Have you decided to save some money and flush your tankless water heater yourself? We will show you how. Follow this step-by-step guide!

In case you own a tank water heater check out our other guide about how to drain and flush your tank water heater.

If you are not comfortable going through this process yourself we recommend you to hire a professional plumber.

Get a free quote from plumbers in your area through this link!

Why Should You Flush Your Tankless Water Heater at Least Once A Year?

Water contains minerals, doesn’t matter where you live. Depending on the area there may be different types and amount of minerals in the water. However, it doesn’t change the fact that the water going through your tankless water heater does contain minerals.

Minerals in water generally speaking are not unhealthy for a human. However, they are not healthy for your water heater. It is because over time a tiny amount of the minerals precipitate within your tankless water heater. Over a long period of time it can cause problems, even ruin your expensive device.

So, it is important to flush your tankless water heater once a year because doing so you can extend its life.

What Do You Need to Do It Yourself?

You don’t need a whole lot of expensive stuff to go through this process yourself:

  • A pump plier to loosen and tighten valves
  • A circulation pump to circulate the fluid through the water heater. This is the most expensive piece you need to buy but the good thing is that you buy it once, you can use it for years to come. Some manufactures make kits that are perfect for this. This kit contains the chemical that you will use to clean the system, too.
  • Chemical for cleaning – if you decide to buy a circulation pump and not a cleaning kit mentioned at the previous point, you will need to buy the chemical separately – or you can use something instead of the chemical and save come bucks. More on that in the guide. J
  • Two hoses that will be connected to the water heater and the circulation pump. Garden or e.g. washing machine hoses will both do the job.
  • A bucket in which first you will mix the chemical with water and later on you will use it to clean the chemical out of the tankless water heater.

Now that you know what you need to go through the steps, let’s jump in!

How to flush your tankless water heater yourself

Find The Flush Valves

Make sure that there are two flushing valves at the bottom of your tankless water heater. The one on the hot side will be the outlet valve the other one on the cold side will be the inlet valve.

Turn Off The Isolation Valves

This is the first important step. You need to turn off the 2 isolation valves in order to isolate the tankless water heater from the house. You want your device to be isolated during most of the flushing process.

Hook Your Hose Connections Up

Hook your hose connections up to the flush valves. Tighten them using the pump plier.

Pro Tip: Don’t Flush The Water Out Of Your Tankless Water Heater at This Point

You may have heard that at this point you should flush the water out of the device before starting the cleaning process itself. We don’t find it to be necessary.

If you don’t flush that water out of the system at this point 1. You will save some time 2. You won’t need to add as much water to the chemical with which you will clean the water heater.

Connect The Circulation Pump

You need to connect the circulation pump to the other end of the hose that is connected to the cold water inlet valve. Attach the hose to the discharge side of the circulation pump. You can use the pump plier to tightly connect it. Once you’re done with the connection, just drop the circulation pump in the bucket.

Drop The Hot Water Hose in The Bucket

Simply drop the non-connected end of the other hose connected to the outlet flush valve of the tankless water heater.

By doing so you will basically create a closed-loop system which looks as follows: The circulation pump pumps the water-chemical mix into the tankless water heater through the valve connected to the inlet flush valve.

The water goes through the heat exchanger cleaning the calcium and magnesium buildup off the inside of it and the “dirty” water leaves through the other hose flowing directly into the bucket.

Add The Water And The Chemical Cleansing Agent to The Bucket

Following the instructions on the chemical cleansing agent you have, mix it with fresh water in the bucket.

Open The Inlet And Outlet Flush Valves

Open first the inlet and then the outlet flush valve using the pump plier.

Start The Circulation

Plug in the circulation pump. This will start the cleansing process.

Remember, in the beginning we isolated the tankless water heater from the house by turning off the isolation valves. Now you know why it is important: you don’t want the chemical to get into the plumbing system of the house, you want it to be isolated within this circulation system you’ve just created.

After you are done with the cleaning process you will be able to easily clean the tankless water heater from the chemical-water mix.

Let the chemical circulate for around 30 minutes.

Unplug The Circulation Pump And Remove The Valve From The Cold Side

  • Once you’re done circulating the chemical through the tankless water heater for about 30 minutes you can unplug the circulation pump. This will stop the circulation.
  • Shut off the valve on the cold side. To this is connected the hose that is also connected to the circulation pump.
  • Loosen and remove the same hose from the cold side inlet valve.
  • Remove the circulation pump from the bucket. Also, empty the bucket which at this point still contain the chemical-water mix.

Flush The Tankless Water Heater

  • After emptying the bucket put the free end of the hose connected to the flush valve on the cold side of the water heater.
  • Open the cold water inlet valve (isolation valve) of the water heater. This way you can flush out the water heater with fresh water. Use the pump plier to open up the valve.
  • Let the valve open until the bucket is almost full. Repeat this process 3-4 times.
  • Caution! The water coming out of the tankless water heater may be really hot because the device is not turned off and it heats the water flowing through it really quickly!
  • Once you’re done flushing the system 3-4 times run a pH test on the water to make sure that all the chemicals have been flushed out.

Final Steps

    • Close the flush valve on the hot side. Remove the hose.
    • Turn the water back on to the house by opening the hot water outlet valve (the other isolation valve).

Congratulations, you are done! Don’t forget to repeat this process once a year!

Some More Pro Tips

  • We didn’t recommend you to turn the water heater off in the beginning of the process. Some people may tell you to do so.Based on information coming directly from manufacturers the flush can be more effective when the heater is on. If you follow our advice and don’t turn the heater off during the process you need to remember one thing though: water going through the heat exchanger will be heated so, be careful how you handle the water coming out of the tankless water heater (into the bucket).
  • You can use vinegar instead of chemical cleansing agent. Doing so you can save some money. You will need around 2-4 gallons of vinegar. If you decide to do so, there are two things to do differently:
    • Turn the power off in the beginning
    • Circulate the vinegar through the tankless water heater for 1,5-2 hours (instead of around 30 minutes).

This is the end of this guide. We hope that you found it to be useful. If you have anything to ask or share please leave a comment and we will get back to you. Remember, you can always ask a professional plumber to go through this process if you don’t have the time, tools or courage to do it yourself.

Get a free quote from local plumbers here!