Heat detectors offer an acceptable, but less sensitive alternative to smoke detectors if environmental conditions rule their use. 601H-R (rate of rise) and 601H-F (fixed temperature) detectors detect abnormally high rates of rise of temperature and abnormally high (static) temperatures respectively. For general use and especially where the ambient temperature may be low, 601H-R is to bepreferred. A fixed temperature limit is also incorporated inthese detectors. In many environments, eg kitchens, canteens and boiler rooms, sudden large changes in temperature are considered normal, therefore, rate-of-rise detectors are generally not suitable in these cases, and a slower fixed temperature detector 601H-F should be used.