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How to Install a Water Heater Expansion Tank? – 10 Steps

Writen by Ronald Brown

Fact checked by Natalie Bridges

how to install a water heater expansion tank

A household expansion tank may resemble a mini boiler or propane tank. But it is a reliable overflow water safety device that reduces the risks of damage to your hot water tank when they thermally expand. It holds excess water to relieve the high-pressure levels inside your water heater.

Learning how to install a water heater expansion tank is relatively easy. You only need to prepare the necessary toolbox. You also don’t need to be a pro to pull off this installation, as the following details can help you handle it nicely.

Table of Contents

What Will You Need in This Tutorial


Depending on your DIY strategy, a thermal expansion tank installation can take a maximum duration of two hours. For starters, you must know how an expansion tank works.

The tank is divided into two compartments separated by the rubber diaphragm. The first side receives the excess incoming water, while the other side is usually dry and has a compressed-air chamber.

As the water pressure rises, the water presses against the diaphragm. There, the diaphragm then begins to contract and expand the air. Then, the compressed air makes room for the extra volume of water to relieve the system pressure.

A typical hot water expansion tank installation is not a complex setup to imagine. Although most manufacturers recommend installing the expansion tank vertically, you can also do it horizontally or diagonally.

When installing or replacing the expansion tank, make sure to prepare the following tools and accessories to complete this small-scale project :

Properly Sized Expansion Tank Unit

Selecting the correct size of your thermal tank is necessary to limit pressure fluctuations in your plumbing system. There are different versions of expansion tanks, and each of them requires a specifically-designed unit.

The consensus is that a hydronic expansion tank size should be double the anticipated expansion volume. Typically, a thermal expansion tank comes in two sizes: 2-gallon and 4.5-gallon.

For instance, a 50-gallon water heater needs a 2-gallon expansion tank as it expands to 52 gallons when water is heated. This size can also accommodate a 40-60 water heater tank.

An 80-gallon hydronic tank should have a 2-gallon pressure tank if your water supply has a default pressure of 40 PSI. Otherwise, if you have an 80 PSI service pressure, go for a 4.4-gallon pressure tank.

Installation Accessories

Generally, when you purchase an expansion tank, it already comes with a complete support kit, including a tee-fitting, female-threaded adapter, and short-length copper pipes. You would also use dielectric unions to keep the copper pipes from touching the steel pipe of your water heater.

  • Power Tools
  • Channel-lock Pliers or Pipe Wrench – You will use channel-lock pliers or a pipe wrench to install the expansion tank accessories. You can use either of them, but I prefer using a pipe wrench because it is mostly used in plumbing pipes (it fits into different sizes too).
  • Screwdriver and Screw heads – A screwdriver will help you lock the screws on the mounting bracket to secure the thermal tank. Don’t forget to prepare a screw head (1 and a half inches) for the mounting bracket.
  • Drilling Equipment (Optional) –You would only use the drill and bits if you need to mount the thermal tank on the wall (if you have limited vertical space above the water heater).
  • Teflon Tape – You would need Teflon tape (also called a plumber’s tape) to seal the dielectric union and threaded fitting of the expansion tank.
  • Mounting Bracket (Optional) –When installing an expansion tank, a mounting bracket is required for horizontal setups.

Step-by-Step Instructions


Now that you have completed the necessary tools and supplies, you are one step away from completing the task. Follow these beginner-friendly steps to install your expansion tank smoothly :

Step 1: Turn off the electric and water supply to your water heater.

Safety comes first before anything else. So, shut off the power supply of your water heater.

Electric shock hazards, explosions, and fires are common among home appliances, and your water heater is not an exception. The water supply should also be disconnected to prevent pipes from bursting/leaking.

Pro Tip: If you have a gas-type water heater, you might want to disable the shut-off valve for added protection if you suspect a gas leak in your home.

Step 2: Prepare the tools and equipment for the installation.

Bring out your thermal pressure tank and unpack the installation kit and tools. It is best to lay the supplies on a clean table.

Next, locate the cold water service line. This component is where you will install the thermal tank. The pipe that is coming from your home’s main water pipe running directly above the water heater is the cold water inlet.

Pro Tip: Before heading out to the installation, you must refer to the expansion tank diagram that fits your water heating system.

Step 3: If you will install the expansion tank vertically, mark the drilling spot on the wall.

Place the mounting bracket on the wall next to your storage tank. Then use a pencil to create the guide hole marks for the fasteners.

Take the bracket aside; Using your drill and bits, bore a hole in the marked areas.

Finally, put the mounting bracket back in place and make sure to secure the bolts, and use a screwdriver to secure the screws within the bracket.

Insert the bolts too (if applicable).

Step 4: Grab the dielectric union and attach it to the cold water inlet port.

Disengage the cold water pipe connected through the cold water inlet port. Apply two to three sheets of Teflon tape and wrap it around the top of the thread of the union clockwise.

Then, using a pipe wrench (or channel-lock pliers), thread it on the cold water entry port.

Step 5: Affix the adapter, copper pipe, and tee fitting.

Connect the female-threaded copper adapter on the reverse end of the dielectric union. You can do this by using a pipe wrench (or channel-lock pliers) to screw it until it fits securely. Lastly, assemble the short-length copper pipe and the tee-fitting on the adapter.

This is optional, but you may consider using sweat-solder equipment or a shark bite slip tee to ensure a steady connection.

Pro Tip: Before inserting the tee-fitting above the water heater, please allow 1 foot or 12 inches height allowance to provide adequate vertical clearance.

Step 6: Set up the thermal expansion tank pipe.

Install the short-length copper pipe on the side open slot of the tee-fitting to arrange the system horizontally. Using a sweat-solder or shark-bite tee, link the female pipe adapter on the opposite end of the expansion tank pipe.

Pro Tip: The copper pipe should be no more than 6 inches in length.

Step 7: Hook up your expansion tank.

For the second time around, use the Teflon tape to coat the threaded fitting of the expansion tank.

After that, secure and hand-tighten the same fitting on the horizontal threaded pipe adapter. You may now position the newly-bought expansion tank on the mounting bracket.

Step 8: Finish the installation.

The only thing you need to do is use appropriate pipes and connectors to attach the top outlet of the tee fitting on the cold water delivery line.

Pro Tip: Most plumbers recommend using a flex copper pipe because it can run around without a problem, making it more desirable for repair works.

Step 9: Open the water supply line and check for leaks.

If you find any leak in some part of the pipelines, turn off the main water supply. Then, retwist the fittings and make them more secure.

Pro Tip: Another way to check if you accomplish the hot water heater expansion tank installation correctly is to check the pressure inside the thermal tank using a manometer. The readings should be the same as the unit’s specifications.

Step 10: When everything works perfectly, restore the power and water supply.

You may now turn on the circuit breaker and the shut-off gas valve for gas water heaters. Then, voila! You just finished your expansion tank system installation!


The purpose of the expansion tank is to alleviate the water pressure and prevent your home’s pipeline system from overworking. As a result, it prolongs the lifespan of your water heater system.

Water is essentially incompressible, and when the pressure buildup inside the tank goes over the limit, there is a tendency for your water heating system to bulge or explode in the worst-case scenario.

Now that you’ve learned how to install a water heater expansion tank, replacement installations should be easier and more manageable for you next time.

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