FST Frost protection thermostat, mechanical

Frost protection thermostat, mechanical

Output: change-over contact 10 (4) A, 250 V AC
Dimensions 108 x 72.5 x 70 mm, IP65
Setpoint range -10..12 °C, hysteresis 1 K


The frost protection thermostat is used for air- or water-side temperature monitoring at heat exchangers, water/air heaters, and for prevention of frost-caused damages.

  • Air handling units, water distribution, hot water circuits.

The switch inside the frost protection thermostat acts (closes contacts C and 3) when the temperature falls below the preset temperature setpoint over a cappilary tube length of at least 40 cm. Simultaneously contact C - 2 opens and can be used as a signal contact.  Resetting follows automatically after the temperature rises above the preset setpoint value plus hysteresis (appr. 2 K) again (on type FST-xD-HR resetting must be done manually vy pressing the reset button). 

FST is “intrinsically safe”, i.e. in case of damaging the capillary tube or membrane system it automatically switches to the frost (alarm) state. The gas-filled (R507) membrane and the capillary tube constitute one measuring unit, which is mechanically coupled to the microswitch.

The capillary tube is laid at the warm side of the air heater to be protected, uniformly over the entire area at a distance of ca. 5 cm across of the heat exchanger tubes. For test purposes it is recommended to make a loop of ca. 20 cm directly underneath the enclosure and before entering the air duct. (This does not apply for outdoor installations!) To avoid damaging of the capillary tube, a minimum bending radius of 20 mm must be observed. Installation is facilitated by using the mounting clamps available under accessories.

The frost situation can be simulated and the functioning of the devices can be tested by dipping the capillary tube testing loop into a pot filled with ice water or using the test spray.

* capillary length 6 m (FST-1D), 1.8 m (FST-3D), 3 m (FST-5D), 12m (FST-7D)