You can and should flush your Rinnai tankless water heater using standard, food-grade distilled white vinegar, which maintains a gentle 5 percent acidity level ideal for dissolving calcium and mineral scale without harming internal components. Rinnai officially recommends regular vinegar flushing as part of standard annual maintenance to protect the copper heat exchanger. You should avoid using high-concentration industrial acids or harsh chemical descalers unless explicitly approved by the manufacturer, as excessive acidity can corrode internal rubber seals, degrade O-rings, and damage delicate metal surfaces.
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