Plumbing, Water Heaters

Water Heater Expansion Tanks: What They Do & Why

Expansion Tank Lg

If you own a water heater, the water inside will expand when heated. An expansion tank helps relieve pressure inside the water heater by providing a space for hot water to expand and release the increased pressure inside the water heater.

However, today’s water system is developed to prevent any backward release of pressure referred to as backflow. There is a check valve employed in these systems to prevent the backflow.

Expansion tanks are safety devices connected to the cold water supply that helps control the thermal expansion of hot water inside a water heater. When water is heated, the pressure can rise beyond the water heaters limit without an expansion tank. Increased pressure can damage the water heater tank, supply pipe joints, and valves in plumbing fixtures. Expansion tanks provide an outlet to prevent hot water backflow and relieve pressure from the water heater. 

Let’s look at what water heater expansion tanks do and why they are necessary.

What is an Expansion Tank?

An expansion tank is developed to lessen pressure in both closed hydronic and potable water heating systems. In addition, it ensures that there is constant pressure in the pipes so that they don’t get damaged by excessive force. 

According to Daniel O’Brian, a technical expert at, an expansion tank is employed in a heating system to protect the entire system from increased volume and pressure. When you heat water, it’ll expand, and in the case of closed heating systems, there isn’t enough space in the boilers and pipes to accommodate the expansion. 

If the water takes up more spaces and lacks enough spaces to occupy, the pressure will increase and damage the system. Damage to the system may result in a pipe burst or leak. Since an expansion tank relieves any stress from the tank, it’s meant to increase the lifespan of the components in the heating system. 

Why was an Expansion Tank Developed?

A few years ago, there were no expansion tanks. However, water heaters have been around for a long time now. 

In the past, when you heated water, the excess pressure could only be relieved by being pushed out of the home into the main water line. This system is referred to as the open system and would at least dissipate the pressure. 

However, due to safety and health concerns, a lot of homes nowadays require a check valve. A check valve has an internal spring that is followed by a rubber gasket. 

The spring is placed on the side of the water pressure that flows into the home. The rubber gasket, however, is placed on the other side. The spring allows water to press upon it and create space between the inner walls and the gasket, thus allowing water to pass through. 

Since the flow is unidirectional, the gasket side functions as a seal that prevents water from flowing in the reverse direction or exiting the building. In addition, it leads to the pressure pushing up on the gasket and forming a watertight seal. In such a case, therefore, the backward flow of water is restricted.

For instance, if a puncture happens in the main water line, a vacuum is created to suck water from the surrounding areas. But, unfortunately, this will also lead to sucking up dirt, substance, and chemicals into the system, thus infecting the water supply. Nobody wants such a case to happen, thus explaining the invention of the check valve

Another scenario involves a case where there is a hot water recirculation system. Assuming you use a water pump, there is a high chance that the hot water is pushed back into the cold water supply. Therefore, it automatically calls for the installation of a check valve.

When you install a check valve, you get a closed system. And if the flow of water pumped into the system is closed off from flowing back, there will be an expansion in the system. It will automatically call for a remedy that absorbs the excess pressure, thus explaining the importance and need of an expansion tank. 

How The Expansion Tank Works

We all know that water isn’t compressible. So when you heat water, it can’t compress, thus builds up pressure as it expands. The process is referred to as thermal expansion. An expansion tank comes in to correct this issue. 

All expansion tanks function by equalizing pressure in the entire system. An expansion tank is simply a smaller tank that has two sections separated by a rubber diaphragm. The first section connects to the pipes of your heating system and has water. There is also a bladder on the interior and is filled with pressurized air at 12 psi

When hot water gets into the heating system, the system pressure automatically increases. An increase in pressure pushes down the diaphragm found in the expansion tank. As a result, the air in the tank is compressed, creating enough space for any excess water to enter. 

The outcome is a relief of excess pressure from the system and ultimate prevention of damage to the pipes. In addition, the bladder restores its normal position as soon as the water starts to cool off. 

The water supply line’s connection to the expansion tank (PEX, CPVC, PVC, or copper) attaches to a Female Iron Pipe (FIP) fitting and then screws onto the tank. You get a watertight seal if you add Teflon tape around the voila and threads.

Installing an Expansion Tank

The process of installing an expansion tank is relatively straightforward and might take you less than one hour. However, there are local building regulations and codes that only permit installations by licensed plumbers. It is therefore essential to confirm everything before you install the tank. 

Assuming that you are allowed to install the tank, you can follow the steps below to manage a successful installation:

Step 1: Disconnect the Power Supply and Water

The first step to installing an expansion tank is to disconnect the water supply connecting to the water heater

In case the unit operates using electric power, you should switch off the breaker. If your system uses gas, turn off the gas supply. You can contact your gas provider to help you if you don’t know how to switch off the gas.

Step 2: Bring out Accessories and Expansion Tank

At this point, you should find an expansion tank that works well for your water system. You can determine the perfect size of the heater required even before getting the tank. 

Remember to get tee fitting and plumber’s tape. It would also help to have some mounting brackets and some screws. 

