A duct extension piece (36) is connected to the air outlet opening (32) and is divided into a first and a second half (40,42) by a partition (38) located at an angle to the dividing wall (20) of the housing. The halves of the channel extension are connected to corresponding halves (28,30) of the mixing chamber. The air outlet is divided into four sub-openings by the two partitions, with the two diametrically opposite sub-openings closed by walls.
