Kitchen Hood Duct Detector and Fuel Shutdown Sequence
A new 200-seat restaurant in a hotel has a 12 ft Type I hood over a charbroiler, a fryer, and two gas ranges. The UL 300 hood suppression system has a manual pull station at the kitchen exit. The exhaust duct runs 25 ft horizontally and 70 ft vertically to a rooftop fan. A duct smoke detector is installed 6 ft downstream of the filters and before the first elbow. It is connected to the building fire alarm system and also to a shunt trip that closes the gas valve and opens the 208 V electric range contactor. During the acceptance test, the duct detector alarms within 40 seconds when test smoke is introduced. Pulling the hood suppression manual station, however, does not shut off the gas or the electric range. The kitchen contractor says the shunt trip is controlled by the duct detector, so the gas will shut off 'when there is smoke,' and that the suppression system's fuel shutoff is outside the fire alarm scope.
1. Evaluate the duct detector location and the use of the duct detector to control fuel and power shutdown.
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The duct detector location is generally correct: it is downstream of the filters and upstream of the exhaust fan, where it can sense smoke in the grease duct and is accessible for testing. Its alarm should report to the fire alarm system.
The duct detector must not be the only means of shutting off gas and electric cooking power. NFPA 96 requires the hood suppression system, on activation, to shut off all fuel and electric power that produces heat. The duct detector may additionally initiate a shunt trip if that is the approved design, but it cannot substitute for the suppression system's shutdown function. The failure of the manual pull station is a critical life-safety deficiency.
2. Diagnose the likely causes of the manual pull failure and list the verification steps.
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First determine whether the manual pull actually discharged the suppression system. If it did not, the problem is mechanical at the pull station, cable, or cylinder. If it did discharge, then the fuel and power shutdown circuit is open. Verify the gas valve is a listed automatic shutoff valve with the correct solenoid, that the shunt-trip coil is powered from the correct source, that any relay or contactor between the suppression system and the valve is energized and not tripped, and that the electric range contactor opens. Check fuses, wiring, and terminal connections.
Test the gas valve and contactor separately, then retest the complete sequence from the manual pull. The deficiency must be corrected by the suppression system contractor, not by relying on the duct detector.
3. Prepare a sequence-of-operations matrix for the hood suppression and duct detector.
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The matrix should include at least these rows:
- Hood suppression activation, automatic or manual: shut off the gas valve and electric cooking power; activate the kitchen alarm; send an alarm signal to the fire alarm control unit; require manual reset of the suppression system and fuel valves.
- Duct detector alarm: send an alarm or supervisory signal to the fire alarm control unit; optionally shut down the exhaust fan or initiate other approved functions; do not use it as the sole fuel or power shutdown.
The matrix should list each initiating device, each output, the supervision status, and the reset procedure. It should be signed by both the suppression and fire alarm contractors.