Heat detector

A heat detector is a device that detects heat and can be either electrical or mechanical in operation. The most common types are the thermocouple and the electro-pneumatic, both respond to changes in ambient temperature. Typically, if the ambient temperature rises above a predetermined threshold, then an alarm signal is triggered.

Heat detectors can also be further broken down into two main classifications, "rate-of-rise" and "fixed" or "rate compensated."

Rate-of-rise heat detectors

Rate-of-rise (ROR) heat detectors react to the sudden change or rise in ambient temperature from a normal baseline condition. Any sudden temperature increase that matches the predetermined alarm criteria will cause an alarm. This type of heat detector can react to a lower threshold condition than would be possible if the threshold were fixed. A typical alarm may sound when the rate of temperature rise exceeds 12° to 15°F per minute.

Fixed temperature heat detectors

Fixed threshold, or rate compensated heat detectors react to a preset threshold and will not activate until the preset threshold is crossed, regardless of the rate of temperature increase. If there is too much thermal lag in the design, the alarm threshold can be exceeded before an alarm condition is indicated.

Heat detector selection

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 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 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 rate-of-rise heat detector may be preferred.

Heat detectors are commonly marked "Not a life safety device". Heat detectors are not meant to replace smoke detectors in life safety applications like in bedrooms. While primarily for property protection, a heat detector will nonetheless notify of a fire in a utility area (i.e., laundry room or attic) faster than just relying on a smoke detector that might be on the other end of the house. This will allow extra time to evacuate the building or put out the fire if possible.

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Rate-of-rise heat detectors

Fixed temperature heat detectors

Heat detector selection

See also