the outlet hose 7 the separating wall 4 is held spaced from the two walls of the hollow wheel 3 by extruberances such as 17a and 17b when the valve ball 9 is closed water is prevented from entering the inside of the rotating heat exchanger 1 and the device acts as a blower for circulating ambient air through the interior of a motor car the heat exchanger draws in air along the path indicated by the arrow 19 which flows from the periphery 20 into the 180 u00b0 volute housing having ducts 21 whence the air is discharged from outlets 22 such air is therefore either heated by the water or remains unheated and relatively cool the stator part of the motor 2 is attached by the spokes 23 to the structure of the volutes 21 whilst the spokes or the plate 24 provide the centering of the ducting and valve elements fig2 shows a view of the motor side partly cut away the separating walls 15 are shown dividing the pipes 14 in the illustrated manner into the in and out flow channels shown by the arrows 16b and 16c instead of the wall 15 the axial flows in the pipes 14 may be conducted by pipes 25 within the pipes 14 also as illustrated in fig2 which illustrates both schemes the pipes 25 in such variant communicate with one of the spaces 4a or 4b and like the separating walls 15 the pipes 25 do not quite reach the closed ends at 14a of the pipes 14 at very high rotational speeds substantial hydrostatic pressure is generated in the space 18 which pressure may be accomodated by the use of the pipes 25 which can extend.