The FS Series controllers are low limit controllers, also known as "Freeze Stats".  These devices were designed for use on HVAC equipment that requires low-temperature cutout protection to prevent cooling coils from freezing.  They should be mounted between the heating and cooling coils on the supply side of the fan unit.  They respond to the lowest temperature sensed along any 1 foot section of the sensing element.  The FS Series has manual and automatic reset versions, as well as, models that feature one or two sets of SPDT contacts. The FS Series are covered by ACI's Two (2) Year Limited Warranty.