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Water Heater Thermal Expansion Tank: Explained

water heater

Clients in Carrollton, TX, and the surrounding areas frequently inquire about their water heater's thermal expansion tank, asking questions such as:

  • What is that strange-looking tank directly above my water heater?
  • Why did my plumber tell me I need a water heater expansion tank?
  • I'm not getting good hot water pressure all the time.. what's going on?

If you've recently wondered about any of these questions, keep reading! The expert plumbers at Jennings Plumbing Services have crafted this guide on water heater expansion tanks just for you.

Note: This article is intended for self-educational purposes only. All Plumbing work should only be done by a licensed, insured plumbing professional. Jennings Plumbing is not responsible for any damage or injury resulting from the use of this information. Use of any information provided on our website is solely at your own personal risk.

How Does an Expansion Tank Work?

While the water inside your water heater won't reach boiling point, it will still expand as it heats up. This phenomenon is known as thermal expansion, and that's where the expansion tank comes into play.

Typically, the extra pressure from thermal expansion is safely discharged into your local municipality’s water supply system without causing any issues. However, if a plumber has installed a check valve or a pressure regulating valve (PRV) on the water supply line to your home, pressure from thermal expansion can build up inside your plumbing system. This trapped pressure can stress your supply lines, fixtures, and appliances, potentially causing significant property damage.

A properly installed thermal expansion tank helps prevent this by releasing the excess pressure before it can damage your home's plumbing. When in use, only half of the expansion tank is filled with water; the other half contains compressed air and a rubber bladder in the middle. As the water in your heater heats and expands, it pushes against the bladder, compressing the air on the other side.

What Does A Thermal Expansion Tank Look Like?

Most expansion tanks intended for residential use look very similar to a small propane tank. The expansion tank pictured above is a 2-Gallon thermal expansion tank that Jennings Plumbing Services installed on a brand new 50-gallon water heater in a residential home. This one as you can see is off-white or beige in color. They are also produced in the color blue but these can be far less common.

On top of each thermal expansion tank, is an air valve like you see on most car tires. This is also known as a Schrader valve. On the bottom, there is a threaded connection where the expansion tank attaches to the water heaters plumbing system. A 2 Gallon expansion tank is roughly a few inches bigger than a football in height and width. The 5 Gallon versions used on dual water heater set up or 100-gallon water heaters are just a little smaller than a water cooler.

Am I Required To Have A Thermal Expansion Tank?

A thermal expansion tank is always a good idea if you have a ‘closed-loop system’.

Much like high blood pressure can eventually wear out your internal organs, excessive water pressure in your plumbing system can also wear down the appliances and fixtures in your home. The weakest parts of your home plumbing system are typically the toilet fill valves, the supply lines, and the solenoid valves found in washing machines, dishwashers, and ice makers. Excessive pressure from thermal expansion is much more likely to cause these items to fail prematurely leading to lost time and money.

If you have excessively high water pressure (anything over 80 P.S.I.) coursing through your home by the city water supply, an expansion tank by itself cannot do anything to correct this. In this particular instance, you will need a PRV to be installed on the incoming water supply lines just after the city water meter. When the pressure supplied to your home is at the proper level (between 40 and 80 P.S.I.) and you have a PRV or check valve, you have met the proper specifications for Thermal Expansion tank operation. The purpose of the Thermal Expansion Tank is to protect against the constant fluctuation caused by your water heater on your properly pressurized hot water system.

In the State of Texas, the requirement of a Thermal Expansion Tank is regulated by your local municipality. For instance, in Frisco, Texas, a PRV and expansion tank are currently legally required on all new homes. Also, at the time of writing this, an expansion tank is required on all water heaters installed or replaced in Little Elm, TX.

Check with your city’s plumbing inspector or municipal website to be sure of the applicable plumbing codes in your area. If you live in or around North Dallas and the surrounding areas you can also call our local office at (972) 458-5584 and we can quickly and easily let you know what your local guidelines are present!

What If I Have More Than One Water Heater?

Depending on the size of your home your plumbing system may have more than one Water Heater. That’s ok! In these instances, it is typically best to use one two-gallon thermal expansion tank per heater or install one 5-gallon thermal expansion tank for both, so long as they are on the same close looped system.

But again, it is always good to check with your local Texas municipality to see what their requirements are! It is also good to check the expansion tank manufacturer’s specifications.

How Long Will My Thermal Expansion Tank Last?

It is very hard to predict the lifespan of a Thermal Expansion Tank.

