The Fenwal Models THD-7052 and THD-7053 are thermistor- based electronic Heat Detectors with a 135Deg F (57Deg C) Fixed Temperature set point. In addition, the Model THD-7052 has a Rate-Of-Rise of temperature detection feature rated at 15Deg F (8.3Deg C) per minute. Both detectors have advanced solid-state, low-voltage, surface- mount circuitry and are designed for 2-Wire and 4- Wire installation using the appropriate Detector base. The detectors are designed for Open Area Protection (UL 521) and may be installed in systems intended for Releasing Device Service through use of a compatible Fire Alarm Control Panel Two Red Light Emitting Diodes are located diametrically opposite each other so as to allow 360-degree viewing. Both LEDs continuously indicate the operating condition of the Detector. During standby, the LEDs flash once every six seconds. During alarm, both LEDs light steady at full brilliance. A double flash every six seconds indicates a detector with a thermistor trouble. An optional base is available to provide remote LED function. A unique gated output circuit design provides improved stability and transient suppression. Special signal processing techniques verify the presence of smoke before the detector will alarm. The detector head is installed into the base with a simple twist-lock action. A locking feature is provided for vandal resistant security. Detector base options are available to provide for auxiliary test, indication and/or control functions Optional bases are available for supplementary 2-wire or 4-wire relay functions. The Model THD-705x Heat Detectors may be interchanged with other Fenwal Series CPD- 705X Ionization and Series PSD-715X Photoelectric Smoke Detectors when using multifunction base configuration