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What is the Purpose of a Water Heater Expansion Tank?

Fact checked by Stephen Conklin

what is the purpose of a water heater expansion tank

What is the purpose of a water heater expansion tank? An expansion tank for a water heater safeguards the plumbing system and the heater against uncontrolled thermal expansion.

It prevents excessively high water pressures by channeling the expanding water into the secondary tank instead of the main supply line. Keep on reading to learn more about the purpose of a expansion tank.

Table of Contents

Purpose of Water Heater Expansion Tanks

The expansion tank purpose centers on water pressure equalization and thermal expansion accommodation. It sounds challenging to digest, so let us dig deeper.

What is a Water Heater Expansion Tank?

As the name implies, a water heater expansion tank is a secondary tank smaller than the water heater storage tank. The device sits alongside the water heater, connected to the appliance by a pipe.

What Does an Expansion Tank Do?


We know that water expands when heated, as does everything else. Sadly, the water heater storage tank is a closed system. Heat-induced water expansion (also called “thermal expansion”) increases pressure within the tank. The water tries to push against the tank walls as a result, potentially damaging them.

Excessive pressure and heat in the tank can also increase pressure within the plumbing system, especially the mains.

Legacy plumbing had no issues with backward pressure because they did not feature a “check valve.” Modern water systems have this component, preventing backflows and potential freshwater contamination by residential wastewater.

Unfortunately, unchecked pressure in modern plumbing means pipes and joints could burst and cause leaks or spills. Increased water compression can also damage the heater, reducing its lifespan.

An expansion tank acts as a safety vessel for the water heater’s thermal expansion tendencies. As water expands, it flows into the expansion tank and prevents back-pressure in the water mains.

Although the principal purpose of an expansion tank is to ensure water heater and plumbing safety, its secondary objective is to comply with regulatory laws. Thermal expansion tanks are required by code (International Residential Code IRC P2903.4) for water heater installations.

Here is a summary of the purposes of a water heater expansion tank.

  • Equalizes pressure between the heater storage tank and plumbing system
  • Manages water’s thermal expansion properties
  • Protects plumbing fixtures from premature wear and damage
  • Complies with regulatory requirements

Water Heater Expansion Tank Benefits


Water heater expansion tanks offer families several benefits, regardless of location, socioeconomic status, and other demographic variables.

1. Safeguards the Family and House

Thermal expansion tanks relieve pressure in the water heater, avoiding catastrophic explosions that might endanger people and properties. Although water heater explosions are uncommon, the US Product Safety Commission reported that four out of 58 (about 7%) water heater-related safety issues in 2021 were explosions.

2. Extends the Lifespan of Water Fixtures and Appliances

Managing thermal expansion and increased water pressure prevents pipes from bursting, joints from leaking, and water heaters from breaking. You will use these fixtures longer, saving you money from untimely replacements.

3. Helps Avoid Regulatory Fines

Many cities have building codes requiring water heater installations to feature expansion tanks. Otherwise, homeowners could be fined.

What Happens if a Water Heater Doesn’t Have an Expansion Tank?

Water heaters without a thermal expansion tank are at risk of damage and reduced operational longevity.

Instead of the water heater lasting ten to fifteen years, its lifespan could be shorter. Water heaters range from $270 (20- to 30-gallon units) to $3,000 (50-plus gallons). The unexpected expense can throw your family budget off.

It is worth noting that expansion tanks equalize water pressure and manage hot water’s thermal expansion properties. Without an expansion tank on hot water tank units, pipes could burst, joints would leak, and valves would wear down faster than normal.

Frequently Asked Questions


How do I know if my expansion tank is failing?

The following signs can indicate that an expansion tank on water heater is failing.

  • Moisture or condensation on the expansion tank
  • The expansion tank makes a dull sound when you tap on it (normal sound should be hollow).
  • More than half of the pressure tank for hot water heater feels warm.
  • Leaks on the expansion tank or wetness on the floor underneath it
  • Water comes out of the expansion tank’s bottom when you push the valve pins
  • Nothing happens when you press the Schrader valve (normally, it should create a “hissing” sound to signify air moving out)

How often do you change an expansion tank on a water heater?

An electric or gas water heater expansion tank lasts five to eight years, depending on brand quality. Hence, you can expect to change the pressure tank every five to eight years.

As a rule, homeowners should check their expansion tank’s function annually. The assessment is as straightforward as pressing the Schrader valve at the expansion tank’s bottom and noting a “hissing” sound. If something else happens, it is a sign you must replace the expansion tank.

Should I replace or repair a failing expansion tank?

Repairing or replacing a failing gas or electric water heater expansion tank depends on the severity or nature of the damage or dysfunction. Homeowners should consider paying a professional to assess the expansion tank and determine if it only needs a fix or replacement.

If the repair cost exceeds or is about $90 to $350, you are better off with a brand-new tank. Check the expansion tank’s age. If it is already within its terminal lifespan, it might be wiser to replace it.

Although you can follow an installation diagram, we recommend professional installation to avoid accidental pressure-related tank explosions.


What is the purpose of a water heater expansion tank? The principal reason for installing a water heater expansion tank is to ensure plumbing safety. This technology prevents unnecessary and uncontrolled pressure increases during water heating, safeguarding the integrity of pipes, joints, fixtures, and appliances.

Secondarily, water heater expansion tanks also ensure compliance with regulatory laws, so they are required in homes. More importantly, they protect the house and everyone living under one roof.

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