A heat detector (also called a heat sensor) is a fire detector that responds to temperature instead of smoke. There are three main types: fixed temperature, which alarms when the air at the detector reaches a set rating such as 135°F or 194°F; rate-of-rise, which alarms when the temperature climbs quickly, about 15°F per minute; and rate-compensated, which alarms at its rated temperature without the lag of a fixed unit. Linear heat detection cable covers long runs instead of a single spot. Heat detectors are best used where smoke detectors would cause nuisance alarms, such as kitchens, garages, mechanical rooms and dusty or humid spaces.
How a heat detector works
Every heat detector has a thermal element and a way to signal the panel. Conventional (mechanical) heat detectors close a contact on an initiating device circuit when they operate. Addressable heat sensors measure temperature electronically and report to the panel over the signaling line circuit. Either way, a heat detector only responds once the fire has produced enough heat at the ceiling, which is why heat detection is slower than smoke detection for most fires.
Types of heat detectors
Fixed temperature heat detectors
A fixed temperature detector alarms when its element reaches the rated temperature. Mechanical types use a fusible alloy or a bimetal element. Units with a fusible element, such as the fixed element in the System Sensor 5600 series, are non-restorable and must be replaced after they operate. Because the element has to heat up, the air around it is usually hotter than the rating by the time it alarms; this is called thermal lag.
Examples: the System Sensor 5604 194°F fixed temperature detector for hot spaces, or addressable fixed sensors such as the Notifier FST-951H.
Rate-of-rise heat detectors
A rate-of-rise (ROR) detector alarms on a fast temperature increase, regardless of the actual temperature. System Sensor's 5600 series uses a threshold of about 15°F (8.3°C) per minute, and its rate-of-rise element is self-restoring. Most ROR detectors are combination units that also have a fixed temperature element, so a slow-building fire still causes an alarm at the rating.
ROR responds faster than fixed temperature to a fast flaming fire, but it can false alarm where temperature normally jumps quickly: next to ovens and dishwashers, at loading dock doors that open to winter air, or near unit heaters. Use fixed temperature in those spots. Examples: the System Sensor 5601P 135°F/ROR and the addressable Fire-Lite H365R.
Rate-compensated heat detectors
A rate-compensated (rate-anticipation) detector uses an element designed to alarm when the surrounding air reaches the rated temperature, whether the temperature rises slowly or quickly. It avoids most of the thermal lag of a fixed unit without the nuisance risk of ROR. These are common in harsh and outdoor locations because many are sealed and rated for wet or hazardous areas. Example: the Thermotech 302-AW-135 all-weather rate-compensated detector.
Linear heat detection (Protectowire)
Linear heat detector cable detects heat anywhere along its length instead of at one point. Protectowire's PHSC cable has two spring steel conductors, each coated in a heat-sensitive polymer and twisted together. At the rated temperature the polymer softens and the conductors short, which the panel sees as an alarm on a circuit that treats a short as an alarm. The section that operated is non-restorable and must be cut out and spliced.
Linear heat is used where spot detectors are hard to place or maintain: cable trays, conveyors, cold storage and freezers, parking structures, tunnels, and outdoor equipment. Protectowire offers ratings from 135°F to 356°F and higher. Its CTI series adds temperature confirmation through a CTM interface module, so the panel can tell a heat alarm from a mechanical short, and can report the alarm location when calibrated. See the PHSC-190-EPC cable, the CTI-155-EPC, and all Protectowire linear heat detection parts.
Addressable heat sensors
Addressable heat sensors report temperature to the panel as data. Depending on the model, the panel can apply a fixed threshold, a rate-of-rise threshold, or both, and the sensor shows its exact location in the alarm message. Examples include the Simplex 4098-9733 TrueAlarm heat sensor, Notifier FST-951 series and Fire-Lite H365 series. Each one only works with its own brand's panel protocol.
