How To Flush Your Tankless Water Heater

If you own a tankless water heater, it is highly recommended that you flush your water heater at least once per year to prevent the buildup of calcium and other minerals. If it is not consistently flushed, corrosion can occur and greatly decrease the longevity of your water heater and reduces your need of a water heater repair service. Luckily, you don’t need a tankless water heater flush kit – you can do this with household items! This process usually takes at least one hour, if you have all your materials prepped.

Required Tools:

  • 1 gal. of white vinegar
  • 5 gal. bucket
  • water transfer pump
  • 2 washing machine supply lines
  • wrench or channel locks

image of the tools needed to flush your tankless water heater on your own


Step 1: Isolate Re-Circulation Pump

Identify every isolation valve in the plumbing line for the re-circulation pump. Each valve should be a ball valve (meaning there is a ball inside the piping used to seal the flow), with a long thin handle, covered with a light colored rubber grip. Turning these a quarter turn (90 degrees) in either direction will stop the flow of water.

image of a ball valve 01 image of a ball valve 02 image of a ball valve 03

Step 2: Shut Off Blue (Cold) Water Valve on Bypass Line of Water Heater

A quarter turn in either direction will turn off the flow of water. If it is difficult to turn, you may need a wrench to make it easier.

image of hand turning blue water valve handle

Step 3: Shut Off Red (Hot) Water Valve on Bypass Line of Water Heater

A quarter turn in either direction will turn off the flow of water. If it is difficult to turn, you may need a wrench to make it easier.

image of hand turning red water valve handle

Step 4: Turn Off Water Heater

This is usually located near the LCD screen which shows the temperature of your water. For gas water heaters, there may be a gas valve to cut flow of gas to the heater instead.

image of a hand turning tankless water heater off

Step 5: Turn Off Re-Circulation Pump

Find the pump control, which looks like a timer with a dial. There should be a plastic cover on it, so if there is, remove it. Then flip the on/off switch to the off position. Put the cover back on so you don’t lose it.

image of finger pointing to on/off switch of recirculation pump timer

Step 6: Turn Off the Temperature & Pressure Release Valve

This valve should look like a metal switch without a colored identifier like the other valves, and if flipped, points outward away from the piping. Flip that switch now.

image of thumb flipping the pressure release valve to off

Step 7: Locate, Remove & Clean the Inline Filter Screen

This should be located directly under the water heater, and usually has a black plastic cap with ridges, allowing you to grip it with your hand or a wrench. Unscrew it, remove it and clean out any gunk from within the tube and screen.

After cleaning it out, put it back in and tighten it. Note: This seals with an O-Ring, so don’t over tighten, or it may leak or crack.

image of unscrewing inline filter to tankless water heater image of inspection of inline filter

Step 8: Empty Vinegar into 5 Gallon Bucket

Empty the entire gallon of white vinegar into the bucket, then add water so the bucket is filled half way, effectively diluting the vinegar.

image of pouring gallon of vinegar into bucket image of filling bucket halfway with water to dilute the gallon of vinegar

Step 9: Hook Up Washing Machine Hoses to Lines

There are two bypass lines, one hot and one cold. They have red and blue valves and a piping line with a plastic or rubber cap at the end. You will need to remove those caps and proceed.

Use the first hose to hook up the hot side return from the water heater, while the other end goes into the bucket.

image of hot water bypass line & valve image of hoses going into and out of bucket

Then, use the other hose to hook up the cold water feed from the water heater and have the other side go into the outlet of the water pump.

image of cold water bypass line and valve image of hose connections to water pump

Step 10: Turn On Bypass Line Valves

These bypass lines allow for you to cut flow to and from external sources, so you can work directly with the water heater locally. By turning on the valves, the water will be redirected to their respective inlet/outlet.

image of turning cold water bypass valve image of turning hot water bypass valve

Step 11: Hook Pump to Suction Strainer

Hook the clear hose included with the pump, from the inlet of the pump to the suction strainer (the triangular piece), which you will place inside the bucket of vinegar-water.

image of hand pointing to inlet of water pump image of water pump suction device submerged in bucket

Step 12: Plug the Pump into a Wall Outlet

Now, the pump should activate (unless it has a power switch, in which case, turn it on now). Double check anywhere for leaks in case of insecurely tightened connections. Then let the pump do all the work. The process should take about 30 minutes.

image of hoses in bubbling bucket

Step 13: Dump Bucket Out, Replace With Fresh Water

Let this fresh, clean water circulate for 15 minutes.

image of replacing contents of bucket with new fresh water

Step 14: Turn Bypass Valves Back Off

It’s best to do this before unhooking the hoses, unless you desire some splash.

image of turning red bypass valve image of turning cold bypass valve

Step 15: Turn On Pressure Release Valve, Disconnect your Hoses

Disconnect both hoses and replace the cap on the line.

image of covering hot water bypass 01 image of covering hot water bypass 02 image of covering hot water bypass 03 image of covering cold water bypass 01 image of covering cold water bypass 02

Step 16: Turn Hot and Cold Lines Back On

This should allow flow to and from external sources again.

image of turning cold water line back on image of turning hot water line back on

Step 17: Remove Air Pockets from Water Heater

To do this, you will need to connect a hose back up to the hot water bypass return line. This will help remove any air pockets from the system and cause it to drain out any remaining water. Use a long hose if you intend to drain outside, or a short one if you want to drain into your bucket. Flip the valve on the return line again to let it drain out.

image of turning hot water bypass valve image of water draining from hose outside from water bypass

Step 18: Final Steps

To finalize the entire process you can now, in this order:

  • Shut off both hot and cold lines
  • Turn off hot bypass return valve
  • Unhook the hose
  • Turn on the power to your water heater
  • Turn on the power to your re-circulation pump

image of turning red hot water bypass valve image of turning blue water bypass valve image of covering bypass valve image of finger turning tankless water heater on image of turning recirculation pump on

And there you have it! You’ve successfully flushed your water heater on your own. Clean up and properly store your tools so that they are available for your next needed water heater flushing.

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If you need help with this process, have a plumbing emergency, or other plumbing problems that needs professional service, Pure Plumbing is available 24/7. Call us at (702) 930-8694 to get help immediately or to schedule an appointment with a qualified Las Vegas plumber. We are here to help!