Check both approvals
Give the provider the heater model and the cleaner's full label and safety information. Ask for the manufacturer's approved product, procedure, and limits.
A. O. Smith's mineral-build-up bulletin discusses deliming products suitable for potable water, meaning water intended for drinking, and following product directions. It is not permission to substitute any household acid. A. O. Smith deliming guidance.
Rinnai's tankless flushing page specifies vinegar and warns against other chemical solutions in that procedure. Its guidance is not a universal recipe for storage tanks or every model. Rinnai flushing guidance.
Different equipment and treatments have different requirements. The actual model's approved instructions govern.
Establish whether treatment is needed
Ask what finding supports chemical deliming (using an approved treatment to remove built-up mineral deposits) rather than routine flushing or another repair. Noise or poor hot water alone does not identify the deposit or prove that chemical treatment will fix it.
Request the scope, product, household-water safeguards, isolation method, discharge handling, and verification before return to use. Have a qualified plumbing or water-quality provider own work outside the heater's service scope.
Do not mix chemicals, remove safety valves to create an opening, or copy a chemical concentration from a different manual.
If something was already added
Stop experimenting and keep the product container. Tell the qualified provider what was used, the amount, timing, and whether water was drawn afterward.
Until you have qualified water-use direction, do not use the affected water for drinking, cooking, brushing teeth, or bathing. Use commercially bottled water for drinking and food, and arrange bathing with a known safe supply. Ask the water-quality provider or health department which fixtures are affected and what is needed before use resumes. Boiling or disinfecting does not make toxic chemical contamination safe. CDC unsafe-water guidance, CDC chemical-contamination safety.
Ask for equipment and water-use direction based on that information. Do not assume a short flush restores drinking-water safety. For a suspected exposure or illness, obtain poison or medical guidance promptly.
Your next step is to request an approved, documented treatment plan before using any cleaner. The plan must protect the heater and the water your household uses.