The Siemens Building Technologies ─ Fire Safety Division Thermal Fire Detectors are fixed temperature or a combination of fixed- temperature / rate-of-rise type. The combination detectors consist of two, independently operated thermal elements. The rate-of-rise element is self-restoring. However, the fixed temperature is of the non-restoring type.
Underwriter’s Laboratories, Inc., recommends the combination-type thermal detector be used to protect a maximum of 2,500 square feet (50-foot spacing), and the fixed-temperature type be used to protect a maximum of 625 square feet (25-foot spacing). However, job conditions and engineering judgment often dictate closer spacing to provide faster detection.