How to Hook Up Water Heaters in Series

Place the second water heater next to the first water heater, about 2 feet apart. Position the tanks so that the cold water from the main line goes into the larger tank, if they are different sizes. If they are the same size, as in this example, keep the main line going into the first tank.

Turn off the power to the water heater at your home's main electrical box. Turn off the valve on the main that feeds cold water into the tank, and open all of the hot water spigots in your home to drain water from the lines. Connect a hose to the drain valve on the side of the tank. Open the valve and drain the tank into a sink or drain. Take care to avoid being scalded by the water.

Cut the main line that feeds water into the first tank with a pipe cutter just before the elbow that directs the water down into the tank. Cut the cold water inlet pipe with the pipe cutter about 6 inches above the tank.

Cut the hot water outlet pipe with a pipe cutter about 6 inches above the first tank. This is the line that carries hot water throughout your home.

Hold a carpenter's level on end against the hot water outlet pipe on the new tank and mark the position of the outlet pipe on the hot water line that leads from the original tank as it passes over the new water heater. Cut the hot water line with the pipe cutter.

Solder a tee fitting onto the main line, with the extension pointing down at the tank. A tee fitting gets it's name because it resembles the letter T, and it is used when branching off a line. In this case, the line being branched off is a bypass so that you may perform maintenance or replace one water heater without losing all of your hot water. Wear safety goggles and gloves when soldering pipe.

Solder a tee fitting onto the hot water outlet pipe of the original water heater, with the extension of the fitting pointed at the new water heater. Solder a tee fitting onto the cold water inlet pipe of the new water heater, pointing it at the original tank.

Solder a tee fitting onto the end of the hot water line, with the extension pointing down at the tank.

Measure the distance between the tee on the main line and the cold water inlet pipe on the original tank, and cut a section of pipe to fit. Cut that pipe in half, and solder each end to a ball valve. This will allow you to bypass the first tank by cutting off the water supply to it, while allowing cold water to go to the new tank.

Solder the assembled valve and pipe to the cold water inlet pipe with a copper sleeve fitting, and to the extension of the tee fitting that points down at the original tank.

Cut copper pipes and connect them to ball valves, then solder them in place to connect the tanks. You need a line that runs between the tees on the hot water outlet pipe of the first tank and the cold water inlet pipe of the second tank, a line that runs between the tees on the cold water main and the hot water line that carries water throughout the house, and between the tee on the hot water line and the hot water outlet pipe on the second tank.

Have an electrician wire the new tank into your home's main electrical box. Open the hot water spigots in your home, and close the drain valves on the water tanks.

Open the valve on the water main to feed water into the tanks, and allow it to keep running until it has flowed out of the hot water spigots for about a minute to remove air from the pipes, then close the spigots. Close the valves for the bypass line.

Turn on the power for the water heaters. Set the thermostats on the water heaters to between 120 and 125 degrees Fahrenheit.