If you have a closed-loop plumbing system, an expansion tank is a small but essential addition to help protect it. Every time water heats up, it expands. In an open system, that extra volume can flow back into the municipal supply. But in a closed system, it has nowhere to go. Without a place to absorb that pressure, your water heater and plumbing can take the hit, leading to leaks, stress on joints, or even a failed pressure relief valve.
In this guide, we’ll explain how an expansion tank works, when you need to install or replace one, and how to install one on your water heater.
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An expansion tank is a small, sealed tank installed on your home's water line that is designed to absorb the pressure of hot water expanding. Inside the tank, a rubber diaphragm separates water from air. As pressure increases, the diaphragm flexes, allowing the tank to take in the extra volume and relieve stress on the system.
Expansion tanks are hydropneumatic, which means they use both air and water to regulate pressure. You’ll also find hydropneumatic tanks used in applications like well systems, reverse osmosis setups, and water hammer arrestors. In the case of a water heater, the expansion tank works quietly in the background as a failsafe to keep everything running safely and smoothly.
How does an expansion tank work?As water heats up inside your plumbing system, it expands and creates additional pressure. To manage this increase, the excess water is directed into the expansion tank, where it's held temporarily. Inside the tank, a flexible diaphragm separates the incoming water from a pocket of air. As water fills the tank, it compresses the air, allowing the system to absorb pressure without straining the pipes. When the water cools or pressure drops, the compressed air pushes the stored water back into the plumbing system, helping maintain consistent pressure and protect components from stress.
Signs you need to install or replace an expansion tankExpansion tanks play a key role in protecting your plumbing system from damage caused by thermal expansion. If you're experiencing any of the following issues, it may be time to install a new expansion tank or replace one that's no longer working properly.
- Your plumbing system is closed-loop. If your system includes a backflow preventer, check valve, or a water meter with a built-in check valve, the expanded water has nowhere to go without an expansion tank.
- Water pressure spikes when hot water is used. As water heats and expands, pressure can rise quickly in a closed system, putting strain on pipes and fixtures.
- The pressure relief valve is dripping or leaking. This valve is designed to open when pressure gets too high. If it's discharging water, it may be compensating for the lack of an expansion tank or for one that has failed.
- A pressure-reducing valve has been added at the main water line. While this helps regulate high incoming pressure, it also creates a closed system that needs an expansion tank to handle thermal expansion.
- Fixtures or appliances are wearing out faster than expected. High pressure can damage seals, joints, and internal components of faucets, toilets, and water-using appliances.
- The existing expansion tank is leaking or waterlogged. A functioning expansion tank should have a cushion of air. If it’s full of water or leaking from the seams, the internal diaphragm may have failed.
Adding or replacing an expansion tank is a simple step that helps prevent long-term damage to your water heater and plumbing.
How much does it cost to install an expansion tank?Installing an expansion tank is a relatively simple and affordable project, especially if your system already has a suitable location for the tank.
Approximate cost of an expansion tank installation: $30–70, plus labor
Here’s a breakdown of the costs:
- Thermal expansion tank: $25 to $60, depending on size and model
- Plumber’s tape or thread sealant: <$1
- Mounting bracket or support strap (if installing horizontally): <$10
If you plan to hire a plumber, expect to pay an additional $100 to $300 for professional installation. The total cost will vary depending on your local rates and whether any adjustments to existing piping are needed.
What size expansion tank do I need?The typical 80-gallon residential water heater will need a 2.1 expansion tank. If you have a water heater larger than 80 gallons, you likely need a 4.8 expansion tank. The size of your expansion tank determines the amount of pressure that can be released from your water heater.
To get the right size for your water pressure and usage, you can refer to the tank manufacturer’s sizing chart or ask a water expert to help you choose the correct size.
How to install an expansion tank on a water heaterWith basic tools and a clear plan, installing an expansion tank is a manageable project for most homeowners and usually takes less than an hour. Follow these steps to install your expansion tank safely and correctly.
1. Prep the workspace
Before beginning any work, make sure your water heater and plumbing system are prepared.
- Turn off power to the water heater. Unplug it or shut off the circuit breaker to avoid electrical hazards.
- Shut off the cold water supply. This is usually connected to the blue pipe feeding into the top of the water heater. Turning it off will stop new water from entering the system during installation.
If your system includes other electrical components nearby, such as circulation pumps or controllers, shut those down as well.
2. Depressurize the hot water heater
Before adjusting your hot water heater, relieve the pressure in the system.
- Open a hot water faucet anywhere in the house. This allows pressure to bleed off from the hot water side of the system.
- Leave the hot water faucet open until water flow slows to a trickle. This indicates that pressure has been released, and it's safe to work on the piping.
The time it takes will depend on the size of your water heater and the flow rate of the fixture you're using.
