A heat exchanger, comprising: a heat exchanger element (23; 73) past which, in use, supply fluid and exhaust fluid is forced; a tubular outer casing for installing in a building or the like by inserting the casing into a mounting bore formed in a wall (62, 63) of the building or the like, the heat exchanger element (23; 73) being mounted within the casing and comprising a plurality of heat exchanger ducts sealingly mounted on the casing using only the engagement of the material of the ducts with the material of the casing, such that the supply fluid and exhaust fluid do not mix, the ducts being spaced apart from one another such that there are gaps between adjacent ducts, the ducts extending along the longitudinal axis of the casing; an exhaust air outlet and a supply air inlet provided at one end of the casing; an exhaust air inlet and a supply air outlet provided at the other end of the casing; and fan means comprising a first impeller to force the supply fluid through the heat exchanger between the supply air inlet and the supply air outlet in a first direction aligned with the longitudinal axis of the heat exchanger and a second impeller to force the exhaust fluid through the heat exchanger between the exhaust air inlet and the exhaust air outlet in an opposite direction to that of the supply fluid, the opposite direction being aligned with the longitudinal axis of the heat exchanger, such that one of the supply fluid or exhaust fluid is forced through the ducts, with the other of the supply fluid or exhaust fluid being forced around the ducts within the casing; characterized by: an exterior grille through which, in use, fluid is forced, the grille being formed from a resiliently deformable material that deforms sufficiently, in use, for the heat exchanger to be installed in the building or the like by inserting the grille first through the mounting bore formed in the wall (62, 63) of the building or the like, the grille expanding to cover the mounting bore when the heat exchanger has been fully inserted. a heat exchanger element (23; 73) past which, in use, supply fluid and exhaust fluid is forced; a tubular outer casing for installing in a building or the like by inserting the casing into a mounting bore formed in a wall (62, 63) of the building or the like, the heat exchanger element (23; 73) being mounted within the casing and comprising a plurality of heat exchanger ducts sealingly mounted on the casing using only the engagement of the material of the ducts with the material of the casing, such that the supply fluid and exhaust fluid do not mix, the ducts being spaced apart from one another such that there are gaps between adjacent ducts, the ducts extending along the longitudinal axis of the casing; an exhaust air outlet and a supply air inlet provided at one end of the casing; an exhaust air inlet and a supply air outlet provided at the other end of the casing; and fan means comprising a first impeller to force the supply fluid through the heat exchanger between the supply air inlet and the supply air outlet in a first direction aligned with the longitudinal axis of the heat exchanger and a second impeller to force the exhaust fluid through the heat exchanger between the exhaust air inlet and the exhaust air outlet in an opposite direction to that of the supply fluid, the opposite direction being aligned with the longitudinal axis of the heat exchanger, such that one of the supply fluid or exhaust fluid is forced through the ducts, with the other of the supply fluid or exhaust fluid being forced around the ducts within the casing; an exterior grille through which, in use, fluid is forced, the grille being formed from a resiliently deformable material that deforms sufficiently, in use, for the heat exchanger to be installed in the building or the like by inserting the grille first through the mounting bore formed in the wall (62, 63) of the building or the like, the grille expanding to cover the mounting bore when the heat exchanger has been fully inserted.