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How to Flush a Tankless Water Heater

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The single most important thing you can do to maintain the health of your tankless water heater is to flush it on a regular basis. Tankless units can have a service life of 20+ years if they are properly cared for, but allow lime scale to build-up within the internal parts of the system and you'll have premature issues. 

Many maintenance tasks can be performed by the homeowner and flushing your tank isn't too difficult. You'll not only save money by doing it yourself, but you're also likely to perform the task more frequently since you won't need to pay a plumber. However, we still highly recommend having your gas unit inspected annually to make sure that combustion is happening as it should and everything else is okay. 

Why Flushing Your Tankless Water Heater is Important

All water based appliances have trouble with two issues: Sediment and limescale build-up. Tank-style water heaters need preventative maintenance to keep these issues from shortening their service life. But tankless units are a particularly a concern.

A tankless water heater is a very expensive and finely tuned system. If the unit isn't flushed, it can result in serious (and expensive) issues. But the good news is, if you take care of a tankless unit, it'll last a very long time!

Limescale build-up occurs when the minerals within the water separate out and begins to form a film within pipes, heat exchangers and anywhere else the water travels. When a tankless unit is flushed, the limescale is flushed from within the unit. 

Most manufacturers recommend flushing every 12 months, however, you should make the decision based on your location. Some areas are prone to harder water than others, which will create more limescale. The higher the mineral content within the water, the more lime scale will build within the unit.

You may want to consider hiring a professional to flush your tankless water heater the first time. You'll have the opportunity to watch the process, ask question and take notes (maybe even record it to video!) 

When it's time to flush your unit again you'll know exactly what needs to be done. Consider it an investment in the health and longevity of your unit. 

Why Flush?

  • Lime scale build-up can wreak havoc on the internal parts of your tankless system and ultimately cause expensive damage.
  • The heat exchanger is extremely sensitive to lime scale. Even a small amount of build-up will lower the unit's efficiency and increase your fuel costs.
  • As the lime scale build-up increases the heat exchanger's burner cycles also increase to compensate for the loss of efficiency. The unit works harder than necessary and the lifespan is shortened.
  • Most manufacturer warranties do not cover damage caused by water quality. This includes lime scale build-up.

Preparing to Flush

The cost of flushing your tankless water heater can be as low as buying 4-gallons of vinegar. There are some supply's you'll need, but once you have them you'll be set for the future. 

Isolation Valves with Service Ports

Isolation valves with service ports are an optional item. Some manufacturers include them with their tankless system, while others leave the decision to the plumber and homeowner.

These isolation valves with service ports come with a hot water pressure relief valve. The yellow handles are the cold and hot water shut-offs, and the red and blue handles open and close the hot and cold water service ports.

3/4 IPS LF EXP E2 With PRV Tankles

Isolation Valves

If your tankless system doesn't have isolation valves with service ports, we strongly recommend having them installed by a plumber. They'll make the flushing procedure much easier.

Necessary Supplies

* Most manufacturers recommend using virgin, food grade white vinegar, however there are some commercial solutions available. Vinegar is environmentally friendly, safe, inexpensive and effective. 

** There is a flush kit available that contains 2 hoses, descale solution, a pump & a bucket.

My PlumbingStuff - Tankless Water Heater Flushing Kit - 32-ounce J.C. Whitlam FLOW32 Flow-Aide System Descaler - Zoeller 42-0007 Floor Sucker Pump - 5-Gallon Bucket - User Instructions - Safe to Use

Flush Kit

You may want to consider purchasing a flush kit that has all the supplies you'll need, including the submersible pump.


  • Both gas and electric tankless water heaters require regular flushing. However, they each use different procedures.
  • Always use caution when working with your tankless system. The water within the unit can be VERY hot.
  • Vinegar is a safe and effective cleaning agent. We strongly recommend NOT using chemicals to flush your system. 
  • Review your owner's manual for specific instructions on your unit. 

How to Flush a GAS Tankless Water Heater

These flushing procedures are for a GAS tankless water heater. If you have an electric system, please follow the procedures outlined for electric tankless water heaters.

The amount of time to circulate the vinegar varies between manufacturers, but as a general rule, longer is always better. 

These procedures should be used as a guide, your owners manual is always your best resource.

Gas Tankless Flushing Procedures

  • Disconnect  the electrical power to the tankless water heater.
  • Remove the access panel. Test  the wires leading into the terminal with a non-contact voltage tester to verify that no electricity is reaching the unit.
  • Close  the hot and cold water shut-off valves. This will keep the cold water from entering the unit. It will also keep water from entering the plumbing system.
  • Open  the hot water pressure relief valve on the hot water line to relieve the pressure. Be careful, the water could be very hot.
  • Hose #1 Connections - Connect one end of hose #1 to the cold water service port and connect the other end to a submersible pump. 
  • Place the submersible pump in a 5-gallon bucket.
  • Hose #2 Connections - Connect one end of hose #2 to the hot water service port. Place the other end of hose #2 into the bucket. 
  • Fill the 5-gallon bucket with 4-gallons of virgin, food grade, white vinegar. Do NOT dilute the vinegar.
  • Open the hot and cold  service ports.
  • Turn on  the submersible pump. The vinegar should circulate thru the unit for a minimum of 60-minutes. This process will break down and flush the limescale from the unit. The pump should run at a rate of 4-gallons per minute. If your pump circulates at a slower rate, allow the vinegar to circulate longer. 
  • Turn off  the submersible pump and dispose of the vinegar.

