Is the electric floor heating system wired correctly?
NOTE: All power to the units should be shut off at the breaker panel before checking any electrical connections.
Have a licensed electrician check to make sure that the heating system is correctly connected to the thermostat and the thermostat to the electrical panel.
Instructions for wiring can be found here if you do not have the copy that came with the heating system and thermostat. Additionally, a video can be viewed below: