Using high-concentration 30 percent vinegar or harsh industrial acids to flush a tankless water heater is strongly discouraged by most manufacturers, as excessive acidity can corrode internal rubber seals, degrade O-rings, and damage delicate metal heat exchangers. Standard distilled white vinegar, which maintains a gentle 5 percent acidity level, is universally recommended because it provides the ideal balance of mineral-dissolving power without harming internal components. If mineral scaling is exceptionally severe, homeowners should opt for certified commercial descaling solutions specifically formulated and approved by the water heater manufacturer rather than guessing with high-strength industrial acids.
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