Abstract: A passive solar assistance device for use with a heat pump having a heat transfer fluid inlet and outlet, with means for circulating the fluid, a tank for containing an amount of the fluid, and at least one solar heat pipe having one end extending into the tank and adapted to be in contact with the fluid. At its opposite end the heat pipe is provided with a solar receiving device, such as an enclosed box having one or more transparent walls facing the sun or a plate construction comprising an extension of the heat pipe itself. In its heating mode, heat pipe is mounted at an angle of at least 30.degree., with the end associated with the solar receiving device in the lowermost position so that the heat pipe acts as a one way heat valve conducting heat from the solar receiving device to the fluid in the tank but not in the opposite direction. The heat pipe may be pivotally mounted to provide for either a heating mode or cooling mode.