Flushing Tankless Water Heaters

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Flushing tankless water heaters – a simple step-by-step guide. Don't guess, find out when and how to flush your tankless heater. If you want to maintain the top performance, efficient heating and long life of your tankless unit, simple removal and flushing out of the limescale and sediment buildup is the way to go.


Every manufacturer of tankless water heaters recommends regular cleaning and maintenance of the unit, and flushing if necessary. Due to the hard water in many communities, especially if the well water is used, the heater might develop limescale and mineral deposits on heater's elements.

Sediment build-up is particularly an issue in narrow water passages, such as the heat exchanger, as it might clog the element and prevent normal heating operation.


Why flushing is important

One of the most important elements in tankless is the heat exchanger. Flushing tankless water heaters whose heat exchangers are blocked by the limescale is very important and must be done before something worse happens or as soon as the problem is detected.

Most modern water heaters are equipped with the sensors and diagnostic system which will react and notify the user when the limescale has developed. In a case of Rinnai, you will see "LC" or "OO" error codes on the remote display, which indicates that there is a problem and that flushing needs to be done.

If you don't flush the water heater, the heat exchanger can be damaged due to high temperatures, as not enough water will pass through to cool the element down.

Keep in mind that any damage related to limescale build-up is not covered by the warranty.

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Video and step-by-step guide how to flush tankless

As said, flushing tankless water heaters is not complicated, and once done properly you have to reset the fault code by resetting the power. Here are the step-by-step instructions (I will consider that isolation or service valves are installed):

  • Turn the power OFF (electricity and gas) to your tankless heater.

  • Close the shutoff valves on hot and cold water lines.

  • Use the service (isolation) valve and connect the circulation pump (such as a pond pump) to the cold water line (garden hose is OK). The circulation pump is used to force the flushing solution through the heater. To make the flushing solution, use approximately 4 gallons of food grade white vinegar.

  • Now, connect the heater to the hose, by using another service valve and terminate it to the appropriate drain bucket.

  • A drain hose and pump hose should end in the same bucket.

  • Open both service valves.

  • Run the circulation pump and let the flushing solution go through the unit for one hour approximately.

  • Turn the pump OFF.

  • Rinse the cleaning solution from the water heater by opening the shutoff valve on the cold water line (close the service valve between the pump and heater) so it runs through the heater and into the bucket (approximately 5 min.) After that open the shutoff valve on the unit and close the service valve between the heater and the bucket.

  • Disconnect all the hoses.

  • The recommendation is also to clean the in-line filter.

  • Resume the power and run the water heater.


The instruction from the above is recommended by the Rinnai manufacturer. This is a safe and effective way of flushing the tankless water heaters, which can be done by its owners or a professional plumber.

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