Abstract: A system is described for use with a hot water supply for hotels, apartment buildings and similar multi-unit structures, which controls the temperature T.sub.1 of water at the outlet of the water tank that circulates past the units and back to the tank, to make the actual temperature T.sub.1 close to a desired temperature DTEMP. The desired temperature at the tank outlet, DTEMP, is adjusted according to the measured temperature T.sub.3 of recirculating water prior to its reentry into the tank. In cold weather, when T.sub.3 decreases below a preset limit such as 105.degree. F., indicating there is a considerable temperature drop along the pipeline before water reaches the last unit, the desired tank outlet temperature DTEMP is raised to more than it would otherwise be. As T.sub.3 increases back toward the limit such as 105.degree. F., the temperature DTEMP is lowered.