Rinse the Vinegar from the Unit

  • Hose #2 - Remove the unattached end of hose #2 from the bucket. Place it in a drain.
  • Hose #1 - Remove hose #1 and close the cold water service port.
  • Open  the cold water shut-off valve. Do not open the hot water shut-off valve.
  • With the cold water service valve open, fresh water will begin to run thru the unit and flush any remaining vinegar thru hose #2.
  • Allow the water to run thru the unit for 10-minutes.
  • Close  the cold water shut-off valve. This will stop the water from flowing thru the unit.
  • When the water stops draining. Remove hose #2 and  close  the hot water service port.

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This video shows you how to flush a gas tankless water heater.

Watch the Video

Best Practice: Remove and clean the cold water inlet filter in between flushings. This easy task will only take a few minutes, but it can make a big difference in the health of your tankless.

Clean the Inlet Filter

  • Remove  the cold water inlet filter. (All tankless water heaters use a cold water inlet filter. Some manufacturers also use a hot water in-line filter. Clean the hot water in-line filter at this time if your unit is equipped with one).
  • Flush the cold water inlet filter with tap water.
  • Replace  the cold water inlet filter and  open  the cold water shut-off valve.
  • Open  the hot water line shut-off valve.
  • Replace  the access panel cover. Turn the electrical power On  to the unit.

This video shows how to clean an inlet filter on your tankless water heater.

Watch the Video

How to Flush an ELECTRIC Tankless Water Heater

These flushing procedures are for an ELECTRIC tankless water heater. If you have a gas system, please follow the procedures outlined for gas tankless water heaters.

These procedures should be used as a guide. You should always check your owner's manual for specific directions on your unit. 

The majority of manufacturers recommend flushing the unit every year but the frequency ultimately needs to be the homeowners decision. If you live in an area with hard water, you should flush your system more frequently.

Checking your owner's manual is critically important with electric tankless water heaters. There are significant design variations between manufacturers and if the unit isn't flushed as advised, the water heater could be damaged and require professional care.

You may want to consider hiring a professional to flush your unit. You can take notes, ask questions and gain the knowledge to do the job yourself next time. 

An Example of Why it's Necessary to Check Your Owners Manual

Manufacturers such as EcoSmart have designed their water heaters with the option of using isolation valves with service ports. The use of isolation valves and service ports will simplify the flushing process since there won't be a need to remove the electric heat elements.

Installing these ports will require a larger upfront investment when installing the unit. If the ports are NOT installed on an EcoSmart, the tankless water heater should be flushed by following the procedures outlined below.

Other manufacturers, such as Bosch, are designed with smaller water flow channels. If these units are flushed incorrectly, the narrow passages and small dip tubes will clog with lime scale during the process and require a professional to repair the unit.

Bosch sells an O-ring kit to be used each time the unit is flushed. Their flushing procedures require the homeowner to remove the electrical heating elements and soaking them in vinegar for 60 to 90 minutes. A brush is used to flick the lime scale from the elements before they are replaced and sealed back into the canister.


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Electric Tankless Water Heater Flushing Procedures

If your tankless water heater has isolation valves with service ports in place, you should follow the flushing steps above for gas tankless systems.

  • Turn off  the electricity to the unit at the circuit breaker.
  • Close the cold water shut-off valve to keep water from flowing into the unit.
  • Open  a faucet inside your home to drain the water from the unit.
  • Close  the hot water shut-off valve.
  • Remove the cover and use a non-contact voltage tester to verify that electricity is not entering the tankless.

Remove the Heating Elements

  • Remove the wires attached to the top of the heating elements by unscrewing the screws.
  • Rotate the hexagon brass top of the heating element counter clockwise to remove the heating element from the copper tank.
  • Look for cracks on the heating elements. If you notice any cracks, replace the element.
  • Place the heating elements inside the cooper chambers of the unit.

Fill Chamber with Vinegar

  • With the heating elements inside the chambers, fill the chambers with virgin, food grade, white vinegar.
  • Leave the vinegar in the chambers for 90 to 120-minutes.
  • Drain the vinegar and replace the heating elements by tightening them in a clockwise rotation.
  • Secure the wires to the top of the heating elements.

Rinse the Vinegar from the Unit

  • Open the cold water shut-off valve. The tank will fill with water.
  • Check for leaks while the tank is filling.
  • Open the hot water shut-off valve.
  • Open several hot water taps. Allow the water to run through the faucets for 5-minutes to flush the vinegar. This will also remove air pockets in the pipes.

Clean the Cold Water Inlet Filter

  • Remove the cold water inlet filter on the cold water line leading into the tank.
  • Flush the sediment and debris from the filter.
  • Replace the filter back on to the cold water line.

This video shows how to flush an electric tankless water heater.

Note: The video encourages you to soak the heating elements in a pan of vinegar. When we contacted the manufacturer, they recommended filling the chamber with vinegar and soaking the elements within the chamber. Doing so will remove any limescale build-up within the chamber as well. However, either procedure will work.

Watch the Video


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