USA Water quality is supported by its audience. If you buy something through our links, we may earn money from our affiliate partners. Learn more

How to Install a Water Heater Expansion Tank in 10 Steps

Fact checked by Stephen Conklin

how to install a water heater expansion tanks

Are you worried excessive water pressure from your water heater tank will damage the unit, water supply pipe joints, and plumbing fixture valves? Did you know you can minimize these problems if you know how to install a water heater expansion tank?

Installing an expansion tank gives your primary water heater a means to minimize pressure buildup in the tank. It saves your water heater, plumbing fixtures, and other water appliances from potential damage.

Thankfully, adding a tank to your existing water heater is less complicated than you think. Let us find out.

Table of Contents

Things You Need for This Tutorial


Adding a thermal expansion tank to your water heater is less complicated than it sounds. You can complete the project in an hour or two, depending on your plumbing knowledge and availability of the following things.

Correct Size of Thermal Expansion Tank

Thermal expansion tanks are smaller than water heaters because they only serve to accommodate excess water pressure coming from the water heater.

Heating the water expands its volume by 2.5% percent for every increase of 100 degrees Fahrenheit.

It is not unusual for a 40-gallon water heater to expand by an extra 0.53 gallons upon reaching its set temperature.

That is why you need a thermal expansion tank to prevent excess water pressure from damaging the water heater and other water appliances. It often works in tandem with the temperature and pressure (T&P) relief valve.

Sizing a thermal expansion tank requires knowledge of water heater capacity.

A 2-gallon thermal expansion tank is suitable for most domestic water heaters with a 40-gallon to 60-gallon capacity and supply pressure of 40 to 50 PSI. If the water pressure is between 60 and 80 PSI, a 3.2-gallon thermal expansion tank is ideal.

An 80-gallon water heater can have a 2-gallon expansion tank if the supply pressure is 40 PSI. Increasing the PSI to 50-60 requires a 3.2-gallon expansion tank. A 4.4-gallon tank is suitable for 80-gallon water heaters with a supply pressure of 80 PSI.

Tools and Materials

Most expansion tanks have fittings and other installation hardware with the package. However, their size might be incompatible with your existing plumbing setup. It would be wise to check them and buy the necessary materials before installing the expansion tank. Click here to learn more about the right-sized tankless water heater for your needs.

You will also need a dielectric union, female copper pipe adapters, short-length copper pipes, tee fitting, and Teflon tape or pipe joint compound.

A pipe wrench and pipe cutter are also essential.

Steps for Installing a Thermal Expansion Tank


Step 1. Turn off the water and power supply to the water heater.

You do not want to create a mess, electrical fires, or a gas explosion thermal expansion tank installation. That is why it would be best to turn off the main water supply line.

You might also want to close the gas supply shutoff valve if you have a gas water heater or switch off the circuit breaker supplying electricity to an electric water heater.

Step 2. Prepare the water heater expansion tank.

Unbox the expansion tank kit and lay the components on a clean surface. Most manufacturers provide a complete installation kit, including tee fittings, fasteners, and extension pipes.

However, they may not supply you with Teflon tape and pipe joint compound. It would be wise to secure these items first before proceeding.

Pro Tip: Study the expansion tank diagram to understand how to install the unit on your water heater.

Step 3. Look for the water pipe supplying cold water to the water heater.

The thermal expansion tank sits between the water heater and the cold water supply line. As such, it would be best to determine the water pipe supplying cold water to the water heater.

Look at the water heater’s top section and check if you see a water tube extending upward from it. This tubing is the cold water pipe. Follow its length until you get to the pipe’s horizontal section.

Step 4. Prepare the area for mounting the expansion tank.

Hot water heater expansion tank installation requires mounting the expansion tank above the water heater. If there is insufficient vertical space, you might want to mount it on the wall next to the water heater.

Position the mounting bracket on the wall and mark the holes for the fasteners. Remove the support and drill holes through the marked spots. Fix the mounting bracket into place, ensuring you tighten the screws or bolts.

Step 5. Secure a dielectric union in the water heater’s cold water inlet port.

Remove the water pipe connected to the water heater’s cold water inlet port.

Wrap Teflon tape around the threads of a dielectric union and twist it into the inlet port. Ensure to wrap pipe-sealing tape on both ends.

Use channel-lock pliers or a pipe wrench to secure the dielectric union.

Step 6. Add a copper adapter and tee fitting.

Twist a female copper adapter onto the opposite end of the dielectric union and secure it with channel-lock pliers or a pipe wrench.

Secure a short copper water pipe to the adapter and add a tee-fitting. It would be best to ensure the tee-fitting is at least 12 inches above the water heater. This distance is sufficient for optimal vertical clearance.

You can sweat-solder the joint between the adapter and the tee-fitting for a more secure connection. Got2Learn has an exciting video on how to solder copper pipes properly.


Step 7. Install the water heater expansion tank pipe.

Insert and secure a short-length copper pipe (not more than six inches long) to the tee fitting’s side outlet to create a horizontal connection. You can use push-fit connectors or sweat-soldering for this.

Secure a female pipe adapter on the expansion tank pipe’s other end using a push-fit connector or sweat-soldering.

Step 8. Install the water heater expansion tank.

You are almost complete with the thermal expansion tank installation.

Wrap Teflon tape around the expansion tank’s threaded fitting before fastening it to the horizontal expansion tank pipe’s threaded adapter.

Most plumbers recommend tightening the connection by hand to avoid overtightening it. Too tight, and you might damage the fittings or the pipes.

Secure the expansion tank in its mounting bracket.

Step 9. Connect the pipe supplying cold water to the tee fitting.

Use the correct pipes and fitting to connect the tee fitting’s top outlet to the pipe supplying cold water to the water heater. Most plumbers use a flexible copper water line for this step.

Step 10. Check your work before restoring power to the water heater.

Open your water supply line and check the water heater and expansion tank joints for signs of leaks. If you find any, you might want to close the water supply again and retighten the fittings.

If all goes well, finish the hot water expansion tank installation by turning on the circuit breaker or the gas supply valve.

You can also observe these steps in replacing expansion tank units.


Adding a water heater expansion tank can extend your water heater’s service lifespan and save you from costly repairs of damaged water appliances, plumbing, and water fixtures.

Did you find our guide on how to install a water heater expansion tank beneficial? Do you think your social contacts will find it advantageous, too? Sharing this tutorial with them is always a fantastic idea.

You can also send me your comments, feedback, or questions below, and I will be glad to answer or shed light on them.

5/5 - (2 votes)