Water Heater Expansion Tanks: A Complete Guide

Last Updated On May 23, 2024

Updated on March 21, 2023



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This is a post about water heater expansion tanks, including sizes and types for installation.

Water heaters are a vital part of our lives. However, they’ve mostly overlooked until something goes wrong. 

We will also focus on an essential element of most water heating systems: the water heater expansion tank. 

So let’s begin!

What's In This Guide?

      What is a Water Heater Expansion Tank? 

      Evident from its name, a water heater expansion tank is an expansion tank for your water heating tank. It is a small secondary tank that is a crucial safety measure for your water heating system. 

      It helps alleviate overflow and excess pressure by acting like a receptacle and prevents damage to your water heating system. Here are the different types we often see.

      Types of Expansion Tanks

      There are two types of water heater expansion tanks or thermal expansion tanks:

      ●            Compression Tanks: 

      Also known as plain steel tanks, a compression tank is usually installed on top of the air separator. The primary function of this tank is to provide space for the water that has expanded due to the increase in temperature. Water increases in volume when heated, so this expansion tank facilitates the system by providing extra space for the heated water to flow into.

      ●            Bladder Expansion Tanks:

      This expansion tank is very similar to plain steel tanks. Still, it is made up of an extra membrane that allows it to separate the water from the air from being compressed. 

      It stops the air from getting absorbed into the system. It also eliminates the chances of corrosion on the tanks and simultaneously prevents water-logging issues.

      Helpful Video of Expansion Tank Installation:

      You can watch this video for further details. Another option is to consult your local plumber and have them inspect and recommend a few options if you are unsure. 

      How Do I Know If I Need a Thermal Expansion Tank? 

      Depending on where you live, you may be required by code to have a thermal expansion tank for your water heating system. Here are a couple of examples:

      • In Frisco, TX, it is mandated by law to have a thermal expansion tank and a pressure-reducing valve installed along with your water heating system.
      • According to the California Plumbing Code Section 608.3, it is mandatory to have a thermal expansion tank installed if your water heating system is a closed system.
      • Again, according to the International Residential Code IRC P2903.4, water heating systems, whether new or replaced, must have a thermal expansion tank installed.

      Regardless of laws, you should always have a thermal expansion tank if your water heating system is closed-loop, has a check valve, or recirculating pump. 

      This is because water expands when it is heated. It means that the volume of water also increases; this is called thermal expansion.

      Thermal expansion creates and builds up extra pressure inside the heating tank. This pressure affects not just the tank but the entire loop since it is closed. The pressure has nowhere to escape, so this can be dangerous. 

      Moreover, since the water increases in volume, it will need more space inside the tank to expand since the heating tank has a limited capacity, typically 50 gallons.

      Installing an expansion tank solves this concern because as the water expands, it can rush into the expansion tank instead of overflowing and building up pressure inside the loop. It alleviates the overall stress inside the loop and prevents any damage from occurring. 

      Now you might be wondering how you can know if your system is closed or open-looped. Here’s how you can tell if you have a closed heating system:

      ●     Ask a local plumber to identify it for you

      ●     The system has a one-way valve meaning water can’t return to the main supply. 

      Besides all this, there are a few other signs that tell you that you need to install an expansion tank:

      ●     Unusually high pressure inside the water heating system

      ●     Rattling noise from the water heater

      ●     Leaks and water dripping from the system at the t&p valve

      ●     It has been 2-5 years since you last repaired or installed your expansion tank

      How Does a Thermal Expansion Tank Work?

      Thermal expansion tanks have only one job. And that is to accommodate the extra expanded water once the main tank is fully utilized and begins to overflow.

      It prevents unwanted damage to your heating system and the entire household because excess pressure may build up to the point that it bursts out and causes leaks and failure in the system. 

      How To Find the Right Water Heater Expansion Tank?

      Finding an efficient water heater expansion tank is a relatively easy task. It is so easy that you can even do it yourself. 

      Ideally, you want a proper-sized expansion tank to get the most bang for your buck. So you should take the following things into account when choosing an expansion tank:

      ●     The capacity of your main tank or water heater

      ●     The pressure of your water supply

      Click on this image to find a variety of sizes and prices to get an idea of what’s available:

      water heater expansion tank options

      What Happens When You Choose the Wrong-Sized Water Heater Expansion Tank?

