Water heaters

Water heater expansion tank problems

An expansion tank absorbs pressure from heated water in a closed plumbing system, and issues usually show up as it becoming waterlogged, leaking, or the main tank's relief valve dripping because the expansion tank isn't doing its job.

Difficulty
Moderate
Time
30–60 minutes
DIY cost
$25–$60
Pro cost
$150–$300

Symptoms

  • T&P relief valve on the main tank drips periodically
  • Expansion tank feels heavy or full of water when tapped
  • Visible leak or rust at the small tank mounted near the water heater
  • Household has a check valve or pressure regulator (closed system)

Likely causes

  • CommonExpansion tank has lost its air charge and become waterloggedThe internal bladder needs a proper air charge to absorb expansion; once lost, the tank can't do its job.
  • CommonFailed internal bladder allowing water to fill the air sideA ruptured bladder makes the tank essentially useless and it will feel completely full and heavy.
  • PossibleExpansion tank undersized for the water heater and system pressureA tank sized for a smaller water heater may not adequately handle a larger unit's expansion.
  • PossibleNo expansion tank installed despite having a closed systemHomes with a backflow preventer, check valve, or pressure regulator need an expansion tank that may never have been added.

What to do

  1. Tap the expansion tank; a properly working one sounds hollow on the top half (air) and different on the bottom (water).
  2. Check the air charge at the Schrader valve on top of the tank with a tire pressure gauge; it should roughly match your home's water pressure, often 40–60 psi.
  3. If the pressure reads zero or the tank feels completely water-filled, the bladder has likely failed and the tank needs replacing.
  4. If there's no expansion tank at all and your home has a check valve, backflow preventer, or pressure regulator, discuss adding one with a plumber.
  5. Check the main water heater's T&P valve for dripping, which often resolves once the expansion tank issue is fixed.
  6. Replace a failed expansion tank rather than attempting to repair the bladder, since these tanks aren't designed to be serviced internally.

Tools and parts

  • Tire pressure gauge
  • Adjustable wrench
Water heater thermal expansion tankSearch: “water heater expansion tank replacement”

When to call a pro

If you're unsure whether your system needs an expansion tank or how to size one, a plumber can assess your plumbing setup and install the correct size.

Safety

Generally low risk

Generally low risk for a careful person. Stop if anything looks different from what this guide describes.

Related guides

Operator provides AI-generated troubleshooting information. It can be wrong. For safety-critical, electrical, gas, structural, medical, automotive safety, or other high-risk problems, consult a qualified professional.