Best Water Heater Expansion Tank (2026) — California Code Compliance Picks

Published 2026-04-30 · Updated 2026-04-30 · 8 min read · By Zomg The Handyman

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TL;DR

California requires expansion tanks on most water heater installs (closed system code). Watts PLT-2 and Amtrol ST-5 lead. Sized to water heater + system pressure.

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California's closed-system plumbing code (post-2008) requires thermal expansion tanks on most water heater installs. They prevent the relief valve from blowing during heating cycles. Easy to skip during DIY install — easy fail at inspection.

Quick Comparison

Pick	Best For	Approx Price
Best Mainstream	Watts PLT-2 Thermal Expansion Tank (2 Gallon)	$45-$70
Best Premium	Amtrol ST-5 Therm-X-Trol Expansion Tank	$65-$90
Best Sizing for Larger Tanks	Watts PLT-5 Thermal Expansion Tank (5 Gallon)	$120-$180

Our Picks

Best Mainstream

Recommended on Amazon

Watts PLT-2 Thermal Expansion Tank (2 Gallon)

Standard 2-gallon expansion tank. Fits most 40-50 gallon water heaters.

$45-$70 View on Amazon →

Best Premium

Recommended on Amazon

Amtrol ST-5 Therm-X-Trol Expansion Tank

Premium build, 5-year warranty. The expansion tank pros prefer.

$65-$90 View on Amazon →

Best Sizing for Larger Tanks

Recommended on Amazon

Watts PLT-5 Thermal Expansion Tank (5 Gallon)

5-gallon for 80+ gallon water heaters or commercial setups.

$120-$180 View on Amazon →

Why Required

Closed-system plumbing (with check valve at meter) doesn't let heated water expand back to the city main. Pressure builds → relief valve opens → water sprays. Expansion tank gives the water somewhere to go.

Sizing

Sized by water heater capacity + system pressure. 40-50 gallon water heater at 60 psi: 2-gallon expansion tank. 80+ gallon: 5-gallon expansion tank.

Install Notes

Mount tank vertically (air bladder up, water connection down). Pressurize bladder to match home water pressure (40-80 psi typical). Install on cold supply line, not hot output.