How to Install a Water Heater Expansion Tank


Installing a water heater expansion tank is an important plumbing task that can help prevent damage and leaks in your water heater system. A water heater expansion tank absorbs excess water pressure created as the water is heated, preventing pipes and fixtures from straining under the increased pressure.

Having the proper expansion tank set up is crucial for any home with a water heater system. In this comprehensive guide, we will walk through the entire process of selecting, preparing, and installing a water heater expansion tank properly. With the right information and preparation, you can complete this project yourself and have peace of mind knowing your plumbing system is protected.

Choosing the Right Expansion Tank

The first step is selecting the proper expansion tank for your specific water heating system. Here are the key factors to consider:

Type of Water Heater

The type of water heater you have determines the size and capacity of the expansion tank required. Common residential water heater types include:

  • Conventional tank – These operate with pressures around 50 psi. A 2-5 gallon expansion tank is usually sufficient.
  • Tankless – Require larger expansion tanks, normally 12-15 gallons, due to higher operating pressures of around 150 psi.
  • Boiler heating system – Needs a tank designed for hot water boiler systems, which operate under higher pressures. Appropriate expansion tank sizes are typically in the range of 25-35 gallons.

Size of Water Heater

Larger water heater tanks or heating systems require larger expansion tanks. Consult the water heater manual or manufacturer recommendations for the proper expansion tank size.

Water Pressure

Homes with higher static water pressure, typically 80 psi or above, should install an expansion tank one size larger than the water heater size recommendation. This provides extra capacity to handle pressure fluctuations.

Once you know your specific water heater type and size, refer to the manufacturer’s literature to determine the recommended expansion tank size. Also consult local plumbing codes for any requirements.


Expansion tanks can be made from:

  • Steel – Durable but prone to corrosion. Requires maintenance to prevent rust.
  • Composite – Made of non-corrosive materials like polypropylene. No maintenance required. More expensive but longer-lasting.

Composite is preferred for longevity and hassle-free operation.


Determine if you need an expansion tank that mounts directly on the water heater, or a remote model that can be located further away. Remote tanks are connected via a pipe called an expansion loop.

Consider existing space constraints in your installation location.

Planning the Installation

Once you have selected the appropriate expansion tank, plan out the installation in advance.

Tools and Materials Needed

Assemble all the necessary installation tools and supplies:

  • Expansion tank
  • Teflon tape
  • Shutoff valves
  • Galvanized steel pipes and fittings
  • Pipe wrench
  • Plumber’s putty
  • Pipe insulation
  • Instant cold packs

Shutdown Planning

The water heater and main water supply lines will need to be shut off during the expansion tank installation.

Plan for:

  • No hot water during the install. Schedule appropriately so it does not impact your household.
  • Isolating and draining water heater.
  • Alternative water source if installation takes an extended period.

Installation Location

Consider the optimal position for the expansion tank based on:

  • Water heater location
  • Available space
  • Pipe routing
  • Avoiding obstacles

The expansion tank should be located close to the water heater. Remote tanks work well when there is limited space near the water heater.

How to Install the Expansion Tank

Once planning is complete, it’s time to install the expansion tank. Follow these key steps:

Step 1 – Turn Off Water Supply and Drain the Water Heater

  • Locate the main water shutoff valve and turn it off to depressurize the system.
  • Open the T&P valve or drain valve on the water heater tank to drain all water from the tank completely.

Step 2 – Disconnect and Prepare Supply Lines

  • Disconnect the cold water supply line from the water heater inlet.
  • Disconnect the hot water outlet pipe as well. Plug the inlet and outlet with caps.
  • Inspect existing plumbing pipes and valves. Replace any corroded or damaged pipes.
  • Have your supplies ready – expansion tank, pipes, fittings, valves, insulation, etc.

Step 3 – Mount the Expansion Tank

  • First, determine the ideal position for the expansion tank and mark the mounting location. Consider existing plumbing runs.
  • Mount the expansion tank securely using appropriate strapping or brackets per the manufacturer instructions. Ensure it is level.

Step 4 – Install Shutoff Valves

  • Install a shutoff valve on the cold water supply line that connects to the water heater inlet. This allows you to easily isolated the water heater.
  • Next, install a shutoff valve on the hot water outlet line.
  • Use Teflon tape on all valve threads to prevent leaks.