3. Pressurize tank to 5–10 psi above house water pressure
Before installing the tank, set the pressure to match your home's water pressure or slightly above.
- Use a pressure gauge to check the water pressure of your home.
- Compare the pre-charged pressure of your tank at the Schrader valve (usually located under a cap on the bottom or side of the tank).
- Adjust the tank’s pressure using a standard air pump. Set it to 5–10 psi above your home’s water pressure to account for minor pressure loss over time.
Note: Make sure to do this step before the tank is connected to the plumbing, when no water is inside. Water inside the tank will cause pressure to increase and potentially cause an inaccurate setting.
4. Apply thread sealant to the tank connection
This step is optional, but it can help create a more secure seal and reduce the risk of leaks over time. Before installing the tank, you can apply plumber’s tape (also called thread seal tape) to the threaded fitting on the tank’s connection. Wrap the tape clockwise around the threads 2–3 times, making sure it lies flat and doesn’t extend over the edge of the fitting.
5. Attach the expansion tank
Once the tank is pressurized, it’s ready to install. Thread the tank onto the pipe fitting by hand. In most cases, hand-tightening is sufficient to create a secure seal, especially if thread tape was applied. Avoid using tools to overtighten, as this can stress the connection or damage the tank.
6. Restore water and power
With the tank installed, you can bring the system back online.
- Turn the cold water supply back on and allow the system to refill.
- Check all connections for leaks, especially at the expansion tank fitting.
- Restore power to the water heater at the breaker or plug.
7. Monitor for leaks
After installation, continue checking the expansion tank connection and surrounding piping over the next 24 to 48 hours. Look for any signs of moisture, corrosion, or slow leaks that may develop under pressure. Early detection helps prevent water damage and ensures the tank is functioning as intended.
Can a thermal expansion tank be installed horizontally?
Yes, but it must be properly supported with a bracket or strap to prevent it from hanging from its connection to the pipe. Without support, the weight of the horizontal tank and any water it holds can put extra stress on the connection point, which significantly increases the risk of leaks over time.
What happens if my expansion tank pressure is too high or too low?If your expansion tank isn’t at a balanced pressure, it can’t function correctly as the buffer for your plumbing.
If the pressure is too low:
- The tank will fill with water, leaving no space for expansion.
- You may see water coming from the pressure relief valve or hear knocking in the pipes.
- Fixtures and appliances may wear out faster due to frequent pressure swings.
If the pressure is too high:
- The tank may not absorb the expanding water at all.
- High pressure can put stress on valves, joints, and fixtures.
- In some cases, fittings can fail or pipes may leak under the strain.
- Most tanks include a manufacturer's warning about the risks of extreme over-pressurization and overheating, including the rare possibility of explosion.
The air pressure in the tank should match your home’s static water pressure or be set just slightly higher, usually by 5 to 10 psi. This cushion helps compensate for the very gradual loss of pressure over time.
Because of this gradual pressure shift, you should check your expansion tank about twice a year to make sure it’s within the correct range. You can check and adjust the air pressure in your expansion tank using a handheld tire pressure gauge and a standard air pump.
Note: Pressure should be measured with the water supply turned off and a hot water faucet turned on. This ensures the expansion tank is empty when applying the tire pressure gauge.
Is a leaking expansion tank dangerous?A leaking expansion tank is not usually dangerous, but it is a sign that something is wrong. Even a slow leak can lead to water damage over time, especially if the tank is installed in a finished space or near electrical components.
Leaks may be caused by:
- Incorrect air pressure inside the tank
- A loose or corroded fitting at the connection point
- Internal wear, especially if the tank has been in service for many years
In some cases, a leak means the tank needs to be replaced. It’s important to inspect the tank and surrounding plumbing if you notice any moisture or signs of corrosion. A properly functioning expansion tank is essential for protecting your plumbing system from thermal expansion.
How long does an expansion tank last?Most expansion tanks are designed to last between 5 and 10 years under normal operating conditions, with some manufacturers offering a 5-year warranty.
Several factors contribute to the full lifespan of an expansion tank:
- Proper installation with adequate support and correct orientation
- Maintained pressure in the range of your home’s water pressure
- Water quality, which can affect internal components over time
- Manufacturing quality, including the materials used in the diaphragm or bladder.
An expansion tank should be inspected regularly for signs of wear, corrosion, or leaks. Repairing or replacing it at the first signs of failure can help avoid larger plumbing problems later on.
Find more expert resources at Fresh Water Systems For more than 35 years, Fresh Water Systems has specialized in water treatment solutions for both residential and commercial applications. Our team offers expert support and a full inventory of components to help you successfully install, maintain, or replace your water system parts.To learn more, browse our selection of thermal expansion tanks or reach out to our water experts for guidance on choosing the right solution for your system.
