What Size Water Heater Expansion Tank Do You Need?

We all hear horror stories of havoc caused by water heaters, and dread the day it happens to us. Luckily, there are safety features in place, and accessories we can install that can help prevent these disasters. 

One safety feature and accessory is a water heater expansion tank. But how am I supposed to know what a water heater expansion tank is, or how to choose one? With just a few pieces of information, you’ll be able to determine what size expansion tank you need.

In general, moderately sized water heaters with 40-60 gallon capacity require 2-3 gallon expansion tanks. Larger homes with 80 gallon water heaters need 3-5 gallon expansion tanks. There are a few pieces of information that can pinpoint what size you need, such as water temperature and pressure relief gauge rating.

Here, we cover what an expansion tank is, what it does for your water heating system, and how to determine what size tank is best for your home.

What is an expansion tank?

You might be wondering what an expansion tank is. While it isn’t a topic we hear about a lot, this appliance plays a very important role in supplying our homes with heated water.

Basically, an expansion tank is an accessory to your water heater. As we know from our science classes, water expands when it’s heated. This thermal expansion can outsize the water heater tank and put undue pressure on the plumbing system. Modern plumbing systems are designed to handle quite a lot, but dealing with excessive pressure on a repetitive basis will cause excessive wear and tear.

Expansion Tank

Prolonged and excessive thermal expansion can also cause damage to your water heater. These appliances have pressure relief valves to release heated water when they detect excessive pressure, but like any other component, wear and tear will cause it to fail. A failed pressure relief valve can lead to costly water damage in your home, on top of the water heater damage.

To solve this problem, engineers designed expansion tanks. Expansion tanks serve to take on excessive water volume created in a closed-loop water system. Heated water is temporarily stored in these tanks, relieving excessive pressure from your appliances and plumbing system.

The Dynamics of an Expansion Tank

Now we know that the expansion tank holds excessive water caused by thermal expansion, but how does it get there? It turns out that an expansion tank isn’t simply a tank, there are internal components to help it work.

In the event that heated water expands beyond the water heater’s capacity, the excess volume will flow out into the plumbing system, applying pressure until a hot water faucet is opened to release this pressure. The expansion tank acts like a buffer between the water heater and the plumbing system to catch the excessive water volume.

Water expansion tanks are divided into two sections by a pressurized rubber diaphragm. The rubber diaphragm reacts to an increase in pressure from the heating system by moving to create negative pressure inside the expansion tank. The space created by the diaphragm allows excess water to flow into the tank, thus relieving the water heating systems from excessive pressure. 

Do I Need an Expansion Tank?

All of this sounds great, but does my water heating system even need an expansion tank? Aren’t there safety features installed in my plumbing system?

There are safety features in plumbing systems, especially modern designs. As we discussed before, water heaters are equipped with pressure relief valves, and modern plumbing systems are equipped with a similar mechanism called a water pressure regulating valve.

While these safety features are effective, they can only do so much if you have a closed-loop system. This is where your system has a backflow preventer, which keeps water in your home from flowing back into the main water supply that services other people. Essentially, if there is excessive pressure in the plumbing system, there is nowhere for it to go except through a faucet in the house.

This is where the expansion tank comes in to relieve your system of the excess water. In some areas, local law actually requires the installation of an expansion tank if you are on one of these closed-loop plumbing systems. So, if you aren’t sure what type of system you have, it’s best to consult a professional plumber. They can tell you what you have, and local regulations regarding an expansion tank.

How to Decide What Size Expansion Tank You Need

Once you’ve figured out whether your water heating system needs an expansion tank, it’s time to get the right one for your home and water heating system. Here are a couple of things you need to do before you make this decision or plan for installation.

Measure your water pressure

To make sure you choose a tank to handle excessive water pressure, you need to know what your static water pressure is first. This is very easy to do, and hardware or home improvement stores sell cheap water pressure gauges that you can attach to spigots with a hose fitting.

Before you measure your water pressure, make sure all faucets are closed, and no appliances are running water, such as your dishwasher or washing machine. Now attach the gauge to the spigot with the hose attachment and read your static water pressure. Take a photo, or write this down because you’ll need this measurement later.

Determine your water heater capacity and temperature setting

Water heaters for the home normally hold between 40 and 80 gallons. You can determine what your water heater holds by locating the label on the appliance. This label is usually on the side of the tank. 

Now that you know the capacity, check to see what temperature setting your heater is on. The temperature ranges between 120 and 140 degrees Fahrenheit, and it is indicated on a dial or digital programmer on the water heater.

Some models have Hot, A, B, and C settings, especially gas water heaters with dials that control settings. In this case, Hot is 120 degrees, A is 130 degrees, B is 140 degrees, and C is 150 degrees. If you are unsure, check with your manufacturer. 

Now locate the pressure relief valve, which is on the side of the tank. The pressure rating of the valve should be labeled on a circular tag near the pressure relief valve. Add this pressure rating to your list of information.

Calculate your expansion tank size

Now you have all of the information you need to decide what size expansion tank you need. There are online calculators available to help you narrow down what is appropriate for your system, and they are very easy to use. Simply enter the information you gathered, and the calculator will figure it out for you. 

To give you an idea of what to expect, here’s a simplified chart. Notice that this doesn’t include water temperature or pressure relief valve ratings.

Water Heater CapacityStatic Water PressureExpansion Tank Size
40 – 60 Gallons40 – 50 psi2 Gallons
40 – 60 Gallons60 – 80 psi3.5 Gallons
80 Gallons40 psi2 Gallons
80 Gallons50 – 60 psi3.5 Gallons
80 Gallons80 psi4.5 Gallons

Is it Important that I Get the Right Size Expansion Tank?

While it is important to install an expansion tank that’s appropriate for your system, there is a little bit of flexibility. Generally, it’s acceptable to oversize your expansion tank, as it should work correctly. Just be sure you don’t install an expansion tank that’s too small, or it may get overloaded and cause expensive damage.

As a rule of thumb, call a professional when you’re in doubt. It really isn’t worth a costly mistake when you can get confirmation from a plumber with training and experience.

Costs Involved in the Installation of an Expansion Tank

The cost of installing an expansion tank is dependent on what size you need, what brand you choose, and the complexity of installation. As you would expect, larger tanks cost more than small ones, and price varies between brands.

You can expect to pay between $250 and $500 for the expansion tank itself, plus any parts required to install it. When it comes to labor, you will save money to install the tank yourself, as professional plumbers charge anywhere between $50 to $200 per hour. Keep in mind that it typically takes 1-2 hours to perform this type of installation.

While professional work is more expensive, it may be worth it if you don’t have much experience with this type of project. A poorly installed expansion tank will only cause you more money and stress down the road in the event that it fails. If this happens, you won’t just need a new expansion tank, you’ll probably have to repair water damage or tend to a damaged plumbing system. 


Now that you have the information you need, you should be able to find a water heater expansion tank that suits your home and plumbing system. With online calculators and the chart we included, you’ll get a general idea of what you need to start the installation process.

The general rule is the higher your water heater capacity, the bigger your expansion tank needs to be. Things like water pressure and temperature can certainly effect this proportion, so it’s best to make sure that you’re aware of your system’s specifications.

As we mentioned before, don’t hesitate to consult a professional plumber if you have any doubts. We all know how devastating and expensive failed plumbing can be, and it’s best to avoid it if possible. Feel free to view our other posts if you have other questions about water heaters as well.

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