Water heater:

Series installation uses heaters that are not identical in both input and storage capacity. An example might be when an expansion or addition is made in an application that will require additional hot water. For example, you add a new bathroom to your home. The existing system will not handle the demand and a new water heater must be installed. The new heater is not identical to the existing heater. In this example, series installation may be appropriate.
When installing water heaters in series, the heater with the largest input should be the first heater in the series, at the cold-water inlet side of the system. Series installation draws hot water from one tank at a time. As hot water is drawn, it is taken from the last heater in the series. For every gallon of hot water drawn, preheated water is introduced into the last heater in the series and cold water is introduced into the first heater in the series.