Water Heater Expansion Tank: What Is It and What Does It Do?

By Michael Franco

September 27, 2024

Water heater expansion tanks are important components in modern plumbing systems, helping to maintain the efficiency and safety of your water heater. If you live in a home that uses a conventional water heater, you likely have one.

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What Is a Water Heater Expansion Tank?

A water heater expansion tank (sometimes called a thermal expansion tank) is a small tank that works to absorb excess water pressure caused by thermal expansion. When water is heated, it expands. In a closed plumbing system, this expansion has nowhere to go, causing pressure to build up inside the water heater and the pipes. This increased pressure can lead to several issues:

Damage to the Water Heater

Excess pressure can put stress on the internal components of a water heater, such as the tank and heating elements. Over time, this stress can cause leaks or even lead to the failure of the water heater.

Stress on Plumbing Fixtures

High pressure can also damage other components of the plumbing system, including pipes, valves and fixtures like faucets and toilets. This can result in leaks, drips and costly repairs.

Reduced Lifespan of Appliances

Appliances connected to the plumbing system, such as dishwashers and washing machines, can also suffer from high water pressure, leading to premature wear and tear.

Safety Concerns

In extreme cases, the buildup of pressure can cause the water heater's relief valve to open, releasing hot water and steam, which could lead to scalding or damage to the surrounding area.

What Does a Water Heater Expansion Tank Do?

A water heater expansion tank helps mitigate these issues by providing a space for the heated and expanded water to go. It consists of a small, air-filled bladder or diaphragm that absorbs the excess pressure. As the water expands, it compresses the air in the expansion tank, relieving the pressure in the water heater and the plumbing system. Once the water cools and contracts, the air pressure pushes the water back into the system, maintaining a balanced pressure throughout the cycle.

Where Is a Water Heater Expansion Tank Located?

A water heater expansion tank is typically installed on the cold water supply line near the water heater. The installation location can vary depending on the plumbing layout, but the tank is usually mounted on a vertical or horizontal pipe within a few feet of the water heater. This proximity allows it to effectively absorb the pressure changes occurring in the heater.

In residential settings, the expansion tank is often installed in utility rooms, basements, or garages where the water heater is located. It is connected via a T-fitting to the cold water inlet pipe leading into the water heater. This setup ensures that the expansion tank can directly interact with the water flowing into the heater, providing immediate relief when pressure builds up.

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Does a Water Heater Really Need an Expansion Tank?

Whether a water heater needs an expansion tank depends on several factors, including the type of plumbing system in the home and local building codes. Here are some scenarios in which an expansion tank is typically required:

Closed Plumbing Systems

In a closed system, where a check valve, pressure-reducing valve, or backflow preventer is installed on the water supply line, an expansion tank is necessary. These devices prevent the expanded water from flowing back into the municipal water supply, causing pressure to build up within the system. The expansion tank provides a safe outlet for this pressure.

High Water Pressure

Homes with high water pressure (typically above 80 psi) can benefit from an expansion tank. High water pressure can exacerbate the effects of thermal expansion, putting more stress on the water heater and plumbing system.

Local Building Codes

Many local building codes now require the installation of a water heater expansion tank in new constructions and when replacing a water heater. Checking with local regulations is important to determine if an expansion tank is required in your area.

Frequent Water Heater Leaks or Relief Valve Discharges

If you notice frequent leaks around your water heater or the pressure relief valve discharging water, this could be a sign of excessive pressure in the system. Installing an expansion tank can help alleviate this problem and protect your water heater and plumbing.

Adding an Expansion Tank to an Existing Water Heater

Adding an expansion tank is a relatively straightforward process that can be done by a professional plumber or a knowledgeable DIY homeowner. If you're comfortable working with plumbing, there should be no reason why you can't install or replace an expansion tank yourself. If not, it's likely a job best left to a professional who will charge, on average, about $325 for the task, not including the price of the equipment itself.

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