Heat detectors may be divided into two categories:
- Fixed temperature devices, which behave rather like thermostats;
- Fixed temperature/rate of rise devices, that will interact with either a quickly increasing temperature or at a predetermined fixed temperature.
Early point heat detectors were typically electromechanical in design.
(E.g. Comprising bi-metallic strips), but modern devices are normally either pneumatic or, more commonly, electronic (e.g. thermistor based) in nature.
Heat detectors are not suitable for Escape Routes
Heat detectors can be utilized for common building protection, however, will usually be considerably slower to activate than smoke detectors. Flames may be as much as one-third of the way towards the ceiling before a heat detector will activate.
As the ceiling height increases, the fire size at the point of detection increases dramatically. If the ceiling height is doubled, for example, the size of the fire at the point of detection is likely to increase by a factor of five to six.
The reaction of heat detectors is simply too slow to be of benefit in escape routes or in areas such as electronic equipment rooms, where a modest fire could result in a substantial damage.
Heat detectors have a place in the design of fire alarms.
Heat detectors are required, however, in locations in which dust, fumes, etc., may prevent the usage of smoke detectors (e.g. kitchens). They may also be used in areas in which a fire is likely to produce a high temperatureoutput rather than smoke (e.g. certain flammable liquids risks). Heat detectors can also be well suited for installation in rooms encapsulated in fire-resisting construction, if the fire protection objective is merely to give a warning ahead of the integrity of the construction is threatened.
Normally, any heat detectors used are of the fixed-temperature/rate of rise point type. However, fixed-temperature detectors ought to be utilized in sudden rises in normal temperature range may occur (e.g. near ovens or in laundry rooms). Line heat detectors are usually used for special applications, in which the geometry of the protected space is especially ideal for their use (e.g. in cable tunnels and under escalators.
Line-type heat detectors could be designed on pneumatic or, typically, electrical principles. Non-integrating line heat detectors usually comprise a length of current-carrying cable, in which the insulation melts within a specified temperature range, resulting in a short circuit. In integrating line heat detecting cables, the capacitance and/or resistance of the insulation changes with temperature.