Step 3: Check the Cold Water Supply Line

Go to the cold water line and identify one that is horizontal with the water heater. This line will be required for connecting the unit.

Step4: Installing the Expansion Tank

Installing the tank should be your top priority at this point. It would be best to have the tank above the pipe supplying cold water closer to the heater. You can then mark a spot and drill holes on the wall where you’ll mount the pipes. 

Finally, screw in the tank and connect the cold water supply line. 

Step 5: Connect the Heater tank

Go to the bottom of the connector of the expansion tank and use a plumber’s tape to wrap the spot. Next, link the connector to the tee fitting and then secure the connection using a wrench. 

Step 6: Restore the Power Supply and Water to the Heater

Start by turning on your water supply unit that connects to the heater. You can follow up with switching on breakers. If the heater operates on gas, turn on the gas. 

Step 7: Check the Expansion Tank

Turn on the faucet to test the expansion tank. You can put your hand below the tap to check whether the water is hot or not. 

When Do You Need an Expansion Tank

It would be best to have an expansion tank on either a closed system or an open system.

On an Open System

A pipe running along the side of a hot water tank is developed either from CPVC or copper. The water line is attached to a pressure relief valve. 

A pressure relief valve is available on all tanks. The valve opens when necessary to relieve any pressure that builds up in the water system. It dumbs water either on your basement floor or into the drain. 

The valves are rated at 150 psi but can reach as high as 160 psi. In some cases, the valve will continuously open up and let water out. It means that the main water supply has a very high pressure together with the thermal expansion of your tank. 

If water comes out of the tank or random wet spots below the tank, you require an expansion tank to build up pressure. 

Closed System

When the thermal expansion occurs in a closed system, it’ll build up pressure in the expansion tank and force water to find relief from the stress. In such a case, it won’t be enough to have pressure relief. 

Several things result from a closed system experiencing a lot of pressure and including: 

  • There will be drips since pressure from the water will be too high for rubber gaskets in the fixtures.
  • Your hot water tank flue might fail, leading to the leakage of toxic gases into your house.
  • The host water tank might rupture
  • The pressure relief valves start dripping off the water.
  • Your toilet ball cock might begin to leak, filling it up and overflowing over time. 

Is an Expansion Tank Relevant If I Have a Pressure Reducing Valve?

The function of a pressure-reducing valve is to reduce high pressure in your home. The valve isn’t found in a house unless the pressure there is above 80 psi. However, pressure in most homes ranges between 40-75 psi. 

A pressure-reducing valve is, therefore, not necessary in most houses. However, in cases where it’s essential, an expansion tank is still required. Consequently, you should contact a professional plumber to advise you on the possible solution in such a situation. 

Choosing Expansion Tanks

Expansion tanks differ in capacity and size. You can use an expansion tank sizing calculator to determine the perfect size for your system. 

There are different types of expansion tanks that you can choose from, including Therm-X-Trol, Amtrol’s Extrol, and Radiant Extrol:


These expansion tanks are ideal for domestic water systems and are developed from non-ferrous materials. Their characteristics include: 

  • Reduces any risky pressure build-up
  • Secure water fixtures and heater
  • Eliminates energy and water waste
  • Keeps relief valves closed

Extrol Expansion Tanks

These tanks are commonly used and have the following characteristics:

  • They are simple to install
  • It offers a permanent separation of water and air
  • Normalizes and controls the pressure of a water system
  • Employs a Butyl/EPDM diaphragm to manage better air retention

Radiant Extrol

Radiant Extrol tanks can function in both closed and open-loop radiant heating systems. Their characteristics include: 

  • They non-ferrous, thus resisting corrosion
  • The tank is used with glycol
  • Appropriate for non-barrier and barrier PEX systems
  • Developed for high-efficiency radiant systems

Cost of Tank Costs

Expansion tanks are more affordable than most people think. With just $30, you can get a small residential tank. However, the price could climb up to $1,000 for large commercial tank systems. In addition, the price varies with both size and brand.

The leading expansion tank brands include the Extrol expansion tank that Amtrol manufactures. It is employed in hydronic heating systems, the Bell & Gossett HFT, and Watts ET series expansion tanks. Another reputable brand, as we’ve seen above, is the Therm-X-Trol which is employed in portable open water systems.

Maintenance of an Expansion Tank

If you have an expansion tank installed in your home, it will help if you check it periodically to function correctly. To do this, place your hand on the tank to feel its temperature. 

The top section of the expansion tank should be warm, while the bottom section should be at room temperature. However, there are cases when you find that the whole tank is warm, mainly due to a failure of the diaphragm. If this ever happens, you should immediately replace the tank. 

If you can replace the tank by yourself, you should contact a professional plumber to help you out. 


Hubert Miles | Licensed Home Inspector, CMI, CPI

Hubert Miles is a licensed home inspector (RBI# 2556) with more than two decades of experience in inspection and construction. Since 2008, he has been serving South Carolina through his company, Patriot Home Inspections LLC. As a Certified Master Inspector, Hubert is dedicated to providing his expertise in home inspections, repairs, maintenance, and DIY projects.