Types of Heat Detectors used in Fire Alarms


Fire Alarm Systems

Fire Alarm SystemsTypes of Heat Detectors used in Fire Alarms

Understand the differences between Smoke Detectors and Heat Detectors.

Heat Detectors are different from Smoke Detectors.  There are 2 basic types of heat detectors; Fixed Temperature and ROR (Rate-of-Rise) detectors.  They have different applications.  Because fire detection could be a matter of life-or-death, make sure you understand how they work, have them professionally installed, and test them frequently.  I have already covered smoke detectors in a previous post.  See Types of Smoke Detectors.

Heat Alarms respond to fire, not smoke. While Smoke detectors get all the attention, heat detectors are another useful component of any modern fire detection system. Some environments can trigger nuisance alarms in conventional smoke alarms due to shifts in temperature and humidity as well as dust, fumes and insects. Heat alarms are intended for use in locations where standard smoke alarms are not suitable because they are virtually unaffected by these adverse conditions. Heat alarms are useful in Kitchens, Unfinished Attics, Basements, Garages, Boiler Rooms and other areas where high levels of dust and fumes are present.

DO NOT install heat alarms in areas with high humidity, like bathrooms or areas near dishwashers or washing machines, air returns, heating and cooling supply vents, fans, decorative objects, window molding etc. that may prevent heat from entering the unit thus interrupting its alarm. Also do not install in rooms where temperatures may fall below -10 degrees F or rise above 100 degrees F or near fluorescent lights – electrical noise and flickering may affect the alarm’s operations.

Fixed temperature heat detectors

This is the most common type of heat detector. Fixed temperature detectors operate when the heat sensitive element 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 sometime after the surrounding air temperature exceeds that temperature.

Heat Detectors and Fire AlarmsRate-of-rise heat detectors

Rate-of-Rise (ROR) heat detectors operate on a rapid rise in element temperature of 12° to 15°F (67° to 83°C) 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 thermocouples/ thermistor. One thermocouple monitors heat transferred by convection or radiation. The other responds to ambient temperature. Detector responds when first’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 says “Not a life safety device”. That is because heat detectors are not meant to replace smoke detectors 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 (i.e., laundry room, garage, or attic), where smoke detectors should not be installed. 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 you 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 thermal lag. In that case the rate-of-rise heat detector may be preferred.

Meeting Fire and Electrical Code

Code dictating the types, quantity, placement, and testing of fire alarm detectors vary greatly.  There are Federal requirements as well as local requirements.  The local codes for Honolulu, Oahu, Hawaii are probably different from other locales.  If you are unfamiliar with requirements for fire alarms, consult a professional.  Call Aloha Alarm at 808-488-4111 and we would be glad to help design and install the proper equipment to help protect your business, home, and family.

Mark Plischke
Aloha Alarm LLC
Business & Home Security and Fire Alarm Systems

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