Plumbing, Water Heaters

Are Water Heater Expansion Tanks Required?

expansion tank

A water heater expansion tank (also known as a thermal expansion tank) is a safety device that can help reduce the risk of damage to your water heater and plumbing system due to pressure built-up in the plumbing system. Right now, you may be wondering to yourself, ‘is it necessary to have one’? Well, let’s find out, shall we?

So, are water heater expansion tanks required? Most cities require residential plumbing systems to be closed water heater systems. When a new water heater is installed on a closed system, an expansion tank is required by building codes. Expansion tanks are not required for open plumbing systems.

If you have a private well water system, you may also be wondering if a water heater expansion tank is required? Most well water systems have a check valve between the pressure tank and the house. In this case, your well water system is closed, and an expansion tank will be required on your water heater.

An open water heater system allows hot water from the water heater to pass freely from the water heater tank into the cold water supply line and (if the pressure is really high) eventually back into the municipal water system. This occurs when there are no valves (such as a shut-off valve, check valve, etc.) preventing water flow on the cold water supply.

A closed water heater system means that the hot water cannot expand into the cold water supply beyond the shut-off or check valves. The problem with a closed system is that hot water becomes trapped with nowhere to expand too. This increases pressure in the plumbing supply piping, which can result in damage to the system.

It is usually required by local building codes to have a water heater expansion tank. Expansion tanks are used to help protect against water heater explosion. Let’s explore this idea below. We will also look at what a water expansion tank is and why it is necessary to install one. So, if you’re ready to learn all you need to know about water heater expansion tanks, then please continue reading…

What is a Water Expansion Tank?

As mentioned above, a water heater expansion tank is a safety mechanism that reduces possible pressure damage to your plumbing system. It is sometimes called a thermal expansion tank and is often required by law for most major North American cities. 

Because water naturally expands when it is heated (a process is known as thermal expansion), extra water volume is created every time the tank fires up.

It has been estimated that a standard 50-gallon water heater tank expands to approximately 52 gallons when heated to a temperature of 120 degrees Fahrenheit. This additional water volume causes an increase in pressure, which, if significant enough, can damage your water heater, plumbing fixtures, and water pipes over time. 

What is the Purpose of an Expansion Tank?

A water heater expansion tank acts as an overflow device. It absorbs the excess created when water volume expands as a result of applied heat. It also accounts for changes in supply pressure caused by new water entering the tank.

In an open plumbing system, expanding water can flow back into the city water supply using a two-way valve, thus eliminating the need for a water expansion tank.

However, many homes still have a closed water supply system, which is basically a system that contains a one-way (backflow, check, or pressuring-reducing) valve. The extra volume has nowhere to go, causing thermal expansion resulting in possible high-pressure damage to your water heating unit.

Do Water Expansion Tanks Hold Water?

A water expansion tank contains air that is, by nature, highly compactible. When water is heated and expands, this excess liquid flows from the heating unit into the expansion tank where the air compresses. Thus, making room for the increased water volume. Therefore, water pressure does not spike to a significant level, helping to negate possible pressure damage to the tank, fixtures, and/or pipes.

What Size Water Expansion Tank Do I Need?

It is important to correctly size and selects your water expansion tank to match your heating unit. Which one to choose depends upon two main factors: the size of the water heater (capacity in gallons) and the water pressure in the piping system. This information can be acquired on the heating unit’s factory label and/or by phoning your local water supply company.

For this article, we will discuss water expansion tanks specifically suited for residential applications. A 40- or 50-gallon heating unit requires an ST-5 expansion tank, whereas a 60- or 80-gallon unit needs an ST-8 tank.

What is the Best Water Expansion Tank?

For a standard 50-gallon tank, the AMTROL ST-5 Thermal Expansion Tank is Amazon’s Choice pick and a best seller! For a portable option, try the Watts DET-5-M1 2.1 Gallon Potable Water Expansion Tank. Let’s review each item in more detail. Best of all, whichever one you chose can be delivered right to your front door in as little as two days! Go online today and check out each model! 

AMTROL ST-5 Thermal Expansion Tank

This expansion unit is affordable and will help protect your heating unit from water pressure damage. It has an overall rating of 4.7/5, with 4.9/5 for ease of installation and 4.7/5 for value. 