      There is no universal tank size for water heaters. So ideally, you would want an expansion tank with enough capacity. It’s always a safer option to get a bigger one because if your expansion tank is small, it won’t be able to do its one job. Make sure it can adequately hold the expanded water and pressure. 

      How Long Do Water Heater Expansion Tanks Last? 

      There is no surefire way to determine and predict how long a tank will last. But since almost every tank has a 1-5 year warranty, be prepared to repair or replace your expansion tank every 2-5 years to be safe. However, many tanks will last over 10 years without an issue.

      Another thing you should do is have your water heating system routinely checked. A general rule of thumb is to inspect your water heating system yearly. This will ensure that everything is running smoothly. 

      On top of this, you’ll be able to repair and take action for any problems identified during those inspections before something more costly goes wrong.

      Can You Install An Expansion Tank Yourself? 

      Yes, while you can install an expansion tank yourself, it is always a wise and better decision to get it installed by a plumbing professional. Even if it costs you a bit of extra money, having a professional do it will eliminate any chances of mistakes. 

      It will ensure everything is set up and installed correctly. Remember, it is always better to be safe than sorry.

      How to Install a Water Heater Expansion Tank

      This supply list and walk-through are for demonstration purposes only. Make sure you are qualified (or licensed) when doing any plumbing work around the household. 

      It’s always easier to first confirm that you have the necessary tools and resources before you even start working. Here is a list of possible things you may need during the installation of this expansion tank:

      ●     Wrenches

      ●     Pliers

      ●     Water heater nipples, small and large

      ●     Flex pipes (copper ones)

      ●     Any fittings, if needed

      ●     Soldering tools

      ●     Plumber’s tape

      ●     Pipe or joint sealant

      Before they install a water heater expansion tank, the individual must perform these preventive measures: 

      ●     Turn off the main water supply line connected to the cold side of the heater.

      ●     Drain the system of any remaining water inside the system and release the pressure.

      ●     After reducing the pressure and draining the water, turn off the drain valves. 

      Special Note: many manufacturers required the tank be installed at least 18” from the cold water inlet above the water heater tank. If an inspection is required, they may require you to relocate it. 

      These are the steps a plumber would typically use to install the expansion tank:

      ●      First, they must remove the copper flex line from the inlet side.

      ●     Next, they need to install a tee where the flex line previously was. Remember to use enough plumber’s tape and joint-sealing compounds to ensure a leak-free connection.

      ●     Now, they need to prepare the ends of the long nipple by wrapping the plumber’s tape around it.

      ●     After that, they need to attach an elbow tightly to one side of the long nipple, and then thread the side of the tee onto the other end of the nipple. Ensure that they are tightly connected.

      ●     Now, for the top tee, they need to apply joint compound or wrap the plumber’s tape around the short nipple, as it will connect in the place of the flex line previously removed.

      ●     Following this, they need to reconnect the main line that supplies water to the short nipple.

      ●     Lastly, they need to screw the expansion tank to the elbow that was installed and ensure all the connections made are tight and leak-free.

      Note: Check the water pressure on the house, and verify the tank is pressurized to this same PSI.

      Water Expansion Tank FAQs 

      Do I Need An Expansion Tank On My Water Heater?

      You only need an expansion tank if and only if:

      -Your water heating system is a closed-loop system

      -Your water heating system has problems like leaks and not working optimally 

      -It is required by local plumbing code

      How Often Do You Change An Expansion Tank On A Water Heater? 

      You should be prepared to check your expansion tank every 2-5 years. Some expansion tanks will last for 10+ years, but others might need to be changed more frequently.

      What Happens When A Water Expansion Tank Fails? 

      When a water expansion tank fails a lot of things can happen. For instance:

      ●     Water leaks through T&P valve

      ●     Pipes bursting

      ●     Rattling noises

      ●     Corrosion

      ●     Explosion due to pressure buildup (rare)

      How Do I Know What Size Expansion Tank I Need For My Water Heater?

      Consult your local plumber. If you want to do it on your own, then you need to calculate the following:

      ●     The capacity of your water heater

      ●     The PSI or pressure of your water supply in your household

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      We write about “all things plumbing,” helping you navigate common questions, repairs, and the best plumbing products on the market.

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