FireClass 611H-F Conventional Fixed Temperature Heat Detector 60øC offer an acceptable, but less sensitive alternative to smoke detectors if environmental conditions rule out their use. The 601H-R (rate-of-rise), 601H-F, 611H-F and 631H-F (fixed temperature) detectors detect abnormally high rates of rise of temperature and abnormally high (static) temperatures respectively. For use particularly where the ambient temperature may be low, a rate-of-rise heat detector 601H-R is to be preferred.