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What Is a Water Heater Thermal Expansion Tank?

And do I need one?

In short, YES! You need a thermal expansion tank for your water heater. It is a critical component of your water heating system that protects the water heater from internal damage. This can protect your house from flooding.

Typical water heater thermal expansion tank, installed


It’s an overflow tank attached to your water heater. Because water expands when it is heated, there must be somewhere for it to go. Water heater tanks usually have very little room to accommodate the expansion. If the expanding water overflows the capacity of the water heater tank, the overflow bleeds into the expansion tank. This relieves the pressure on the water heater and pipes.

Water heater tanks are built tough, but over time, excess pressure and stress can create leaks. A thermal expansion tank can prevent this!


Before leaving a residence we perform the following complimentary 3-point inspection for every customer:

  • First, we check the water pressure. We’ll verify that building water pressure is between 60 and 80 psi. If the water pressure being delivered to your house is too high, it can cause pressure problems in your water heater.
  • Second, we’ll check your drains and fixtures. If there are slow drains or deteriorating fixtures, we’ll identify them so you can prevent a problem in the future.
  • Third, we’ll inspect your water heater and thermal expansion tank. We’ll check the age, condition and code compliance of your water heater and thermal expansion tank system.

When our technician performs a water heater inspection, they will review with you the condition of your thermal expansion tank and water heating system.  If there is a problem, your technician will provide you with options to repair, replace, or upgrade your existing tank.