you MUST have a double security. I would suggest a good old fashioned bi-metal thermostat that cut the heater off if the temp get over a ceartain limit. Amother good old fashioned bi-metal thermostat that sound an alarm if the temperature foes under another limit. Your stuff WILL, regardles of how well it is designed, fail. With the 2 theremostats also have a "test buttor that via the uC cut the heater off (check thermostat 2) and another that keep the heater on (check thermostat 1) These buttons should be key operated.

Erik