Szerdi heaters are designed to warm up the air flown into rooms. They are installed directly in round ventilation ducts, produced in the following sizes: Ø100/ Ø315 and of power from 0,3/ 12kW. They are used in residential buildings, public utility buildings, and industrial buildings in order to increase the ventilation air temperature to its required value. They are also used in the chimney heating system CIEPLIK of DOSPEL to heat the air, when the fireplace does not light. In standard, heaters possess two levels of thermal protection against overheating. The first one, that can be automatically reset disconnects the power supply of the heater in the temperature of 70 ºC. The second level, that is reset manually, disconnects the electrical circuit after 80ºC is exceeded.