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Expansion tank
An expansion tank is a small tank that gives hot water room to expand when pressure rises.
Why it matters
Without it, pressure can build and stress valves and fixtures, especially in closed plumbing systems.
Where people get this wrong
Not every system needs one, but many modern systems do. A leaking relief valve can be a clue that expansion pressure is not controlled.
Real-world example
The water heater relief valve drips every morning after showers. That can be the system shedding pressure because there is no good place for expanded water to go.
Where this hits your build
This comes up during rough-in and final fixture installation. Accessibility for future repairs matters as much as the initial install.
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