HEAT DETECTORS

Heat detectors respond to the thermal energy signature from a fire and are generally located on or near the ceiling. They respond when the detecting element reaches either a predetermined fixed temperature or a specified rate of temperature rise occurs. Knowing the difference between types of detectors is very important.

Heat detectors warn of fire when the temperature in the area around the smoke detector reaches a certain level. The static response temperature of a heat detector should be a minimum of 29°C above the maximum ambient temperature likely to be experienced for long periods of time and 4°C above the maximum temperature likely to be experienced for short periods of time.

Periodic tests must be made of all detectors. Applying a safe source of heat can test restorable detectors, while non-restorable detectors must be tested mechanically or electrically. It is important to know which types of heat detectors are installed so that tests can be made on all restorable heat detectors, but not on the fusible elements of non-restorable detectors.

Heat detectors are highly reliable and have good resistance to operation from non-hostile sources. They are also very easy and inexpensive to maintain. On the down side, heat detectors do not notice smoke. They do not function until room temperatures have reached a substantial temperature, at which point the fire is well underway and damage is growing exponentially. Subsequently, thermal detectors are usually not permitted
in life safety applications. They are also not recommended in locations where there is a desire to identify a fire before substantial flames occur, such as spaces where high value thermal sensitive contents are housed. However, a heat detector could be valuable additional protection in areas such as kitchens and attics, where smoke detectors are not recommended. They are not recommended for the use in bedrooms or sleeping areas. There are several types of heat detectors including:

Heat detectors respond to the thermal energy signature from a fire and   are generally located on or near the ceiling. They respond when the detecting element reaches either a predetermined fixed temperature or a  specified rate of temperature rise occurs.

Fixed-temperature heat detectors:

These detectors initiate an alarm when the detecting element reaches a predetermined fixed temperature. Because of inherent thermal lag, when the detector actually operates, the temperature of the air surrounding the detector has always extended considerably higher then the set point of the detector.

One form of a spot-type fixed temperature detector uses a fusible element made from a eutectic metal alloy that melts rapidly at a predetermined temperature (commonly 135°F). Automatic sprinklers, fire dampers and door fusible links commonly use a similar
material. The operation of the detector destroys either the entire unit (or at least the operating element) which the person who maintains the system must replace. Another form of spot-type fixed-temperature heat detector uses a bimetallic element. After operating, the bimetallic type automatically restores when the temperature falls to a point below the set point of the detector

Fixed temperature heat detectors operate when the sensing mechanism reaches its specific temperature threshold. Usually there is a fusible metal element which melts and causes a short on the initiating circuit.

The most common units are fixed temperature devices that operate when the room reaches a predetermined temperature (usually in the 135°-165°F/57°-74°C). Normally fixed temperature detectors employ a fusible alloy element which must be replaced after the detector has operated. Different temperature rated elements are available to take account of varying ambient air temperatures. A typical set temperature might be 57.2ºCentigrade. These detectors are non-restoring type (because it is destroyed when activated) and have to be replaced, if another setting is required.

When a fixed temperature device operates, the temperature of the surrounding air will always be higher than the operating temperature of the device itself. This difference between the operating temperature of the device and the actual air temperature is commonly spoken of as thermal lag, and is proportional to the rate at which the temperature is rising.

  • •Activate by one or more of three mechanisms
    • •Fusible device •
    • Frangible bulb •
    • Continuous line detector
Rate-of-rise (ROR) heat detectors

The second most common type of thermal sensor is the rate-of-rise detector, which identifies an abnormally fast temperature climb over a short time period. Rate of rise detectors also have a fixed temperature backstop to ensure that even very slow increases in temperature will eventually raise an alarm, if the increase continues for a sufficiently long period. Rate of rise detectors are not usually used for suppression systems because they operate on a 12 to 15ºF temperature rise per minute. This makes them too sensitive to sudden environmental changes causing false alarms and unexpected discharges.

The rate of rise type is the most sensitive type of heat detector, particularly when used in areas where the ambient temperature can reach low levels and therefore create a large difference between the ambient temperature and the trigger temperature of a fixed temperature detector. In order to avoid false alarms rate of rise detectors should not be used in areas subject to frequent temperature swings, such as in kitchens, boiler rooms and warehouses with large doors to open air. In most of these detectors, when the rate of rise element alone has been activated, the detector is self-restoring.

Both rate of rise and fixed temperature heat detectors are “spot type” detectors, which mean that they are periodically spaced along a ceiling or high on a wall and are suitable for inclusion in open, closed or line monitored systems.

A rate-of-rise detector will operate when the rate of temperature increases from a fire exceeds a predetermined level, typically around 5°F in twenty seconds or 15°F per minute. Small, normal changes in ambient temperature that can be expected under nonfire conditions will not operate the detector. These detectors are available as both linetype or spot-type detectors, and are restorable. Linear

  • •Operate on assumption that temperature in room will increase faster from fire than from normal atmospheric heating
  • •Designed to initiate signal when rise in temperature exceeds 12° to 15°F (-11°C to -9°C) in one minute
  • •Can be initiated at room temperature far below that required for initiating fixed-temperature device
  • •Reliable, not subject to false activations
  • Pneumatic rate-of-rise spot detector
Considerations in Selecting Heat Detectors

Each type of heat detector has its advantages, and one cannot say that one type of heat detector should always be used instead of another. If you were to place a rate-of-rise (ROR) heat detector above a large, closed oven, then every time the door is opened a false alarm could be generated due to the sudden heat transient. In this circumstance the fixed threshold detector would probably be best. If a room is protected with a fixed heat detector filled with highly combustible materials, then a fast flaming fire could exceed the alarm threshold due to thermal lag. In this case the ROR heat detector may be preferred.

A general comparison of smoke v/s heat detectors is as follows:
  • A smoke detector transmits a signal to the control unit when the concentration of airborne combustion products reaches a predetermined level. A heat detector transmits a similar signal when the temperature reaches a predetermined level or when there is an abnormal rate of temperature rise.
  • The key advantage of smoke detectors is their ability to identify a fire while it is still in its incipient. As such, they provide added opportunity for emergency personnel to respond and control the developing fire before severe damage occurs. Smoke detectors give the earliest warning of fire, typically responding to a fire 1/10th of the size as that required to operate a heat detector.
  • Heat detectors are not prone to false alarms although it is rather insensitive to smoldering fires of low temperature. Heat detectors are therefore preferred for the environments where the ambient conditions might cause false alarms.
  • Heat detectors must be mounted closer together than smoke detectors, so whilst the mounting bases are compatible for all types, care should be taken to ensure that the spacing between detectors is appropriate for the detector type fitted. With analogue systems it is possible for the photo thermal detector to act as a thermally enhanced smoke detector during certain times and as a pure heat detector at other times. If this mode of operation is envisaged then spacings must be those appropriate for heat detectors.
Linear Heat Detector:

For some applications, the use of a linear heat detector is an option to consider. These may be installed in head to reach areas,or areas that are subject to high heat. The detector is contained within a cable which when exposed to heat that is greater than its rating, will short circuit, causing an alarm