Flushing Tankless Water Heater Instructions

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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 cleaning and flushing out the limescale and sediment buildup is the way to go.

Tankless water heaters, as opposed to tank-type heaters, do not store hot water. The cold water runs through the heat exchanger, where it is heated by the gas burner, usually located underneath. The heat exchanger utilizes a set of water pipes supported by a number of thin fins to enhance the heat transfer.

Why flushing a tankless water heater is important

Rinnai tankless water heaterRinnai tankless water heater

As the tankless heater can be affected by water quality, every manufacturer of tankless water heaters recommends regular cleaning and maintenance, where flushing is the most thorough. As hard water is found in many households, primarily if the well water is used, the heater might develop limescale and mineral deposits on the heater's elements.

Sediment build-up is mainly an issue when water is flowing through the narrow water passages of the heat exchanger, as it might clog the element, prevent regular heating operation, and other problems. Mineral buildup can even produce hot spots and corrosion, which will wear down the combustion chamber walls, affecting the lifespan of the unit.

As it can be seen, the most important element in tankless heaters is the heat exchanger. Flushing tankless water heaters whose heat exchangers are blocked by the limescale is crucial and must be done when suggested or as soon as the problem is detected. If not, the problem will aggravate.

Most modern water heaters are equipped with sensors and a diagnostic system that reacts and notifies the user when the limescale develops. In the case of Rinnai, on the remote controller or display, you can see "LC" or "OO" error codes, which indicates that there is a problem that indicates that flushing is required.

If you don't flush the water heater, the heat exchanger can damage due to high temperatures, as the insufficient flow of water will pass through and won't cool the element down.

Note: Keep in mind that the warranty does not cover any damage related to limescale buildup.

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Required tools

  • Adjustable wrench
  • Plumber wrench
  • Screwdriver
  • Multimeter
  • Bucket
  • Small submersible pump
  • Garden hose (2 pieces) (or a kit)

Instructions on how to flush a tankless water heater

As said, flushing a tankless water heater is not complicated, and once done correctly, 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 off the power (both electricity and gas) to your tankless heater.
  • Close the valves on both 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. Alternatively, you can buy a delimer from the tankless manufacturer. It is recommended to get a non-toxic, non-corrosive, and biodegradable cleaning solution.
  • Pour the cleaning solution (vinegar or delimer) into the bucket.
  • Now, connect the hose to the heater using another service valve and terminate it to the appropriate drain bucket.
  • Drain and pump hoses should end in the same bucket and into the cleaning solution.
  • Open both service valves on the hot and cold water lines.
  • 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 a bucket.
  • Disconnect all the hoses.
  • The recommendation is also to clean the in-line filter.
  • Resume the power and run the water heater.

The Rinnai manufacturer recommends the instruction as described in the above text. This is a safe and effective way of flushing tankless water heaters, which its owners or a professional plumber can do.

You can also buy a flush kit, which generally includes a pump, hoses, hose adapters, a bucket, and descaler as an option. The kits are compatible with tankless valves and can be easily connected.

Manufacturers of the tankless water heaters recommend flushing the mineral deposits at least once a year, or more often if you leave in an area with hard water.


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