Why Is My Water Heater Expansion Tank Leaking?

In this article, we’re tackling a somewhat complex issue. We’re talking about why your water heater expansion tank might be leaking and how you should fix this problem. We’ll give you a full guide, starting with how these tanks work, why they might leak, and the proper next steps to take.

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Your thermal expansion tank is likely leaking for one of two reasons.

The first possibility is that the point of connection between the hot water tank and the expansion tank is loose, causing the tank to leak and corrosion at the connection point.

Alternatively, the rubber bladder inside the tank might have corroded over time, causing leaks from within the water heater tank itself and rendering it useless. In either case, you’ll likely need a new tank. But there are steps you can take to address this problem before it occurs.

Below, we’ll give you more details on expansion tanks, proper care, and how to prevent leaks going forward.

What is a Water Heater Expansion Tank?

water heater expansion tank (if you have one) sits slightly above your water heater and connects to the water heater tank via a pipe. The tank itself looks like a propane tank and has a pressure relief valve attached to the top.

The expansion tank stores excess pressure created by the water heater. As your water heater heats the water, the water begins to expand. As the water expands, so does pressure within the tank.

In the past, much of this pressure can travel back into the municipalities’ plumbing, and you wouldn’t have to worry. But nowadays, many municipalities install check valves, which prevent the pressure from traveling back out of your home. As a result, this pressure needs to go somewhere, so the water heater expansion tank was created.

The water heater expansion tank takes on this excess pressure. The expansion tank contains both water and air. The expansion tank slowly releases some of this pressure to keep the water pressure in your home at acceptable levels.​

Without this tank, the water pressure in your home might become too high, which can cause wear and tear on plumbing and various fixtures over time. While it might not be immediately noticeable, there are very real long-term benefits of an expansion tank.

Why Is My Water Heater Expansion Tank Leaking?

As you can see, the water heater expansion tank serves a vital purpose in your home. Alleviating excess pressure from the tank essentially alleviates this pressure from your entire home. But what if your expansion tank is faulty?

Leakage is one of the most common problems that your expansion tank might experience. It might be especially prone to failure given the intensity of the pressure passing from your hot water tank to the expansion tank.

You can usually identify this problem visually, as you’ll notice leakage at the point where the hot water tank connects to the expansion tank, which might eventually start to rust or corrode.

Your expansion tank will usually start to leak in one of two ways:

The rubber bladder inside the actual tank wears out, which might eventually cause leakage from inside the tank itself. Once this bladder has worn out, the tank is no longer useful and will need to be replaced.

The point of connection between the hot water tank and the expansion tank begins to leak. This problem often occurs when the connection between the two components weakens over time, allowing water to escape, which eventually causes the connection point to corrode. This corrosion further weakens the connection which further diminishes the tank’s effectiveness.

How Do I Prevent This Issue Before it Starts?

As you can see, each of these issues stresses the importance of buying a high-quality expansion tank and installing it properly the first time. You can avoid both of these issues before it was is late. An experienced professional (a plumber) will install your expansion tank above your water heater when possible because this minimizes the likelihood of leaks. A good plumber will also ensure the connection is secure.

A high-quality tank will also use a steel threaded connection and high-quality materials. Generally, if you look for a tank with a long-term warranty, you can be sure it is built to last.

How Do I Stop My Expansion Tank From Leaking?

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Generally speaking, once your expansion tank has begun to corrode, it will be too late to salvage it. That said, if you catch the issue early enough, you might be in luck. If you begin to spot minor leaks, it might be a matter of simply tightening the connection between the water heater and the expansion tank.

This fix will be similar to the way the tank was installed and will be a quick touch-up. Contact a professional if you don’t feel comfortable handling this yourself. If your leaky water heater tank is too far gone (see the two scenarios above), then it’s probably time to replace it.

Replacement, whether you have an electric water heater or a gas water heater, is a relatively straightforward task for a professional, but you likely won’t be comfortable doing it yourself. A plumber knows how to address this situation and to safely and functionally install your tank.

Is a Leaking Expansion Tank Dangerous?

A leaking expansion tank can be dangerous over time. The expansion tank itself is a safety issue. So if the tank isn’t functioning properly, the water pressure running through your home might be too high which can eventually lead to plumbing and fixture issues down the line.


Thanks for reading our guide to hot water heater expansion tanks. These are an easily forgotten yet highly important add-on for your water heater. It might even be the case that your area requires them by law. Whatever you do, try to make sure you install your expansion tank properly the first time. Extra attention now could avoid a major headache down the line.

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