Water Heater Expansion Tanks — What Are They?

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Categories Water Heating
That extra tank next to your water heater is more important than you think and here’s why!

Do you have a closed water supply system? If you do, does it need a water heater expansion tank?

How necessary are they and can you get away with not having one? In this article, we’ll discuss everything you need to know about water heater expansion tanks.


Open vs. Closed Water Supply Systems

Whether or not you need a water heater expansion tank depends on both your unit type and your water supply system. If you have a tankless water heater, you don’t need to worry about installing one.

You also don’t need to worry about it if your home is part of an open water supply system. When a plumbing system is “open,” it has the ability to flow back into the city’s water supply.

A closed system is just the opposite. Water can flow into your home, but the only way it can flow out again is through your faucets or shower.

What Are Water Heater Expansion Tanks?

In order to heat the water in your home, your water heater uses thermal expansion (1). This works exactly as it sounds. The water inside of your unit expands as it gets warmer.

Thermal expansion will cause strain on your pipes if the water doesn’t have anywhere else to go.

Because most households have a closed water supply system, expansion tanks are required with any traditional tank water heater. These contraptions are essential in increasing the longevity of your unit and will save your pipes too.

An expansion tank comes in handy because it can take the extra volume of water after it’s expanded. It also serves as a resource to help regulate the pressure of your water too.

How to Install a Water Expansion Tank

Once you’ve determined that you need an expansion tank, your next step is to figure out what size tank you’ll need. The correct size will depend on the size of your water heater as well as the water pressure in your system.

There’s no one way to figure out the size you need, but each expansion tank manufacturer provides a guide. This guide will tell which of their tanks will fit the requirements you have.

An expansion tank that is bigger than necessary won’t cause any issues. However, a tank that’s too small can cause pipe leaks as the pressure relief valve overworks to keep up.

To check your water pressure, use a pressure gauge on your pressure relief valve and periodically check the pressure over 24 hours. The recommended water pressure is 50-60 PSI. If it’s any more than that, you’ll want to make sure you also have a pressure reducing valve.

Pressure reducing valves help provide steady pressure to your system. It regulates the water hammering and works with your pressure relief valve to protect your water heater and pipes (2).

Most expansion tanks will be available in either a two or five-gallon size. Once you have your tank, you’re going to connect it to the cold water line. Here is what you should do to retrofit it to your system.

  1. Solder a T fitting or use a saddle valve that bolts onto the pipe.
  2. Adjust the air pressure in your expansion tank with a tire gauge. The air pressure should match the maximum water pressure in your home.
  3. To add more air into the tank, you should use a hand pump rather than an electric pump. Using an electric air pump can cause the tank to blow.
  4. Attach the tank to your cold water line. It should fit right into where you installed your T fitting or saddle valve.
  5. Turn your hot water faucet on until a steady stream flows.

Replacing an Old Tank

If you find yourself in a situation where you need to replace your existing expansion tank, here’s what you should do (3):

  1. Locate your old tank: They’re usually found right next to your water heater.
  2. Record your old tank’s measurements and information: You’ll need the dimensions and connections to make sure you buy the right size.
  3. Find your desired expansion tank.
  4. Turn off your power and water supply: You can do this by closing the shut-off valve. You’ll need to give your water heater around an hour to cool down.
  5. Drain the water from your water heater: Open the bleeder valve and let the water drain into a bucket.
  6. Disconnect your old expansion tank.
  7. Pressurize your new tank: Make sure the tank starts at zero and use a bicycle pump to get it up to 12 PSI or the amount recommended by the manufacturer.
  8. Check the pressure and use Teflon tape: The tape should only be wrapped around the ends of your connectors.
  9. Install your new expansion tank.
  10. Turn on your water supply: Open a faucet and see if you have running water. You should also check for any leaks.
  11. Turn on the power supply and water heater: Once the system has heated up, test your hot water.

How Much Do Expansion Tanks Cost?

Expansion tanks will typically cost between $40 and $70 (4). When talking budget, you should decide whether you plan to install the system yourself or if you’ll call a professional. If you aren’t comfortable dealing with electrical and plumbing lines, we recommend the latter.

But if you do decide to hire someone, you’ll also need to factor in the cost of installation. Labor costs vary for each company. After scouring the internet, we’ve found companies that charge anywhere from $150 to $800+ to install an expansion tank.

Troubleshooting Your Expansion Tank

No system is perfect and there are sure to be times when you experience trouble with your expansion tank. Problems could stem from the wrong size tank, but in some cases, it means something is wrong with your unit.

If your expansion tank loses its air cushion, the pressure during each “on cycle” can be too high. This can wreak havoc for your water heater’s pressure relief valve (5).

Older expansion tanks also don’t have an internal bladder to separate air and water. So, if your unit is older, you may experience a loss of hot water. This commonly happens when air mixes with your hot water in the system.

Waterlogging and How to Deal

Over time, your expansion tank can become waterlogged. This can happen for a few reasons.

In older systems without a bladder, waterlogging happens when air finds its way in and out of the tank. When you lose so much air, you lose air pressure, causing the water in the tank to be stagnant. If this happens, you may be required to drain your system.

However, if you have a newer system and your tank gets waterlogged, it’s very possible that the internal bladder has ruptured. Waterlogging in a newer system could also be due to pinhole leaks in your tank.


The Unsung Hero to Your Hot Water

Who knew just how important expansion tanks could be? They aren’t just another add-on that plumbers try to upsell you on. These contraptions are necessary for keeping your home “hot water plentiful” and essentially flood-free.

For those of you with traditional tank heaters, go check right now to make sure you have an expansion tank. If you do, great, but if not, check if you have a closed-supply system. If you don’t have an expansion tank and your water supply system is closed, call a professional or go buy one.

Any more burning questions about water heater expansion tanks? Leave us a comment below and we’ll answer anything you want to know!

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About the Author

Peter Gray

Peter has been a homeowner for 35+ years and has always done his own repair and improvement tasks. As a retired plumber, Peter now spends his time teaching others how they can fix leaks, replace faucets, and make home improvements on a budget.
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