With so many variables that can factor into how long one lasts, such as the manufacturer's material or production quality, water quality, and proper inflation of the tank. It is not uncommon for tanks to fail in as little as two years, and in other situations, there are some that have lasted eight years or longer. At Jennings Plumbing Services we recommend the replacement of your thermal expansion tank as soon as the manufacturer’s warranty expires.

Most of the tanks we install are normally backed by a one or five-year manufacturer’s warranty.

Do Thermal Expansion Tanks Leak?

Yes, like every other part of your home’s plumbing system, the expansion tank is subject to failure with use.

They normally fail in two ways. First, the rubber bladder inside them wears out and the tank ceases to function as a way to mitigate thermal expansion. Second, the point of connection between the water piping and the tank can corrode and begin to leak.

One thing you can do to ensure damage is not caused by the expansion tank is to have it properly installed by a skilled, licensed plumber. A good plumber will install the tank, making sure it is properly supported and has a good connection. He will also position it (when possible) over the water heater pan or somewhere it will minimize the risk of damage if it leaks.

Another thing you can do to boost the longevity of your thermal expansion solution is to purchase a quality tank with an extended manufacturer warranty (5 years). Good tanks have a stainless steel threaded connection and are made from quality materials that will give the expansion tank some longevity.

Most manufacturers recommend checking expansion tanks yearly when new. They also recommend checking them more often as the tank gets older.

Is My Thermal Expansion Tank Working?

NOTE: Before testing, confirm that there is a functioning shut-off located on the waterline preceding the expansion tank itself. If a failure occurs while you are testing, it could cause severe damage to your home or other property. We always recommend using a licensed professional for any testing. Should you see any signs of corrosion, shut off the water and call in a licensed professional. Jennings Plumbing Services does not accept responsibility for damages caused by improper testing or failure of fittings during testing with the use of this information.

There are a couple of different things you can do to tell if your expansion tank is working properly:

Method 1: Checking Water Pressure

One of the easiest ways to tell if your thermal expansion tank is working or not is to monitor the water pressure visually. To perform this visual test use the hot water for a longer period of time, and then do not use any hot water for a while thereafter. In this instance, if your thermal expansion tank is working properly there will be no difference in pressure. However, should your expansion tank be faulty you will then notice there is higher pressure that will gradually decrease back to normal as the water continues to run.

We recommend turning on a faucet in order to visually inspect the water pressure for this test. Please note that this is not completely definitive, considering there are other factors that can affect the water pressure in your home.

Method 2: Sound Test

Another quick and easy way to check the integrity of your thermal expansion tank is to take a metal object like a coin and tap both the top and bottom of the expansion tank. The top side of the expansion tank is supposed to be filled with air so there will be a distinctly hollow ring to it when you tap the top. Tapping the bottom of the tank should result in a clang that is indicative of water being present on the underside of the bladder within the expansion tank. If you notice zero difference in the sounds when the observation is made then you likely have a thermal expansion tank that may be compromised.

Method 3: Schrader Valve Test

One other quick and surefire way of testing your thermal expansion tank's integrity is to perform a quick test on the valve on top of the tank. By opening this Schrader valve and releasing air you confirm that the tank is operational. Should water be emitted during the test instead of air then your expansion tank has been compromised.

Pressure Check: Advanced

The things you'll need for this test are:

1x Bike Tire Pump
1x Water Pressure Gauge

  1. Turn on your hose bib and let it run for about 15 seconds. This will relieve any excess pressure created by thermal expansion. The remaining pressure will coincide with just the pressure your PRV is set to.
  2. Attach your water pressure gauge to the hose bib and determine the water pressure.
  3. Turn off the water to your house with the main shut-off valve that is typically located in your front yard.
  4. Open up a faucet inside your home and drain all of the pressure off of your system.
  5. Go to the expansion tank and connect the outlet of your bicycle pump up to the Schrader valve.
  6. The gauge on your pump will tell you how much air pressure is in the tank.
  7. If the pressure reading you got in step 2 does not match the tank's pressure reading, you can adjust it with your pump to the proper P.S.I.
  8. After disconnecting your pump, you can turn the water supply to your house back on and double-check for leaks.

After performing this procedure, you can be assured your expansion tank is now working as it should. It is very common for plumbers to overlook this step during the installation or replacement of your expansion tank.

Have More Questions?

At Jennings Plumbing Services, our skilled technicians are fully trained to diagnose and resolve any expansion tank issues and ensure proper installation. If you have any questions or concerns about your expansion tank, don't hesitate to reach out to us—we're here to help. Also, be sure to check out our customer reviews!

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