Informational Guide

Do I Need An Expansion Tank On My Water Heater?

by Andrew

A water heater does what the name suggests: it gives you warm water, mainly in domestic settings. However, they also serve the purpose of supplying water continually.  

If you are installing one for the first time, you might be wondering: do I need an expansion tank on my water heater? This guide will give you the answers to all your questions. 

To understand what expansion tanks are, you must have a fair idea of how water heaters work. During the heating process, the water heater expands.  

This is due to a phenomenon called ‘thermal expansion,’ which causes strain on the plumbing system. 

So suppose your water heater can hold around 40 gallons. However, the capacity will balloon to approximately 42 gallons when it experiences substantial warming.  

If there is no place for that extra volume to go, your plumbing system will be negatively impacted. An example to help you understand this: if you keep folding a paper clip, it will eventually break.  

Taking the heater as the paper clip — If enough pressure is put on it during the expansion and contraction process, the tank can burst or start to leak. 

This is where an expansion tank is needed. It helps you protect your system by taking on the excess volume and water pressure fluctuations.  

It is essentially a backup system that prevents your plumbing from being overworked. When the volume expands due to heat, its excess gets stored in the expansion tank, allowing the pressure within the water heater to reach a safer level.  

This is the primary purpose of an expansion tank on a water heater. 

expansion tank water heater

When Do I Need An Expansion Tank Installed On My Water Heater? 

This is where you’re probably wondering, “when is an expansion tank needed on a water heater?” Well, several instances would necessitate the installation of the tank.  

For A Closed Water Heating System 

Firstly, several US states necessitate a closed water heating system, and those need an expansion tank. So, you may be required by law to install one. You can check if your code necessitates it or not.  

Expanding on the last point, if your home is on an open system instead of a closed one, the extra water created during thermal expansion flows back to the mainline.  

Closed-loop systems keep expanding and contracting as the water has no place to go and causes the unit to break down prematurely.  

Slows Deterioration 

Even if you aren’t required by law to install an expansion tank, it is a good idea to have one anyway, as they will prevent your water heater from deteriorating quickly.  

Help Regulate Water Pressure 

Expansion tanks help regulate the water pressure. So if your water pressure is exceptionally high or changes quite regularly, the expansion tank can ensure that your heater isn’t damaged because of it. 

TPR Leaks 

Another essential thing to note is that if your heater’s temperature/pressure relief valve (TPR) leaks a lot, then you need to get an expansion tank.  

Why? Well, because the TPR valve is the safety mechanism activated when the heater's pressure or temperature becomes dangerous. 

 It will start discharging hot water so that pressure is regulated. It may cause a mess, but it ensures your water heater doesn’t explode.  

But, if you find it frequently leaking, for safety reasons, an expansion tank should be installed. 

water leak

What Size Expansion Tank Do I Need? (Guide For Your Needs) 

If you have concluded that the repercussions of no expansion tank on the water heater aren't worth the damage to your plumbing system, you need to figure out the size of tank you'll have to get. 

Basically, you will have to see what expansion tank size will fit in the closed water supply system. To figure out the size, consider the following two factors: 

  1. 1
    How much water pressure there is. In some cases, it doesn’t exceed 30 psi. But, it would help if you got a more accurate measurement of it by calling your providers or through a portable gauge. 
  2. 2
    How many gallons worth of water can your heater contain. This can be found on the printed label stuck to your water heater. 

There are several reasons to get the right size expansion tank. Firstly, a smaller tank may result in excess water pressure, becoming problematic.  

When purchasing one, you can contact companies who, in most cases, do offer a chart that lets you know what model and size you should get depending on your system.  

The rule of thumb is to get a bigger expansion tank if you aren’t sure what exact size you should choose, as there are less likely to be issued.  

Also, oversized tanks are likelier to handle all your closed water system needs.  

water heater enclosure

People Also Ask (FAQs)  

How do I know if my water heater expansion tank is bad?  

When an expansion tank goes bad, you may see water dripping from it. This can happen due to debris. If you have an old expansion tank, you should check if its pre-charge pressure is equivalent to your home system’s water pressure. Otherwise, the tank’s diaphragm may get damaged. 

Can you install an expansion tank yourself?   

The expansion tank on the water heater cost is pretty low, especially if you install it yourself – you can get everything done within $70. And it is pretty easy to do it!  

How should an expansion tank be mounted? 

The direction of an expansion tank isn’t important. It can be mounted horizontally, upright, or upside down, but it will continue working without a hitch. If you keep the tank horizontal or sideways, just ensure you have something supporting it. 

Do I need a thermal expansion tank with well water? 

A check valve will likely be present near the pressure tank on a well water system. This will indicate that the water system is closed, so you must purchase an expansion tank. 

Can expansion tanks cause low water pressure? 

An expansion tank’s air pressure is generally lower than the water pressure in your home. Thus, the expansion tank will likely cause the water pressure to lower. 


So, to heat your water safely, if you have a closed-loop system, you need to ensure you install an expansion tank.  

This is because when water heats up, it expands, causing the excess to pressurize the plumbing system. The tank takes the excess so that your system remains protected. 

Get one as soon as possible to avoid excessive future repairs!