How to descale a tankless water heater for efficient hot water every time

How To Descale A Tankless Water Heater

Tankless water heaters are becoming more and more popular each year. This is because they have a lot of benefits over traditional tank water heaters. However, one downside to tankless water heaters is that they need to be descaled regularly to ensure proper function. 

Regularly scheduled descaling is essential to prevent the blockage of water pipes, allow an efficient transfer of heat, and lengthen your water heater’s life. Learning how to descale your tankless water heater is practical, and taking preventative measures to slow the scale accumulation is wisdom.

In this blog post, we will walk you through the step-by-step process of how to descale a tankless water heater. We will also provide some helpful tips on preventing scale buildup in the future. Let’s get started!

How to Descale A Tankless Water Heater Step by step

Tankless water heaters are designed to be descaled every 12 to 18 months. Before starting the procedure, ensure that you have all the necessary tools or equipment to do the job.

You can buy and use a tankless water heater flush kit or use your tools and equipment.

If you opt to use your equipment, you will need the following tools.

  • One big bucket, at least 5 gallons.
  • A small submersible pump.
  • Two short hoses with bibs on their ends.
  • A descaling agent such as 3 gallons of vinegar.
  • Toolbox: pliers, flat head screwdriver, channel lock, or groove joint pliers.

1. Turn off the tankless water heater

The essential first step is to turn off the power source to your tankless water heater.

For a water heater, make sure to shut off the gas valve, and for an electric tankless water heater, be sure to shut off the main circuit breaker.

2. Close Water Pumping Into Water Heater

The next step is to shut off the water going into and out of the tankless water heater. You can do this by closing the cold water valve into the tankless heater and closing the hot water valve for water going out of the heater.

You might need to use a wrench to turn these knobs. Be gentle. It should also be perpendicular to the system, “tank.” You should find this under the tank. Disconnecting hot water from going out and cold water from coming into the unit creates a loop internally.

3. Connect The Pump

Connect one of the hose connections to the hot water drain valve. Then take the second hose connection and connect it to your cold water drain valve. You might want to use a channel lock to reasonably tighten the hose connection ends.

Once the hoses are connected, take the hose connected to the cold water drain valve and connect it to the discharge side of the submersible pump. Once connected, you can drop the pump into the five-gallon bucket.

The next step is to place the hose connected to the hot water drain valve inside the bucket, which now creates a close loop system.

With the loop you’ve created, this setup allows a cleaning circulation to go from the top of the heating unit, through coils, and back out into your collection bucket. This cleaning process will take about an hour to complete.

4. Open The Heating Unit’s Hot Water End

With both hoses connected and fastened adequately, open each isolation valve near the hot and cold drain valves. You want the water that was in the tank to drain out. Water should start draining out even before you the pump gets turned on.

5. Pour Descaling Agent Into Bucket

Pour your descaling agent or 3 gallons of vinegar into your bucket. This is what will help clean and descale the tank during the circulation.

6. Turn The Pump On

Turn on the submersible pump and keep it on for at least an hour. This will give the descaling agent enough time to remove any scale buildup inside the tankless water heater.

7. Flush Out Descaling Agent

Once the descaling process has been completed, you can turn off the pump and disconnect the hose from the cold water drain valve that lets the descaling agent in.

Once that is done, you can remove the pump from the bucket and dump the bucket’s contents.

Once that is done, you can turn the cold water main valve back to its open position to allow fresh cold water to enter the tankless water heater. Keep the second hose inside the bucket to catch the cold water.

Once the bucket is filled, close the cold water valve and dump the bucket’s contents. Do this multiple times until the water heater is completed flushed of the descaling agent.

Once completely flushed, close the cold water valve one last time.

8. Clean the filters

Remove the cold water inlet filter and the hot water inline filter. Rinse these filters at the sink using running water to remove dirt or debris. Once thoroughly rinsed, place the filters back and secure them properly.

9. Turn The Water Heater’s Gas Back On

Make sure that the isolation valves are now closed and go and turn the power back on for the tankless water heater.

10. Turn Water Back On

Open your hot and cold water valves. Wait for the system to switch back on and for the water to fill the system.

Hot water should start being produced again without fear of scale ruining your faucets.

11. Check the System For Any Leaks

While running the hot water heating unit, check around the valves and under the unit for any leaks that aren’t residue water from the cleaning process. If the coast is clear, the chore is done.

How To Prevent Scale From Building Up?

Preventing scale from collecting can help lengthen the life of your water heater. It is far easier to perform preventative maintenance to prevent scale from building up than going through the descaling process.

Below are some ways to prevent the scale buildup in your tankless water heater to prolong its life and avoid descaling.

  • Use a water softener if you have hard water. This will remove minerals before they ever have a chance to build up and cause problems.
  • Flush the system regularly according to the manufacturer’s instructions—this is generally once per year.
  • Use distilled water in the heating unit instead of regular tap water—change this out every few months.
  • Install an inline filter on the cold water line going into the unit. This will help remove sediment before it has a chance to enter the system and wreak havoc.


Descaling your tankless water heater is essential to prolonging the life of your unit and keeping it running efficiently. By following the steps in this guide, you can easily descale your water heater and get it back up and running like new again.

Be sure to perform preventative maintenance regularly to avoid going through this process too often.

Frequently Asked Questions (FAQs)

How much does it cost to descale a tankless water heater?

The cost of descaling a tankless water heater will vary depending on the severity of the scale buildup and the type of system you have. A professional technician may charge anywhere from $200 to $500 for the service.

If you decide to do it yourself, expect to pay $100 to $200 for a Tankless Water Heater Flush Kit.

What do you use to descale a tankless water heater?

There are a few different products that can be used to descale a tankless water heater. The most common is white vinegar. You can also use CLR (Calcium, Lime, and Rust) or Muriatic Acid.

Can you use CLR to descale a tankless water heater?

Yes, CLR (Calcium, Lime, and Rust) can be used to descale a tankless water heater. Just be sure to follow the instructions on the product label and take all necessary safety precautions.

How often should you descale a tankless water heater?

The frequency of descaling will vary depending on the hardness of your water and how often you use your tankless water heater. In general, you should descale your unit every 12 to 18 months.

If you have hard water or use your system frequently, you may need to descale more often.

Do I need to descale my tankless water heater if I have a water softener?

If you have a water softener, you may not need to descale your tankless water heater as often. However, it is still good to check the unit regularly for any scale buildup.

How long does it take to descale a tankless water heater?

The length of time it takes to descale a tankless water heater will vary depending on the severity of the scale buildup. In general, it should take about four hours to complete the process.

Will vinegar descale a tankless water heater?

Yes, vinegar can be used to descale a tankless water heater. Just be sure to use white vinegar and follow the instructions in this guide.

The H2Home Lab team is dedicated to helping you find the best solutions to your specific water needs, as well as provide troubleshooting tips and guides for water filtration, heating, softening and plumbing. We hope you will find the info you need on our site from any one of our buying guides or subject matter articles.

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