Do I Need an Expansion Tank on My Water Heater?

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To determine whether you need an expansion tank on your water heater, you first need to understand what type of water heater you have, what an expansion tank is, and what size of an expansion tank you may need. Read on and we will answer these questions and give you a better understanding of what is going on with your water heater.

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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.

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 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.

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  1. …water does not expand when you heat it. you would need to exceed 212 degrees (depending on the pressure you have it will likely be slightly higher than that) to get the water to turn to steam before you increase pressure.

  2. Jim Mooney it’s like most other fluids. Temperatures directly effect pressure in a sealed environment such as refrigeration or water system

  3. I have 5) 50 gallon electric water heaters in 4-plex. One meter, cold supply header to 3 in basement and other cold supply to attic on the other two.
    Do I need expansion tanks on hot side of each water heater? Seems like it is all one big system with check valve back at city meter ?? so any expansion could push back into. Single, perhaps bigger, expansion tank?

    • When selling our house the Buyers inspector insisted we have an expansion tank installed on our HW water well system which also had a pressure tank. Called our local plumbing and said in the 25 years of doing work on well systems he had never installed an expansion tank on said well. Contacted the Buyer and their inspector and they agreed after a little more research on their part. it was not needed. Just one opinion here.

  4. If your tank, have a pressure relief valve, and a pan under the tank with drain to the outside as required by plumbing code in my area; What do you need, value gained, from installing the pressure tank?

    • Thanks for that. I have a similar situation and my expansion tank for my well pump system is much larger than I would put on my new NG fired water heater. I’m told it’s now our county code but it’s silly because it’s redundant, and we’re not risking backing up water into the city system.

    • Good question. I don’t know of anyone who’s added an expansion tank to these small units. With that much water, there should be much expansion going on. You’ll still have a pressure relief valve at the top which might dribble out some water in rare cases. If that happens, then you can decide.


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