Expansion Tank - Why Do I Need One?

WHAT IS AN EXPANSION TANK — AND WHY DO I NEED ONE?

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An expansion tank is a small steel tank installed on the cold-water line near the water heater. Inside is a flexible rubber bladder (or diaphragm) that is pre-charged with air. When water is heated it expands. That extra volume has to go somewhere. The bladder compresses and absorbs the expanded water so pressure inside the house does not rise sharply.

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Why a closed system needs one

 

City water can flow into the house, but it cannot always flow freely back out. A check valve, backflow preventer, or (in practice) a pressure-reducing valve (PRV) at the meter restricts reverse flow. That is what creates a closed system.

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Most Phoenix-area homes on municipal water have a PRV because street pressure often runs high. Plumbing codes (IPC and UPC) treat a PRV the same as a check valve or backflow preventer for this purpose: once one is in place, expanded water is effectively trapped in the house and an approved method of controlling thermal expansion is required. An expansion tank on the building side of the valve is the usual solution.

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Note: Many modern residential PRVs include a small integral bypass that can allow limited reverse flow if house pressure exceeds street pressure. This helps limit how high the pressure can climb, but it does not eliminate the need for an expansion tank. Codes still require one, and the tank provides reliable, controlled expansion capacity.

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The air charge in the tank should be set to match your incoming house pressure (the pressure on the downstream side of the PRV).

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Tank vs. tankless

A traditional tank water heater holds 40–50 gallons of water that is kept hot all day, so it produces more expansion. A tankless unit heats water only on demand, so it produces less expansion on its own.

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A tankless still needs an expansion tank when:

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• the home is a closed system (PRV, check valve, or backflow preventer), or

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• a hot-water recirculation loop keeps reheating water in the pipes.

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That is how we install them, and it matches local code requirements in many Arizona jurisdictions.

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Well homes

 

Homes on a private well usually already have a pressure tank. That tank often absorbs thermal expansion, so a separate expansion tank is usually not required. If extra check valves or other devices have closed the system, we check the setup rather than guess.

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What goes wrong without one

 

The extra pressure has to go somewhere. It stresses the water heater, fittings, supply hoses, and fixtures. The most common warning sign is a T&P (temperature-and-pressure) relief valve that drips or weeps. That valve is a last-resort safety device, not an everyday pressure-control device. Repeated dripping lets Arizona minerals collect on the valve and can seize it.

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If the T&P valve is dripping, pipes bang when the heater cycles, or pressure climbs when no one is using water, have the system checked.

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Call us to inspect, size, or install an expansion tank: 602-833-1533