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Moen Solutions

Thermal Expansion

This article includes detailed information on the: 

  • Flo by Moen app, also known as the Smart Water Network app or the Moen Smart Water app 
  • Flo Smart Water Monitor and Shutoff, also known as the Flo by Moen Water Shutoff 


 Why You Need a Thermal Expansion Tank

Have you ever noticed the small tank that sits right next to your water heater? Ever wonder what the heck it actually does? Well, it’s called a thermal expansion tank, and it’s actually kind of a big deal. 

The thermal expansion tank does more to protect your home than you might realize. It helps keep your home’s water pressure in check — and water pressure, if left to its own devices, can do some serious damage. 

In this article you’ll learn about thermal expansion and water pressure, and how thermal expansion tanks keep your plumbing system from breaking down.  

What Is Thermal Expansion?

Thermal expansion is the tendency for matter to expand in mass or volume as its temperature increases. It’s one of those universal laws of science that applies to just about anything. The biggest victim of thermal expansion in your home is water. Since your home’s water is all being held up in extremely non-flexible pipes, it’s something you might want to know about. 

Water heaters are one of the foundational aspects of our modern lifestyle, and one we take for granted quite often. It’s working all the time, heating up water for our sinks, showers and appliances. And every time it kicks on, the water in that tank expands. 

Where does that extra water volume go? 

You’d think it just flows right out of your house, but it’s not that easy. That’s because most home’s operate on a closed plumbing system, meaning that excess water is essentially trapped in your home.

Closed Plumbing Systems

Plumbing systems are either open or closed. In an open system, water can flow in and out of your home through the water main. If there’s an increase in water pressure due to thermal expansion, water will naturally flow out of the system since the incoming water is flowing at a lower pressure. 

In a closed system, this can’t happen. And the reality is that most homes in the U.S. operate on a closed system. There’s a couple reasons for this. The first is that when it comes to municipal water supplies, having an open system could pose a possible public health risk. Sediment and debris from old lead pipes can contaminate drinking water. So home plumbing systems have a check valve to prevent backflow into the municipal supply. 

The other reason most homes operate on closed systems is because of pressure reducing valves. Since municipal water suppliers usually pump out water at extremely high pressures, homes have pressure regulators to dial down the water pressure to safer levels. This protects your pipes and fixtures from wear and tear due to too much pressure. 

So if water can’t leave your home, how does your plumbing network deal with the excess pressure and volume caused by your water heater? It relies on a thermal expansion tank.

What Is A Thermal Expansion Tank?

Thermal expansion tanks are the unsung heroes of modern plumbing. They give all that extra water a place to go so the rest of your plumbing system can go about its business without having to worry about high water pressure.

A thermal expansion tank is a small tank that is installed on the inlet line going into your water heater. Inside the tank is compressed air and a bladder. The compressed air is pressurized to match your home’s water pressure. 

When your water heater fires up and the water in its tank starts expanding, the water pressure starts to exceed the pressure of the compressed air in the expansion tank. That high-pressure water makes its way into the expansion tank’s bladder. As the bladder fills up, the overall water pressure in your plumbing system starts to go back down.

But an expansion tank isn’t foolproof. If your water heater is set too high, there might be so much expansion going on that there isn’t enough room in the expansion tank for all that water. The water heater manufacturers know this, so they put a pressure release valve on their water tanks. 

Pressure release valves are the last line of defense for your water heater. They do their job in a more primitive way by simply letting the excess water spill out into a pan — or sometimes just onto the floor.  

So why worry about all this? What’s the big deal with high water pressure?

Why You Should Worry About Pressure 

You might think that having high water pressure isn’t that big of a deal. The truth is, water pressure is one of the most important aspects of your home’s plumbing system. 

When your water pressure is too high, fixtures, appliances and pipes are all put under extra stress. Your toilets, shower heads and faucets are all designed to operate under a certain pressure threshold. Exceed that pressure and they start to break down and need to be replaced. 

Having chronically high water pressure can also lead to bigger problems. Small leaks can develop that can go undetected for months if they’re behind walls, above ceilings or under floors. Those small leaks eventually lead to structural damage, rotting out your home’s wood frame and encouraging toxic mold growth. 

High water pressure can also lead to sudden pipe bursts that can cause catastrophic damage and loss of property.  This is especially true in the winter, when freezing temperatures can cause ice buildup in pipes that lead to even higher water pressure.

Fortunately, monitoring your home water pressure has come a long way since the days of manual pressure gauges. Smart leak detectors like the Flo Smart Water Monitor and Shutoff device  monitor your home water pressure, temperature, and scan for leaks all in real-time. And all that data is available to you 24/7 on the Flo by Moen app.  

Avoid Common Problems – Expansion Tank Maintenance

Unfortunately, thermal expansion tanks don’t last forever. Most tanks eventually become waterlogged, making them unable to perform their function. Professional plumbers recommend replacing them every five to ten years. Here are a few ways to look after your expansion tank:

  • Watch out for condensation. This can be caused by a waterlogged expansion tank or could just be the result of a very humid climate. Either way, it could pose a danger if your water heater is situated near any electrical devices.
  • Do the “tap test.” Simply tap or knock on your expansion tank and listen to the sound it makes. If it makes a hollow, ringing sound you know there’s still air inside it. If it makes more of a dull sound, it means your tank is full of water and needs to be replaced. 
  • Feel the tank. Feel the top and the bottom of your expansion tank. The top of the tank, which should be filled with compressed air, should be cool to the touch. The bottom of the tank, where the water goes, should be warm. If the top of the tank feels the same as the bottom, you know you’ve got a problem. 
  • Pay attention to your water heater’s pressure release valve. Remember the pressure release valve? It starts releasing water if there’s too much pressure for the expansion tank to handle. If you notice the release valve is constantly dripping, it could mean your expansion tank isn’t working anymore. 
  •  Upgrade your tank before it fails. You don’t have to wait until you notice a problem to invest in a new expansion tank. A good rule of thumb is to replace your tank whenever you replace or service your water heater. 
  •  Replace a faulty tank as soon as possible. If your expansion tank isn’t doing it’s job, your home’s at risk for water damage or, even worse, a water heater explosion. As soon as you notice that something’s not right, get a hold of a plumber and start taking steps to get a new expansion tank installed. 


As you can see, thermal expansion is always at work in your home, and your thermal expansion tank is one of your most important plumbing components. It keeps your pipes and fixtures in good shape and protects your water heater. Take good care of it and it will take care of you.

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