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Do I Need an Expansion Tank on My Water Heater?

Note: This post may contain affiliate links. This means that at no cost to you, we may receive a small commission for made purchases.

Read on and we will answer these questions and give you a better understanding of what is going on with your water heater.

What Type of Water Heater Do You Have?

This may seem like an easy question but it is also one that many people do not actively think about. If you have a tankless water heater, you don’t have to worry about an expansion tank because you don’t have a primary tank to begin with.

However, if you have a traditional tank-style water heater, you will most definitely need to consider if an expansion tank is needed.

What Does an Expansion Tank Do?

First, you must understand how your water heater works. When water is heated inside your water heater, it expands. This process is called thermal expansion and it can cause strain on your plumbing system.

water heater expansion tank code

The way that it works is this: if your water heater holds 50 gallons of cold water, it will expand to about 52 gallons when it is heated. If the extra volume has nowhere to go it pushes on the walls of your plumbing.

You can think of this process as bending a paper clip; with enough bending, the paper clip will break. Your water heater is this paper clip; given enough pressure from the expanding and contraction, the tank itself can start leaking or possibly even burst.

There are some homes that run on an open water supply system, where the water pushes back into the city water supply and there is no additional strain on the home’s plumbing. Unfortunately, many homes have closed water supply systems, wherein the water supply has a one-way valve.

Many cities where homes have closed systems require the home owners to invest in an expansion tank. As a matter of fact, the manufacturer of your water heater may even void your warranty if you do not have an expansion tank with your closed system.

There are ways to test this and determine if you have a closed or open water supply system, but the process may be difficult because the backflow prevention devices tend to vary in appearance. However, you can always contract a plumber to perform this particular task.

Then where does the expansion tank come into play? Your expansion tank can help you to save your system. The expansion tank serves to take on the extra volume as well as any fluctuations in the incoming water supply pressure.

The expansion tank serves as a backup system to prevent overworking your plumbing. The excess water volume, instead of causing too much pressure on the plumbing, rushes into the expansion tank and lowers the pressure in the water heater to safer levels. This ultimately protects it from damage.

What Size Expansion Tank Do I Need?

There will be two factors that impact the size of expansion tank (see also ‘Expansion Tank Installation‘) that your house requires. The first of these will be the size of water heater that you have. You can look for this information on the factory label of the water heater itself. The second factor is the water pressure in your system. With these two factors, you can then make a decision about what size of expansion tank to get.

Each expansion tank manufacturer will have a calculator or chart which will tell you the recommended size and model of expansion tank you’ll need. There’s no universal approach so you’ll have to a do a bit of research or simply contact a plumber (click here for a free quote). Here are a couple examples:

You may, however, arrive at the situation where you are unsure if the size of the expansion tank is too small. The standard is to select an expansion tank that is larger than necessary rather than select one that is too small. This is because a tank that is too small can cause relief valve discharge. On the other hand, if the tank is too big, it has no negative impacts on your plumbing.

So Do You Need an Expansion Tank?

If you have a normal storage tank water heater, most likely you do. To be sure, do your due diligence on your particular model of water heater or contact a plumbing expert. This is one of those things you want to make sure you have right.

author avatar
Anthony Barnes
Anthony Barnes is the founder of Water Heater Hub and a second-generation plumber by profession. Before developing Water Heater Hub, Anthony Barnes was a full-time plumber, and he has undertaken a wide variety of projects over the decades. As a second-generation plumber, it was easy for Anthony to get used to the technicalities of all from a tender age


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