Heating or air conditioning device with a drum valve

The heating or climate control unit, especially for a motor vehicle, has a housing accommodating a first and/or a second heat exchanger, and a drum valve (7) serving as a mixing valve which is installed in a mixing chamber after the first and/or second heat exchanger in the region of the inlet of a first and second air duct. The drum valve in the region of at least one longitudinal edge (16) of its partial hollow cylindrical section (15) has a protruding section (17) which in the end position of the drum valve covers a passage lying upstream of the mixing valve between the first and second air ducts. The first heat exchanger is an evaporator and the second a radiator.