Smoke alarms in homes can go off whenever there's smoke present. If a person smokes cigarettes, burns food or lights incense, a smoke alarm might go off. A heat detector, on the other hand, is indifferent to how much or what type of smoke is in the air. It will only react when it detects a change in heat, associating that heat change with the possibility of a fire in the area.
Most heat detectors function off of photoelectric sensors. They can be powered by the electricity in a home, from a battery, or in some cases from both sources, with the battery being a backup. A heat detector's sensor is geared to take readings of the ambient heat in a room and to test that heat against a predetermined number in its inner workings. Once the heat in an area reaches that predetermined number, the heat detector sends a signal. The signal might set off an alarm, or it might move through a security system to warn those monitoring for dangers. Sometimes, both of those events occur.
With heat detectors, it's about the temperature. According to firesnoop.com, many heat detectors come with a built in number of 120 degrees Fahrenheit. Once the ambient temperature of a room reaches that degree, the signal goes out and the alarm goes off. Heat detectors might have these numbers changed by the owner to lower levels. Say for instance that a heat detector wasn't meant to monitor for fire, but rather to be sure that a cold room for storage doesn't rise above a certain temperature. This will trigger an alarm and inform people that the cold room is no longer serving its purpose.
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