Abstract: A horizontal in-ground heat exchanger is disclosed which comprises an input line, a return line and a plurality of heat exchanger pipes connected between the input and return lines. The heat exchanger pipes are divided into a plurality of groups, each including several pipes. An input manifold is connected to at least some pipes in each group and a return manifold is also connected to at least some pipes in each group. The cross-sectional dimension of each manifold is selected to be substantially smaller than a crosswise dimension of each pipe group lying in a horizontal plane. Each pipe has a curved end portion connected to its respective input and return manifold.