Heat detector comparison
| Type | Alarms on | Restorable | Best for | Avoid |
|---|---|---|---|---|
| Fixed temperature | Reaching the rating (e.g. 135°F, 194°F) | Fusible types: no. Bimetal and electronic: yes | Kitchens, boiler rooms, attics, spaces with normal temperature swings | Where fastest heat response is needed |
| Rate-of-rise (with fixed) | About 15°F/min rise, or the fixed rating | ROR element: yes | Garages, storage rooms, unheated areas with stable temperature | Near ovens, dock doors, heaters |
| Rate-compensated | Air reaching the rating, with little lag | Yes | Outdoor, wet, corrosive and hazardous locations | Few limits; check listing |
| Linear heat cable | Rating reached at any point along the cable | Operated section: no (PHSC) | Cable trays, conveyors, freezers, tunnels | Short runs a spot detector covers |
| Addressable heat sensor | Panel-set fixed and/or ROR threshold | Yes | Any heat application on an addressable system | Mixing with another brand's panel |
Heat detector temperature ratings
NFPA 72 groups heat detectors into temperature classes and requires the rating to be at least 20°F above the maximum expected ceiling temperature. Spot fixed temperature and rate-compensated detectors carry a color code. From NFPA 72 (2016) Table 17.6.2.1:
| Class | Rating range | Max ceiling temp | Color |
|---|---|---|---|
| Ordinary | 135–174°F | 115°F | Uncolored |
| Intermediate | 175–249°F | 155°F | White |
| High | 250–324°F | 230°F | Blue |
| Extra high | 325–399°F | 305°F | Red |
In practice, 135°F is the standard rating for normal rooms. A 190°F to 200°F class detector is used where the ceiling can reach 115°F to 155°F, such as boiler rooms, attics under a roof deck, commercial kitchens and some process areas. Keep the rating as close to the ceiling temperature as the 20°F rule allows, because a higher rating responds later. Check the edition of NFPA 72 your jurisdiction has adopted for the full table.
Heat detector vs smoke detector: where heat detectors are best used
Smoke detectors respond much earlier to most fires, so they are the choice for life safety in occupied spaces and escape routes. Manufacturers state that heat detectors are intended for property protection and are not a substitute for smoke detection where life safety is the goal. Heat detectors are best used where a smoke detector would give nuisance alarms or would not survive:
- Kitchens and break rooms with cooking appliances, toasters and steam.
- Garages and loading areas with vehicle exhaust.
- Dusty or dirty spaces such as woodshops, warehouses and manufacturing.
- Humid areas like laundry rooms, locker rooms and pool equipment rooms.
- Unheated or hot spaces, including attics and mechanical rooms, outside a smoke detector's listed temperature range.
- Elevator hoistways and machine rooms where heat detectors near sprinklers start elevator power shutdown (shunt trip). Follow the NFPA 72 and ASME A17.1 requirements adopted in your area.
For a full comparison of detection methods, browse our smoke detectors and multi-criteria detectors, which combine smoke and heat sensing in one head.
Spacing basics
Each spot heat detector has a listed spacing from its UL 521 test, such as 50 ft by 50 ft for the System Sensor 5600 series. NFPA 72 reduces that spacing for high ceilings, beams, joists and sloped roofs. Use the spacing printed on the detector's installation sheet and apply the NFPA 72 reductions before you lay out the room.
FAQ
What is the most common heat detector rating?
135°F, in the ordinary class. Use 190°F or 194°F class detectors in spaces where the ceiling can exceed 115°F.
Is a heat sensor the same as a heat detector?
Yes. "Heat sensor" usually refers to an addressable or analog device that reports temperature to the panel, but the terms are used interchangeably.
Can a heat detector replace a smoke detector?
Only where the code or design allows heat detection for that space. Heat detectors respond later and are not a life safety substitute for required smoke detection.
Do heat detectors need to be replaced after an alarm?
Fusible fixed temperature detectors and operated sections of PHSC linear heat cable do. Rate-of-rise elements, rate-compensated detectors and electronic heat sensors reset.
Where should I not use a rate-of-rise heat detector?
Where temperature changes quickly in normal use, such as next to ovens, dishwashers, unit heaters or loading dock doors. Use fixed temperature there.
Can heat detectors be tested with a heat gun?
Restorable detectors are functionally tested with a heat source per NFPA 72 and the manufacturer's instructions. Non-restorable fixed elements are not heat tested; they are tested mechanically and electrically.
Shop all heat detectors by brand, rating and type.