Heat detectors may operate using fixed temperature, rate of rise, or both. Which statement is true?

• A. They are all only fixed temperature
• B. They operate only on fixed temperature
• C. They operate only on rate of rise
• D. They may use fixed temperature or rate of rise

Answer: (D)

Heat detectors come in different sensing methods. Some monitor ambient temperature and trip when it reaches a fixed preset threshold. Others look for a rapid rise in temperature over a short period and will alarm even if the absolute temperature isn’t extremely high yet. Many devices combine both ideas, so they can alarm if the temperature hits a set point or if it climbs quickly. Because of this variety, the statement that heat detectors may use fixed temperature or rate of rise, or both, is accurate. The other options imply exclusivity to one method only, which doesn’t reflect how detectors are designed to match different environments and reduce false alarms.