Illustration of a typical descaling flush kit: isolation valves, hoses, a pump, and a bucket.

How to Flush a Tankless Water Heater

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Flushing a tankless water heater means circulating a descaling solution through it via the isolation valves to dissolve mineral scale before it restricts flow or causes overheating faults. Most manufacturers recommend this annually in hard-water areas, less often in soft-water areas.

Illustration of a typical descaling flush kit: isolation valves, hoses, a pump, and a bucket.
Illustration of a typical descaling flush kit: isolation valves, hoses, a pump, and a bucket.

Step-by-Step Diagnosis

  1. DIY SAFE
    Turn off power/gas and shut both isolation valves

    Most tankless units have dedicated hot/cold isolation valves specifically for this procedure — if yours doesn’t have them, that’s worth adding.
  2. DIY SAFE
    Connect a flush kit’s hoses to the isolation valve service ports

    A flush kit is just a small pump, a bucket, and two hoses —ee our flush kit review for tested options.
  3. DIY SAFE
    Fill the bucket with a descaling solution (food-grade citric acid or white vinegar)

    Follow your unit’s manual for the recommended concentration — don’t guess.
  4. DIY SAFE
    Run the pump to circulate the solution through the unit for the time specified in your manual

    Typically 45 minutes to an hour, depending on scale buildup and manufacturer guidance.
  5. DIY SAFE
    Flush the solution out with clean water for several minutes

    Don’t skip this — leftover descaling solution shouldn’t go straight into your hot water lines.
  6. DIY SAFE
    Reopen isolation valves, restore power/gas, and check for proper operation

    Run a hot tap and confirm normal heating before considering the job done.

FAQ

How often should I flush my tankless water heater?

Most manufacturers recommend annually in hard-water areas; soft-water areas can often go longer — check your specific manual.

Can I use vinegar instead of a commercial descaling solution?

Many manufacturers explicitly approve food-grade white vinegar or citric acid — check your manual, since concentration and flush duration can differ from a commercial product.

Written by HotWater Decoded — water heater repair specialists. Technically reviewed by a licensed technician on our team.

Sources

  • Standard tankless flush procedure, consistent across Navien, Rinnai, and Rheem manufacturer manuals

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