It provides a secure connection to water heaters while preventing dangerous water pressure build-up levels. It comes pre-charged at 40 psi and complies with the Lead-Free Law. Choose from either a 2-gallon or 4.5-gallon option made from high-quality butyl rubber and 304-grade stainless steel fittings for long-lasting performance. 

Watts DET-5-M1 2.1 Gallon Potable Water Expansion Tank

This expansion tank is both affordable and portable. It has an overall rating of 4.5/5 and is great for any home that suffers from prematurely damaged fixtures (such as a water heater) or appliances (including washing machines and dishwashers).

It also helps with annoying (and often costly) plumbing issues such as leaking faucets, running toilets, or banging pipes. It comes pre-pressurized at 20 psi and is easy to install. It is a great value at just under $70 (excluding shipping costs).

How to Install a Water Expansion Tank

Installing a water expansion tank is best when done immediately following the installation of a new water heater. Most tanks should come with their own instruction manual, but I have outlined an easy-to-follow, step-by-step process below in case they don’t. The parts required for this DIY project include:

  • ¾ by 4” galvanized nipple
  • ¾ galvanized tee
  • ¾ galvanized elbow
  • 2” galvanized nipple
  • Pipe joint compound
  • Plumber’s tape

The four necessary installation steps for a water heater expansion tank include the following:

Step One

Install the tee on top of the water heater to use plumber’s tape and pipe joint compound on the nipple attached to the unit. Be sure to face the tee in the direction you want the tank to go and then tighten it securely. 

Step Two

Put plumber’s tape on both sides of the nipple and then apply pipe joint compound over the top of the plumber’s tape. Fasten the elbow to the nipple and screw one side to the tee. Be sure it is facing straight up before tightening.

Step Three

Prepare the threads on the water expansion tank with both plumber’s tape and pipe joint compound. Fasten the tank to the elbow securely. 

Step Four

Prepare the end of the nipple going on top of the tee with the plumber’s tape and pipe compound. Fasten the nipple to the tee connecting it to the water heater flex line. 

Plumbing Issues Caused by High Water Pressure

High water pressure can cause a variety of plumbing issues in your home. Water pressure should not exceed 80 psi. If it does, fixtures (including water heaters), faucets, shut-off valves, and toilet parts will break down early. That is why it is important to install a water expansion tank. It will help in the long run by preventing both costly plumbing repair bills and premature appliance replacement. 

The following is a list of household problems caused by thermal expansion:

  • Banging pipes
  • Short water heater life
  • Short washing machine life
  • Short dishwasher life
  • Leaking faucets
  • Spitting from faucet aerators
  • Running toilets


In conclusion, a water expansion tank is also called a thermal expansion tank. It is basically a safety device that can help reduce the risk of pressure damage to your plumbing system. Most cities require (by law) that pumping systems be closed and equipped with expansion tanks. 

In an open plumbing system, expanding water can flow back into the city water supply. However, in a closed system similar to that in most North American homes, the excess water volume has nowhere to go, therefore requiring a water expansion tank to reduce the risk of high-pressure damage to your water heating unit.

A water expansion tank could save you money in the long run by reducing the costly plumbing and appliance repairs often associated with thermal expansion. If you have banging pipes, leaking faucets, running toilets, or a defective water heater, washing machine, or dishwasher, chances are your home is suffering from the after-effects of thermal expansion. 

Installation is fairly easy and safe if you follow the steps outlined above or in the unit’s accompanying instruction manual. Units such as the AMTROL ST-5 Thermal Expansion Tank and the Watts DET-5-M1 2.1 Gallon Potable Water Expansion Tank are durable, affordable, and available online at Check them out today! Good luck with your future home installation projects.


Hubert Miles | Licensed Home Inspector, CMI, CPI

Hubert Miles is a licensed home inspector (RBI# 2556) with more than two decades of experience in inspection and construction. Since 2008, he has been serving South Carolina through his company, Patriot Home Inspections LLC. As a Certified Master Inspector, Hubert is dedicated to providing his expertise in home inspections, repairs, maintenance, and DIY projects.