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How to Flush a Tankless Water Heater in 7 Simple Steps

Fact checked by Stephen Conklin

how to flush a tankless water heater

Did you notice your tankless water heater is not giving you hot water anymore? Did you also observe a reduction in the water flow coming from your water heater? You can address these issues if you know how to flush a tankless water heater.

Hard water minerals can deposit in the water heater’s heating elements and other components, reducing its water heating efficiency. That is why tankless water heater manufacturers recommend flushing and cleaning tankless hot water heaters at least once annually. Also, you can find out more water heaters specially designed for hard water here.

This article will help you flush your tankless water heater and keep it running like new.

Table of Contents

Things You Need for This Tutorial


Flushing a tankless water heater is not complicated if you prepare the correct materials before heading to the actual procedure. Here are some items you will need to clean a tankless water heater.

Owner’s Manual

Tankless water heater manufacturers have their recommendations for cleaning and flushing their products. They also have preferences for the cleaning solution you can use for the water heater.

I suggest reading your manual’s maintenance instructions on flushing the unit. You will also learn what tools you need for such an activity and the safety precautions you must observe.

Cleaning Solution

Many manufacturers only recommend pure, undiluted, virgin, and food-grade white vinegar for cleaning the tankless water heater. This vinegar type removes mineral deposits and limescale buildup without risking your family’s health.

However, some manufacturers recommend using a specific cleaner for their tankless water heaters. That is why it is always an excellent idea to read the owner’s manual for specific instructions and recommendations.

Other Tools

If you have a gas tankless water heater, you will need a large bucket, a submersible pump, and water hoses to connect to the water heater’s service ports.

A screwdriver is also essential for opening and closing the water heater’s access panel. A non-contact voltage detector will also be beneficial in ensuring your water heater is powered off completely before cleaning.

Steps for Flushing a Tankless Water Heater


Step 1. Read the manual.

Each manufacturer has its procedure for cleaning tankless water heater systems. For example, Rinnai may have a different method for cleaning its tankless water heaters compared to Rheem or other brands.

That is why I always recommend checking the owner’s manual for specific instructions related to the product you want to clean.

Step 2. Disconnect the tankless water heater from its power source.

Contrary to what some people think, even a gas tankless water heater has an electrical connection that powers its electronic components. That is why I suggest unplugging the device from the electrical outlet.

It would be best to shut off the tankless water heater’s power supply from the circuit breaker panel. You will perform this procedure for an electric tankless water heater.

Open the access panel and use a non-contact voltage detector to double-check the electrical current in your system before proceeding. The Fluke 1AC-A1-II VoltAlert is a reliable device homeowners can use to determine voltage presence in their water heater.

Step 3. Close the gas valve, and the water supply line, and drain the water.

Locate and turn off the tankless water heater’s water shutoff valves. If your unit does not have these mechanisms, I suggest closing the main water supply line’s shutoff valve.

I also recommend closing the water heater’s gas isolation valve if you have a gas tankless water heater before shutting off the water supply.

Open all taps to empty the water in the supply line. I also suggest carefully opening the hot water pressure release mechanism to relieve pressure in the system.

Step 4. Prepare the tankless water heater.

You can forego this step if you have a gas tankless water heater.

Remove the electric water heater’s heating elements by loosening the screws securing the wires and unfastening the element’s hexagonal brass top.

Check the water heater elements for cracks and other signs of damage. You can replace the heating element with a new one after the tankless water heater cleaning.

Place the water heater elements in the copper heating chamber, allowing you to clean them, too.

Step 5. Set up the flushing system.

  • For Gas Units

Locate the tankless water heater’s cold and hot water service ports, and connect flexible tubings to both.

Get a large bucket and place a submersible pump in it. Then, attach the cold water tube to the pump. Place the hot water hose’s free end in the container or bucket.

Fill it with four gallons of an appropriate tankless water heater cleaner or pure, food-grade, virgin white vinegar. A few water manufacturers have recommendations for a descaler or cleaner. However, most companies recommend only white vinegar.

  • For Electric Units

Open the copper heating chamber and fill it with pure, food-grade virgin white vinegar. You can also use a manufacturer-recommended tankless water heater descaling solution.

Step 6. Flush the tankless water heater.

  • For Gas Units

Open the tankless water heater’s cold and hot water service ports and power the submersible pump on. Let the white vinegar or cleaner circulate throughout the system for an hour.

You might notice the water from flushing the tankless water heater with vinegar turns blue. It is not a cause for alarm because it is a sign that the solution contains dissolved copper oxide from your tankless water heater.

Power off the pump and drain the cleaner or vinegar solution from the tankless water heater. Next, turn off the service port for cold water and detach the tubing from the pump.

Turn on the supply line’s shutoff valve to flush any remaining vinegar or cleaner and let it run for at least ten minutes.

Shut off the valve and continue draining the water. Turn off the service port for hot water and detach the tubing.

Check the inlet for cold water and then the hot water supply line for any filter. Remove these filters and clean them in the sink, ensuring no sediments or debris remain.

  • For Electric Units

Let the white vinegar or cleaner sit in the copper heating chamber for at least 90 minutes. However, most experts recommend two hours to ensure the complete dissolution of limescale and mineral deposits on the heating elements.

Replace the water heater elements and refasten the electrical wires. Next, open the cold water line’s shutoff valve to fill the heating chamber with water. After a few minutes, open the hot waterline’s shutoff mechanism and all faucets in your house.

Continue running the water for five to ten minutes or until you no longer smell vinegar or cleaner from the water. Doing so also purges air from your water line.

Step 7. Finish up.

  • For Gas Units

Replace and secure the filters to the hot water line and cold water inlet before opening the shutoff valves of the cold and hot water lines.

Place the access panel back and secure it with screws. Turn on the gas supply line’s shutoff valve, and plug the heater unit back into the electrical outlet. You can also turn the power on at the circuit breaker panel.

  • For Electric Units

Close all faucets and shut off the cold water valve.

Remove and clean the cold water supply inlet filter to ensure no debris remains before re-securing it.

Secure the access panel and power on your water heater at the circuit breaker panel.

I find Silver Cymbal’s video on cleaning and flushing tankless water heaters informative. I suggest you consider watching the five-minute clip.


How to flush a tankless water heater is as breezy as closing a few valves and pouring the vinegar or appropriate cleaning/descaling solution into the heating chamber or a bucket.

Letting the solution sit for several minutes before dumping the cleaner rounds up the tankless water heater flushing process.

I hope you enjoyed this tutorial and share it with your acquaintances. You can also send me questions or comments about this guide, and I will be happy to share my thoughts with you.

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