shut down, during a fire event. Dedicated smoke-control panels (per NFPA 92) handle that logic, but they communicate with the FACP via relay outputs in the same general architecture as shutdown relays.
Does the relay need its own backup power?
The relay derives its power from the fire alarm system, which satisfies NFPA 72 §10.6 secondary power. The HVAC motor does not need backup power for shutdown — on utility power loss the motor stops on its own, which also accomplishes the intent.

References

1. NFPA 72 (2022), §23.8 — Supervisory signal-initiating devices and relays.

2. NFPA 90A (2024), §6.4 — Detection, Alarm, and Control.

3. NFPA 92 (2021) — Smoke Control Systems.

4. International Mechanical Code §606 — Smoke Detection Systems Control.

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Discussion (2)

You
CI
Controls Integrator

The biggest gotcha is the interlock with the BMS. In large buildings the AHU sequence is managed by the BMS, and a fire alarm shutdown command has to override the BMS. If the BMS is programmed to auto-restart on a "loss of command" signal, the fan can bounce back on a few seconds after shutdown. The right answer is a hard-wired interlock that the BMS cannot override.

▲ 0Reply
HE
Hospital Engineer

OR suites are tricky because the pressure relationships are life-critical. Shutting down the supply fan on a duct detector alarm can drop a positive-pressure OR to neutral within 60 seconds, and that is a real infection-control problem. Coordinate with the clinical engineering team on post-alarm HVAC sequencing — it is not always as simple as "fans off."

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