it is stopping any power output to the device that's connected to it, so yes the heat is turned off. those thermostats operate in an on/off style: full power or no power, with a 2° ∆ (delta; this means the device waits for the temperature to be 2° above or below the target temp before switching off/on.).
you might need to up the temperature set on the thermostat. what's the temperature reading at the bottom of the enclosure above the UTH?