Water Heater Expansion Tank: When You Need One and How to Size It
An expansion tank is code-required on most closed systems. What thermal expansion does, how to tell if your system is closed, and how sizing works.
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Water Heaters: Thermal Expansion and Pressure
Thermal expansion, expansion tanks, pressure and the code requirements behind them.
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How to choose
Quick answer
Water gains roughly 2% in volume when heated to typical water heater temperature. On an open system that volume pushes back toward the supply. On a closed system — one with a backflow preventer, a check valve, or a pressure-reducing valve with a built-in check — it has nowhere to go, and pressure climbs every time the heater fires. An expansion tank absorbs that, and on most closed systems it is a code requirement rather than an upgrade.
What to check first
- Whether your system is actually closed. Look for a backflow preventer, check valve or PRV, or watch a pressure gauge while the heater runs with no fixture open.
- Your home's static pressure. The tank's air pre-charge has to match it, and most tanks ship set to a value that will be wrong for your house.
- Potable versus hydronic rating. They look alike and are not interchangeable.
- Your local code and your heater's warranty terms. Adoption is broad, but the specific requirement is set locally, and some manufacturers make the tank a condition of coverage.