How to Install a Water Heater Expansion Tank

Expansion tank installed over water heater with copper pipe

The Spruce / Kevin Norris

A water heater expansion tank is a safety device that protects pipes and fixtures in plumbing systems that are fitted with a pressure-limiting valve or back-flow preventer. Installation of an expansion tank may be required by your local building code when you are installing a new water heater, or it may be a retrofit project installed to add protection to an existing water heater.

The project involves splicing into the cold water delivery pipe above the water heater and installing a small air-filled tank that serves as an expansion chamber to accept water that increases in volume as it is heated. This is a moderately difficult DIY project that shouldn't be tackled unless you have some experience with plumbing work. Novice DIYers may want to have this work done by a professional plumber, but more experienced DIYers can usually do the work in an afternoon.

How an Expansion Tank Works

An expansion tank can help your water heater last longer and it may even be required in some circumstances. Water expands as it heats up, and without an expansion tank attached to the water heater, a closed plumbing system can experience damage from this thermal expansion. For example, when water is heated from the temperature of 50 to 120 degrees Fahrenheit, the volume increases by approximately 2 percent. An expansion tank provides space for this expanding water, thereby reducing pressure on the water heater tank, and on the pipes and fixture.

An expansion tank is most critical in closed plumbing systems, where some form of pressure-limiting valve or back-flow valve prevents the expanding water from backing up into the municipal water main. Inside an expansion tank is a flexible rubber diaphragm that divides the tank into two sections—one of which accepts expansion water as it heats, the other which provides an air chamber that becomes slightly pressurized as the diaphragm expands into it.

An Expansion Tank May Be Mandatory

Since water heater installation generally requires a plumbing permit, you will learn about any expansion tank requirements when applying for a permit to install the water heater. An expansion tank is usually not required if the system does not have some type of pressure-limiting or back-flow valve that keeps the system closed. But if your system does have some type of limiting valve, it is a good idea to install an expansion tank, even if it is not required by the local code.


The installation described here uses existing copper tubing and sticks to general guidance. Your installation may be different depending on the size of the pipes in your situation and other factors; if you're uncertain about how to proceed at any point, call a professional.

Tools and Supplies You Will Need

  • Thermal expansion tank
  • Dielectric water heater union
  • Pipe wrench or channel-lock pliers
  • (2) 3/4-inch copper female-threaded unions
  • Pipe-sealing tape
  • 3/4-inch copper tee-fitting
  • 3/4-inch copper pipe
  • Additional 3/4-inch copper fittings (as needed)
  • Plumbing soldering equipment (if needed)
  • Copper flex pipes (if needed)
Materials and tools into install a water heater expansion tank

The Spruce / Kevin Norris

Instructions (New Water Heater Installation)

An expansion tank is generally installed directly above the water heater by means of a tee-fitting installed in the cold water delivery pipe. The expansion tank is usually installed vertically, though it is acceptable to install it horizontally if it is necessary because of space limitations. The plumbing fittings you will need depends on the type of plumbing pipes you have and on how the expansion tank is oriented, but most commonly the connections are made with copper pipes and fittings. The expansion tank itself usually has a 1/2-inch or 3/4-inch threaded fitting that is joined into the cold water pipe by means of tee fitting and short lengths or threaded or sweat-soldered pipes.

  1. Attach a Dielectric Union

    After the water heater is correctly positioned, attach a dielectric union to the cold water entry port. Wrap the threads of the union with pipe-sealing tape, then thread it into the cold water inlet port on the water heater, using a pipe wrench or channel-lock pliers.

    Dielectric union wrapped with pipe-sealing tape on cold water entry port

    The Spruce / Kevin Norris

  2. Attach Copper Adapter

    Wrap several loops of pipe-sealing tape around the top threads of the union, then thread a female threaded copper adapter onto the union.

    Copper adapter threaded on dielectric union

    The Spruce / Kevin Norris

  3. Tighten Adapter

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

    Channel-lock pliers tightening dielectric union to adaptor

    The Spruce / Kevin Norris

  4. Attach Tee Fitting

    Install a short length of copper pipe and a tee-fitting to the adapter on the water heater's dielectric union. Normally, the tee-fitting must be about one foot above the water heater to provide sufficient clearance for the expansion tank. Most plumbers make these connections with sweat-soldering, but they can also be done with push-fit connections (sometimes known as "shark-bite").

    Copper pipe and tee-fitting attached to water heater's dielectric union adaptor

    The Spruce / Kevin Norris

  5. Attach Expansion Tank Pipe

    Using sweat-soldering or push-fit connections, attach a short length of horizontal copper pipe to the side outlet on the tee-fitting. The length of this copper pipe will depend on available space, but it is best to keep it as short as possible—no more than 6 inches or so. At the end of the horizontal pipe, attach a female threaded adapter, using sweat-soldering or a push-fit connection.


    It is best to work with brass fittings and brass nipples whenever possible. The weight carried by the expansion tank is better supported with brass than copper. It is always best to provide pipe support by way of a clevis hanger from a beam.

    Short copper expansion tank pipe attached to tee-fitting

    The Spruce / Kevin Norris

  6. Attach the Expansion Tank

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


    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.

    Expansion tank attached to adaptor on horizontal pipe

    The Spruce / Kevin Norris

  7. Complete the Installation

    Complete the installation by connecting the top outlet on the tee-fitting to the cold water supply pipe, using whatever pipes and fittings are necessary. Many plumbers make this connection with a copper flex line rather than rigid pipes.

    Also complete the hot-water pipe connections to the water heater, then turn on the water heater, open the water valves, and test the operation while looking for leaks.

    Copper flex line connected to top outlet of cold water supply pipe

    The Spruce / Kevin Norris

Variations for an Existing Water Heater

When adding an expansion tank to an existing water heater, the main challenge is finding room for it. Space can get a little tight, and you may need to use various pipes and fittings to find room for the expansion tank. Often, this will mean replacing the copper flex line with a shorter or longer one, and sometimes adding additional elbows and pipes to the cold water pipe. If necessary, the expansion tank can be installed a foot or two away from the water heater, provided it is properly spliced into the cold water line. Most professional plumbers choose to replace the dielectric union with a new one when an expansion tank is added to an existing water heater.

  • What size expansion tank do I need for my water heater?

    A 2-gallon expansion tank is the adequate size for water heaters that are 40- to 50-gallons.

  • How much does an expansion tank cost?

    The price for an expansion tank can go from $40 to $200 or more, depending on the size you need for your water heater.

  • Should a professional plumber install an expansion tank?

    Installing an expansion tank can be done by yourself if you have moderate to expert plumbing knowledge and experience. Otherwise, let a professional plumber do the job for you.