Bi-directional air flow separator for a double-flow installation, in particular a thermal installation, intended to be arranged at the interface inside and outside of a building, of the type comprising a casing which comprises: - a main opening, intended to be connected to an internal installation, - at least two secondary openings (230, 240), external, separate from the main opening, - a means for separating flows comprising at least two deflectors (260, 270) which extend, from a common edge, dividing the main opening into two basic passages (224, 226), in order to define two diverging channels (280, 290) respectively going from a basic passage (224, 226) of the main opening to one of the external secondary openings (230, 240), the main opening, intended to be connected to an internal installation, falling into a vertical plane, characterised in that the deflectors and the means for orienting the inlet and outlet flows which pass through the external secondary openings and travel through the channels together define a double divergence: - a first horizontal divergence, imposed by the deflectors, enabling to direct the inlet and outlet flows on either side of a vertical plane passing through the separator, in other words, a divergence in a horizontal plane, the deflectors (260, 270) extending in horizontally divergent directions, and - a second divergence, imposed by the orientation means, enabling to direct the inlet and outlet flows on either side of a horizontal plane passing through the separator, in other words, a divergence in a vertical plane, the means for orienting the flows being adapted such that the inlet and outlet flows which travel respectively through one of the two channels, have tilts (-α, +α), respectively opposite, with respect to the horizontal at the level of the external secondary openings. - a main opening, intended to be connected to an internal installation, - at least two secondary openings (230, 240), external, separate from the main opening, - a means for separating flows comprising at least two deflectors (260, 270) which extend, from a common edge, dividing the main opening into two basic passages (224, 226), in order to define two diverging channels (280, 290) respectively going from a basic passage (224, 226) of the main opening to one of the external secondary openings (230, 240), the main opening, intended to be connected to an internal installation, falling into a vertical plane, characterised in that the deflectors and the means for orienting the inlet and outlet flows which pass through the external secondary openings and travel through the channels together define a double divergence: - a first horizontal divergence, imposed by the deflectors, enabling to direct the inlet and outlet flows on either side of a vertical plane passing through the separator, in other words, a divergence in a horizontal plane, the deflectors (260, 270) extending in horizontally divergent directions, and - a second divergence, imposed by the orientation means, enabling to direct the inlet and outlet flows on either side of a horizontal plane passing through the separator, in other words, a divergence in a vertical plane, the means for orienting the flows being adapted such that the inlet and outlet flows which travel respectively through one of the two channels, have tilts (-α, +α), respectively opposite, with respect to the horizontal at the level of the external secondary openings.