Flushing Tankless Water Heaters

Flushing tankless water heaters - step by step guide. Don't guess, find out when and how to flush your heater. The simple limescale and sediment removal ensures the top performance, efficient heating and long life.


Every manufacturer of tankless water heaters recommends regular cleaning and maintenance, and flushing if necessary. Due to the hard water, especially from the well, the heater might develop lime and mineral deposits on heater's elements. Sediment build-up is especially dangerous in narrow water passages as it might clog the element and prevent further normal heating operation.


Why flushing is important

One of the most important elements is the heat exchanger, which has those narrow water passages. Flushing tankless water heaters whose heat exchangers are blocked by the lime scale is very important and must be done before something happens.

Most modern water heaters are equipped with the sensors and diagnostic system which will react and notify the user when the lime scale has developed. In a case of Rinnai, you will find "LC" or "OO" error code, which indicates these problems and that flushing needs to be done.

If you don't flush the water heater, the heat exchanger can be damaged as not enough water will pass through and cool it down. Any damage which is related to lime scale build-up is not covered by the warranty.


emergency plumber

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 pond pump) to the cold water line (garden hose is OK). The circulation pump is used to force the flushing solution through the heater.

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

  • Drain hose and pump hose should end in the same bucket.

  • As the flushing solution use approximately 4 gallons of food grade white vinegar.

  • 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 the heater) so it runs through the heater and into the bucket (approximately 5 mins.) After that open the shutoff valve on the unit and close the service valve between the heater and the bucket.

  • Disconnect all hoses.

  • 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 tankless water heaters, which can be done by its owners or if not than the professional plumber.

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