A system produces domestic hot water and includes a tank having top and bottom portions and containing heat storing fluid, and a tapping unit including first and second tapping coils. The first tapping coil is immersed in the bottom portion of the tank. The second tapping coil is fluidly connected in series to the first tapping coil by a connection pipe, and to the hot water port, and is immersed the top portion of the tank A switching device switches between a first state in which water flows from the cold water port through only first tapping coil to the hot water port, and a second state in which water flows from the cold water port through the first tapping coil and, after flowing through the first tapping coil, through to the second tapping coil to the hot water port.