Answer:
The FST-851 has a fixed 135°F (57°C) setpoint for ambient temperatures up to 100°F (38°C); the FST-851R adds a 15°F (8.3°C)/min rate-of-rise circuit to the 135°F threshold; and the FST-851H is a high-temperature detector with a 190°F (88°C) fixed setpoint for spaces up to 150°F (66°C).
The Honeywell Notifier 851 Series thermal detector platform consists of three specific models tailored to distinct ambient environments and fire response profiles:
- FST-851 (Fixed Temperature): Features an electronic thermistor calibrated to a factory fixed-temperature trip point of 135°F (57°C). It is rated for operating ambient conditions from –4°F to 100°F (–20°C to 38°C). Best suited for areas where stable ambient temperatures are maintained and smoke detectors would be subject to false tripping.
- FST-851R (Rate-of-Rise with Fixed Setpoint): Combines a 15°F (8.3°C) per minute rate-of-rise thermal detection circuit with a 135°F (57°C) fixed-temperature backup trip point. Operating ambient rating is –4°F to 100°F (–20°C to 38°C). Engineered for spaces where rapid temperature surges indicate fast-flaming fires prior to reaching the fixed temperature setpoint.
- FST-851H (High-Temperature Fixed): Features a high-temperature fixed-point thermistor set at 190°F (88°C) and an expanded operating temperature range of –4°F to 150°F (–20°C to 66°C). Designed specifically for unconditioned spaces, boiler rooms, commercial kitchens, mechanical utility plants, or drying rooms where ambient temperatures regularly exceed 100°F (38°C).
