Descaling Heat Exchangers on
Bosch Tankless Water Heaters

Step-by-step guide on descaling heat exchangers on Bosch tankless water heaters, for better performance and to maximize the efficiency. Find out who should perform descaling and detail instructions about the procedure and required tools.

The article about removing the limescale build-up covers most of the non-condensing and all condensing water heaters from Bosch and can be also implemented on other brands and models.

Why descaling

Descaling water heaters, especially tankless, is very important to keep the performance high and unit functional for many years. According to the manufacturers, flushing limescale should be done periodically, at least once a year.

Heat exchanger is very important element on every tankless water heater, including Bosch, and it has to withstand high thermal and mechanical stress. Due to its design, which includes small water passageways, the heat exchanger is very sensitive to limescale and mineral build-up.

The scale build-up, if not properly removed, will not only reduce the flow rate, heater's efficiency or make the disturbing noise, but will shorten its life. This is especially important if you live in the area where the water supply has a high mineral content, also known as hard water.

Descaling procedure

Tools required

  • A set of plumbing wrenches

  • Bucket with the capacity of several gallons
  • Pump

  • Three hoses

  • Filter

How to remove the scale build-up

  • Turn the unit OFF and shut the water supply OFF.
  • Relieve the water pressure by opening hot water taps.

  • Disconnect the water inlet and outlet and drain water.
  • Use one of the three hoses and connect the inlet of the water heater and outlet of the pump.

  • Take the second cable and connect one end to the water heater outlet and the other end submerse in the bucket with the descaling solution.
  • Take the third cable and connect one end to the inlet of the pump and the other put in the bucket. This end will have a filter to reduce the larger particles to enter the system again.

  • Make sure that all connections are not over-tight and there is no leak.
  • Run the pump through the tankless unit and its heat exchanger. Depending on the amount of the lime build-up the returning solution might be unclean; it may take some time before it comes out clear.

  • After you flushed out all the scale from the heat exchanger, it is time to reconnect the cold water line and flush the unit with the fresh, clean water.
  • Reconnect the hot water line and open all the water valves.

  • Resume the power and run the unit.
Descaling heat exchanger is a very important step in keeping the water heater efficient and long-lasting. The higher the temperature is, the greater the risk of lime build-up is. According to the manufacturer, problems due to the water quality may void the warranty, so they also recommend installing a water softener.

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