A heat recovery ventilation unit comprising - a polygonal heat exchanger and a housing, wherein the polygonal heat exchanger and the housing define - a first inlet and a first outlet defining a first airstream pathway from the first inlet to the first outlet through the polygonal heat exchanger; - a second inlet and a second outlet defining a second airstream pathway from the second inlet to the second outlet through the polygonal heat exchanger; - the housing further comprises - a bypass channel arranged to bypass the polygonal heat exchanger along the first airstream pathway, thereby defining a bypass airstream pathway, - an airstream swap unit downstream to the polygonal heat exchanger and the bypass airstream pathway, the airstream swap unit being configurable between a heat recovery position, where the bypass airstream pathway is blocked, and a bypass position, where the first airstream pathway is blocked. characterised in that the housing is substantially formed by insulation material, and the airstream swap unit comprises a stationary grate and a slidable grate, the grates (66, 68) comprising a series of complementary apertures at a bypass section facing the bypass channel and at an exchanger section facing a side of the polygonal heat exchanger, wherein the apertures are offset between the bypass section and the exchanger section, wherein - at the heat recovery position, the slidable grate is positioned such that the apertures at the heat exchanger section overlap and the apertures at the bypass section are blocked, and - at the bypass position, the slidable grate is positioned such that the apertures at the bypass section overlap and the apertures at the exchanger section are blocked. - a polygonal heat exchanger and a housing, wherein the polygonal heat exchanger and the housing define - a first inlet and a first outlet defining a first airstream pathway from the first inlet to the first outlet through the polygonal heat exchanger; - a second inlet and a second outlet defining a second airstream pathway from the second inlet to the second outlet through the polygonal heat exchanger; - a first inlet and a first outlet defining a first airstream pathway from the first inlet to the first outlet through the polygonal heat exchanger; - a second inlet and a second outlet defining a second airstream pathway from the second inlet to the second outlet through the polygonal heat exchanger; - the housing further comprises - a bypass channel arranged to bypass the polygonal heat exchanger along the first airstream pathway, thereby defining a bypass airstream pathway, - a bypass channel arranged to bypass the polygonal heat exchanger along the first airstream pathway, thereby defining a bypass airstream pathway, - an airstream swap unit downstream to the polygonal heat exchanger and the bypass airstream pathway, the airstream swap unit being configurable between a heat recovery position, where the bypass airstream pathway is blocked, and a bypass position, where the first airstream pathway is blocked. the housing is substantially formed by insulation material, and the airstream swap unit comprises a stationary grate and a slidable grate, the grates (66, 68) comprising a series of complementary apertures at a bypass section facing the bypass channel and at an exchanger section facing a side of the polygonal heat exchanger, wherein the apertures are offset between the bypass section and the exchanger section, wherein - at the heat recovery position, the slidable grate is positioned such that the apertures at the heat exchanger section overlap and the apertures at the bypass section are blocked, and - at the bypass position, the slidable grate is positioned such that the apertures at the bypass section overlap and the apertures at the exchanger section are blocked. - at the heat recovery position, the slidable grate is positioned such that the apertures at the heat exchanger section overlap and the apertures at the bypass section are blocked, and - at the bypass position, the slidable grate is positioned such that the apertures at the bypass section overlap and the apertures at the exchanger section are blocked.