Abstract: A secondary heating source is used to preheat the water supplied to a conventional hot water tank. Three two-position valves permit the water from the water supply line to be supplied directly to the hot water tank during part of the year (such as the winter), with the secondary heating source having its inlet and its outlet connected to a drain to drain all water from it so that it does not freeze in the winter. The second position of the three valves causes the water from the water supply line to be diverted through the secondary heat source prior to supplying it to the inlet of the hot water tank, so that supplemental preheating of the hot water is effected. Manually controlled valves are used to accomplish this purposes and each of them includes indicia on it to indicate the position of operation to simplify the change-over from the "summer" mode of operation to the "winter" mode of operation.