How to Install a Water Heater Expansion Tank

Do You Need an Expansion Tank?

An expansion tank has a flexible rubber diaphragm dividing the tank into two sections: one allows water to enter as it heats, and the other provides an air chamber for the diaphragm to expand into.

It's typically required for standard water tanks and closed plumbing systems where water only flows inward, and expanded water has no place to go. These types of systems have a mechanism called a pressure-limiting valve—or back-flow valve—preventing expanded hot water from re-entering the municipal water line.

Water heater installation usually requires a plumbing permit, so you'll find out about any expansion tank requirements when you file for your permit. An expansion tank may not be required for open plumbing systems, but it's still a good idea to install one to help preserve your home's plumbing.

Instructions (New Water Heater Installation)

An expansion tank is usually installed directly above the water heater using a tee-fitting installed in the cold water delivery pipe. It typically sits vertically— but can also sit horizontally if there are space constraints and your specific tank allows. You will typically need copper pipes and fittings for your expansion tank's connections, but exact materials can vary depending on the types of pipes in your home's system.

The expansion tank itself also usually has a 1/2-inch or 3/4-inch threaded fitting joined into the cold water pipe by means of a tee fitting, and short lengths or threaded or sweat-soldered pipes.

1. Turn Off the Water Supply

   Before you begin the expansion tank installation process, you'll need to locate your home's main water shutoff valve, and switch it to the "off" position. This valve can typically be found in the basement, crawlspace, utility room, or yard,

2. Attach a Dielectric Union

   A dielectric union is a fitting used to separate pipes made from different metals and prevent corrosion. Once your new main water heater tank is in place, wrap the union with pipe-sealing tape, then thread—or tighten—it into the cold water inlet port using a pipe wrench or channel-lock pliers.

3. Attach Copper Adapter

   Wrap the pipe-sealing tape four to six times around the top threads of the union, then thread a female threaded copper adapter onto it.

4. Tighten Adapter

   Tighten the copper adapter securely with a pipe wrench or channel-lock pliers.

5. Attach Tee Fitting

   Attach about one foot of copper pipe and a tee-fitting to the adapter, in order to provide enough room for the new expansion tank. Plumbers will usually make this connection by sweat-soldering, but it can be done with push-fit connections (sometimes known as "shark-bites") instead.

6. Attach Expansion Tank Pipe

   Using either sweat-soldering or push-fit connections, attach a short length of horizontal copper pipe— no more than six inches long— to the tee-fitting's side outlet. Next, attach a female threaded adapter to the horizontal pipe using your preferred connection method.

7. Attach the Expansion Tank

   Wrap four to six loops of pipe-sealing tape around the threaded fitting on the expansion tank, then screw it onto the horizontal pipe's adapter. It is generally sufficient to tighten by hand— but take care not to over-tighten, which can damage the pipes or fittings.

   Tip

   Consider installing an extra valve before the expansion tank, which will allow you to isolate the tank from the system for easy replacement when necessary.

8. Complete the Installation

   Connect the top outlet on the tee-fitting to the cold water supply pipe, using any pipes and fittings necessary. Many plumbers opt to make this connection with a copper flex line instead of a rigid pipe.

   Finally, complete the hot-water pipe connections to the water heater, turn everything on, open the water valves, and check for leaks.

Variations for an Existing Water Heater

When adding an expansion tank to an existing water heater tank, the main challenge is finding the space for it. You may need to use different pipes and fittings to create room for the expansion tank, which could mean replacing the copper flex line with a shorter or longer one, or adding additional elbows and pipes to the cold water pipe.

If necessary, the expansion tank can also be installed a foot or two away from the water heater, as long as it's properly spliced into the cold water line and there are no valves between the two tanks. Most professional plumbers choose to replace the dielectric union with a new one when an expansion tank is added to an existing water heater.