The units incorporate specially manufactured heating elements. During operation to provide security against overheating, there exist 2 thermostats connected to each other in series. Adjustable type first thermostat cuts the current in case of overheating, and starts the operation again when cools down. In case of malfunction of this thermostat, the security is provided by the second thermostat, which cuts the current and provides a definite protection, until the reset buttons pressed. Depending on demand, electrical connections are produced gradually. The electrical connections are made via the terminals in the terminal box on the product on the field.