The professional carbon monoxide warning device with internal CO sensor, whose measuring cell, evaluation electronics and software have been tested by TÜV Süd according to the standard, is here equipped with an additional external city and natural gas sensor, which makes the entire system the perfect warning device for the boiler room in which a gas heating system or boiler is installed. The alarm relay, which is energised in monitoring mode, drops out when gas is detected (methane, butane, propane, ethanol) or when the CO concentration specified by the standard is exceeded, and can thus switch off the heating system with its solenoid shut-off valve in the event of danger to reliably prevent further inflow of toxic or flammable gases.