Abstract: A compact highly efficient heat exchanger is used to utilize waste heat from a motor vehicle engine cooling system to heat a source of water for use with a shower or the like in a recreational environment. The heat exchanger comprises a hollow cylinder having a cylindrical wall to define an annular space therebetween. Within the annular space is located a neatly fitting helical tubular coil with spaced helixes to define a helical pathway between adjacent coil helixes working fluid passes through the tubular coil and process fluid passes through the helical pathway to effect heat exchange between the working and process fluids.