Step 5 – Install Expansion Tank on the Cold Water Line

  • Cut the cold water supply line as required to install a tee fitting to accommodate the expansion tank connection.
  • Route copper or galvanized steel piping as needed to connect the expansion tank to the tee fitting on the cold water supply. Use elbow joints as required.
  • Install a shutoff valve after the expansion tank to allow isolation if needed for maintenance.

Step 6 – Connect Hot Water Outlet

  • Run a pipe from the water heater hot water outlet to the home’s hot water lines. Install any elbows required.
  • Insulate any new hot water outlet piping to minimize heat loss.

Step 7 – Reconnect Water Supply and Refill System

  • Confirm all valves are closed before turning main water supply back on.
  • Slowly open the cold water supply shutoff valve to refill the tank. Open valves to bleed air from the lines.
  • When water runs freely from the hot water line, open hot water outlet valve.
  • Allow water heater to fully heat up, then check carefully for any leaks and fix as needed.

Step 8 – Adjust Expansion Tank Pressure

  • Check the tank’s pre-charge pressure using an air pressure gauge on the tank’s air valve.
  • Adjust the pre-charge to match your incoming water pressure. This is typically around 2 psi lower than the incoming cold water line pressure to the house. Add or remove air as needed.
  • Remove air gauge and replace the protective cap on the tank’s air valve stem.

The expansion tank is now installed and ready to protect your water heating system by absorbing pressure fluctuations!

Troubleshooting Expansion Tank Issues

If you experience any problems after installing the expansion tank, here are some steps to troubleshoot:

  • Noisy tank – Tank air charge is too low. Check and increase tank pre-charge pressure if needed. Ensure air valve cap is tight.
  • Water leaks – Tighten pipe connections. Replace leaky valves. Check for cracks in the tank.
  • Low water pressure – Make sure tank size matches water heater requirements. Check for blockages. Inspect shutoff valves.
  • Pressure relief valve dripping – Expansion tank is undersized or faulty. Install properly sized tank. Replace defective tank.
  • Banging pipes – Uneven pressure is causing pipes to flex. Check expansion tank air pre-charge pressure. Add air to tank if needed.

Calling a professional plumber is recommended if problems persist after attempting the troubleshooting steps.

Maintaining and Replacing Expansion Tanks

Routine maintenance is required to keep your expansion tank working properly for the long run:

  • Check air pressure annually – Use an air pressure gauge to check the pre-charge pressure on the tank’s air valve. Adjust as needed to match your incoming water pressure.
  • Inspect visually – Check for any signs of rust, corrosion, bulges or leaks. Address any issues discovered immediately to prevent tank failure.
  • Flush sediment – Open drain valve annually to flush out mineral sediment that can accumulate inside the tank.
  • Replace every 5-7 years – Standard composite expansion tanks will wear out over time. Proactively replacing them on this schedule prevents costly leaks or bursts.

Following this preventative maintenance routine will maximize the service life of your expansion tank.

FAQs About Installing Expansion Tanks

What size expansion tank do I need?

The correct expansion tank size depends on factors like your water heater capacity, type, operating pressure, and incoming home water pressure. Consult manufacturer sizing guidelines to select the recommended tank.

Where is the best place to locate the expansion tank?

Ideally install the expansion tank close to the water heater on the cold water supply line. Make sure it is easily accessible for maintenance.

How do I know if my water heater needs an expansion tank?

All modern water heaters require an expansion tank, with the exception of some tankless designs with built-in expansion systems. Adding a tank is recommended anytime an old water heater is replaced.

What temperature range do expansion tanks operate in?

Expansion tanks are designed to operate in water temperatures ranging from 40°F to 200°F. They can handle the hottest water heater temperatures.

Can I install the expansion tank myself?

Yes, installing an expansion tank is a project a handy homeowner can tackle with proper planning and preparation. Make sure to follow all safety procedures when working with plumbing systems.

How can I quiet a noisy expansion tank?

Loud banging or hammering noises usually indicate insufficient air pressure in the tank. Use an air pressure gauge to check the pre-charge, and add more air as needed until the tank quietens down.


Installing a properly-sized water heater expansion tank is vital to protect your plumbing and fixtures from excess water pressure damage, as well as prevent nuisance leaks. By following the instructions outlined in this guide, you can confidently install an expansion tank on your own. Just take your time, exercise safety precautions, and refer to the steps to complete this project successfully. With routine maintenance, your expansion tank will provide many years of reliable service life. Our tips will help you troubleshoot any issues to keep your hot water system operating optimally.