A heat detector is a fire alarm device designed to respond when the convected

thermal energy The term "thermal energy" is used loosely in various contexts in physics and engineering. It can refer to several different well-defined physical concepts. These include the internal energy or enthalpy of a body of matter and radiation; heat, de ...

of a fire increases the temperature of a heat sensitive element. The

thermal mass In building design, thermal mass is a property of the mass of a building that enables it to store heat and provide inertia against temperature fluctuations. It is sometimes known as the thermal flywheel effect. The thermal mass of heavy structura ...

and conductivity of the element regulate the rate flow of heat into the element. All heat detectors have this

thermal lag Thermal lag describes a body's temperature with respect to time as a result of its thermal mass. A body with high thermal mass (high heat capacity and low conductivity) will have a large thermal lag. :Thermal \ lag (s) = α = Thermal diffusivity ...

. Heat detectors have two main classifications of operation, "rate-of-rise" and "fixed temperature". The heat detector is used to help in the reduction of property damage.

Fixed temperature heat detectors

This is the most common type of heat detector. Fixed temperature detectors operate when the heat sensitive

eutectic alloy A eutectic system or eutectic mixture ( ) is a homogeneous mixture that has a melting point lower than those of the constituents. The lowest possible melting point over all of the mixing ratios of the constituents is called the ''eutectic tempe ...

reaches the eutectic point changing state from a solid to a liquid. Thermal lag delays the accumulation of heat at the sensitive element so that a fixed-temperature device will reach its

operating temperature An operating temperature is the allowable temperature range of the local ambient environment at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the de ...

sometime after the surrounding air temperature exceeds that temperature. The most common fixed temperature point for electrically connected heat detectors is 58°C (136.4°F).

Rate-of-rise heat detectors

Rate-of-Rise (ROR) heat detectors operate on a rapid rise in element temperature of 6.7° to 8.3°C (12° to 15°F) increase per minute, irrespective of the starting temperature. This type of heat detector can operate at a lower temperature fire condition than would be possible if the threshold were fixed. It has two heat-sensitive

thermocouple A thermocouple, also known as a "thermoelectrical thermometer", is an electrical device consisting of two dissimilar electrical conductors forming an electrical junction. A thermocouple produces a temperature-dependent voltage as a result of th ...

s or

thermistor A thermistor is a type of resistor whose resistance is strongly dependent on temperature, more so than in standard resistors. The word thermistor is a portmanteau of ''thermal'' and ''resistor''. Thermistors are divided based on their conduction ...

s. One thermocouple monitors heat transferred by convection or radiation while the other responds to ambient temperature. The detector responds when the first sensing element's temperature increases relative to the other. Rate of rise detectors may not respond to low energy release rates of slowly developing fires. To detect slowly developing fires combination detectors add a fixed temperature element that will ultimately respond when the fixed temperature element reaches the design threshold.

Heat detector selection

Heat detectors commonly have a label on them that reads "Not a life safety device". That is because heat detectors are not meant to replace

smoke detector A smoke detector is a device that senses smoke, typically as an indicator of fire. Smoke detectors are usually housed in plastic enclosures, typically shaped like a disk about in diameter and thick, but shape and size vary. Smoke can be detecte ...

s in the bedrooms or in the hallway outside of the bedrooms. A heat detector will nonetheless notify of a fire in a kitchen or utility area, ''e.g.'', laundry room, garage, or attic, where smoke detectors should not be installed as dust or other particles would affect the smoke detector and cause false alarms. This will allow extra time to evacuate the building or to put out the fire, if possible. Mechanical heat detectors are independent fire warning stations that — unlike smoke detectors — can be installed in any area of a home. Portability, ease of installation, and excellent performance and reliability make this a good choice for residential fire protection when combined with the required smoke detectors. Because the detectors are not interconnected, heat activation identifies the location of the fire, facilitating evacuation from the home. Each type of heat detector has its advantages, and it cannot be said that one type of heat detector should always be used instead of another. If one were to place a rate-of-rise heat detector above a large, closed oven, then every time the door is opened a nuisance alarm could be generated due to the sudden heat transient. In this circumstance the fixed threshold detector would probably be best. If a room filled with highly combustible materials is protected with a fixed heat detector, then a fast-flaming fire could exceed the alarm threshold due to

. In that case the rate-of-rise heat detector may be preferred.

Fixed temperature heat detectors

Rate-of-rise heat detectors

Heat detector selection

See also