A heat detector responds to the air temperature at its own sensing element, and that element has thermal mass, so it always trails the gas moving past it. A fixed-temperature detector operates when its element finally reaches a set point, commonly about 57 degrees Celsius (135 degrees Fahrenheit) for ordinary rooms, with higher-rated elements near 90 degrees Celsius (194 degrees Fahrenheit) for hot spaces; because of the lag, the surrounding air is already hotter than the set point when it operates, an effect called thermal lag. A rate-of-rise detector operates when the temperature climbs faster than a set rate, often about 8 degrees Celsius per minute, so it responds to a fast-growing fire well before the fixed point is reached, and ignores a slow seasonal warming of a roof space. Most devices combine both, with the fixed point as a backstop for a fire that grows too slowly to trip the rate element. The essential limitation is that heat detectors are late by design: they are chosen where smoke detection would nuisance-alarm, such as kitchens, garages, dusty workshops and boiler rooms, and not where early warning is the goal. After this Concept you can choose between the two operating modes for a space and explain why either is later than a smoke device.
Heat detectors are fire-alarm-system devices that sense the thermal effect of a fire and trigger an alarm, classified by their sensing mechanism into fixed-temperature (activates once ambient heat re…