Motor vehicle heating, ventilation and/or air conditioning device which contains a first heat exchanger and a second heat exchanger, the device comprising a housing defining a flow channel for a stream of air which has a. at least a first duct for the flow of a first stream of air, defining a first air outlet, b. at least a second duct for the flow of a second stream of air, defining a second air outlet, c. at least one mixing chamber communicating with the respective air outlets of said ducts (52, 54), the first duct directly connecting the first heat exchanger to the mixing chamber, bypassing the second heat exchanger, and the second duct connecting the second heat exchanger to the mixing chamber, and d. at least one mixing flap comprising a first sliding door arranged so as to control the apportionment of the first stream of air and second stream of air in said at least one mixing chamber, said at least one mixing flap being arranged so as to be movable between a first end position, in which it closes the first duct, and a second end position, in which it closes the second duct, e. the device comprising a third duct connecting the first heat exchanger to the second heat exchanger, f. the first heat exchanger being arranged vertically in relation to the vehicle in the mounted state, g. the device comprising at least one air outlet opening (12, 14, 16) arranged downstream of the mixing chamber, each air outlet opening (12, 14, 16) being configured to guide the stream of air in particular towards various regions of a vehicle passenger compartment, and at least one air outlet duct running along a wall of the flow channel so as to direct the stream of air in a direction substantially away from the direction of the stream of air as it passes through the first heat exchanger, characterized in that: h. said first sliding door has at least one recess, so as to fluidically connect the first air duct and the second air duct, when said at least one mixing flap is arranged in an intermediate position between the first and the second end position, i. the second heat exchanger is arranged horizontally in relation to the vehicle in the mounted state, j. the device comprises a first distribution flap comprising a second sliding door arranged so as to be able to at least partially close another of said air outlet ducts (12, 14). a. at least a first duct for the flow of a first stream of air, defining a first air outlet, b. at least a second duct for the flow of a second stream of air, defining a second air outlet, c. at least one mixing chamber communicating with the respective air outlets of said ducts (52, 54), the first duct directly connecting the first heat exchanger to the mixing chamber, bypassing the second heat exchanger, and the second duct connecting the second heat exchanger to the mixing chamber, and d. at least one mixing flap comprising a first sliding door arranged so as to control the apportionment of the first stream of air and second stream of air in said at least one mixing chamber, said at least one mixing flap being arranged so as to be movable between a first end position, in which it closes the first duct, and a second end position, in which it closes the second duct, e. the device comprising a third duct connecting the first heat exchanger to the second heat exchanger, f. the first heat exchanger being arranged vertically in relation to the vehicle in the mounted state, g. the device comprising at least one air outlet opening (12, 14, 16) arranged downstream of the mixing chamber, each air outlet opening (12, 14, 16) being configured to guide the stream of air in particular towards various regions of a vehicle passenger compartment, and at least one air outlet duct running along a wall of the flow channel so as to direct the stream of air in a direction substantially away from the direction of the stream of air as it passes through the first heat exchanger, characterized in that: h. said first sliding door has at least one recess, so as to fluidically connect the first air duct and the second air duct, when said at least one mixing flap is arranged in an intermediate position between the first and the second end position, i. the second heat exchanger is arranged horizontally in relation to the vehicle in the mounted state, j. the device comprises a first distribution flap comprising a second sliding door arranged so as to be able to at least partially close another of said air outlet ducts (12, 14).