How to Flush Your Tankless Water Heater: Gas and Electric

Tankless Water Heater Maintenance

Flushing a tankless water heater on a regular basis could be the single most important task in keeping your unit running at peak performance. Failure to flush your tankless can result in a reduced service life and unnecessary fuel expenses. Although, we highly recommend having a professional check your unit annually, the flushing task is not too difficult and can save you money if you do it yourself.

Many of the maintenance tasks necessary to keep your unit running at peak performance can be completed by the homeowner. Whether you plan to do the work yourself or hire a professional, knowing what needs to be done can be a contributing factor to getting the most from your tankless system.

  • Click HERE to read our post regarding other maintenance tasks for tankless water heaters.
  • Click HERE to read our post regarding Sediment Filters.
  • Click HERE to read our post on hard water and water softeners. 
  • CONSIDER: Hiring a professional to flush your unit the first time. You'll be able to observe, ask questions, and take notes. It's money well spent and you'll have the peace of mind that you know exactly how to perform the task.

Why Tankless Water Heater Descaling is Important​

Tankless water heaters are very different from their cousins, the traditional tank style water heater, but both​ are negatively affected by limescale build-up. Although the tankless units won't leak when their preventative care isn't occurring, they will have very serious issues that can be quite expensive and annoying. 

Flushing your tankless water heater is a critical part to ​keeping your system performing at it's optimal level. Manufacturer recommendations vary, but most suggest flushing your system every 12 months. 

However, the recommended flushing frequency is likely best determined by the hardness of your water supply. Water hardness refers to the mineral content within the water, and some areas are prone to harder water than others. ​

Put it simply, when water has a high mineral content,​ lime scale will build-up inside the unit. The heat exchanger is very sensitive to lime scale and even small amounts of build-up will negatively impact the units efficiency.

Overtime the lime scale build-up will increase which will cause the heat exchanger's burner cycles to also increase in order to compensate for the lost efficiency. Because the unit needs to work harder than necessary, the lifespan will be shortened. The majority of manufacturer warranties do not cover damage caused by lime scale build-up.

Some manufacturers design their units so that they will display a diagnostic code when the limescale build-up reaches a level that requires flushing. However, even with this feature in place, it is always a best practice to flush your unit prior to the code triggering.

How to Flush Your Tankless Water Heater

Tankless Maintenance - Inlet Filter

Lime scale build-up can wreak havoc and cause damage as well as negatively impact the efficiency and service life. Many manufacturers are beginning to design models that help reduce lime scale build-up from forming. But even with this new technology, it's a good idea to flush your unit on a regular basis. 

  • IMPORTANT:  Both gas and electric tankless water heaters need to be flushed regularly, but it's important to note that the procedures are different. The process for both are outlined in separate sections below.

There are some commercial solution products available that remove lime scale build-up, however, most manufacturers recommend using vinegar.  It's safe, environmentally friendly, inexpensive, and very effective.

Once you have the supplies needed, the cost of performing your tankless water heater maintenance can be as low as purchasing 4-gallons of vinegar!

  • WARNING: Use caution when working with your tankless water heater. The water within the system can be VERY hot, especially if the unit was just operating.
  • WARNING: We strongly recommend NOT using chemicals to flush your tankless water heater system. Vinegar is a safe and effective cleaning agent. It is environmentally friendly and recommended by most manufacturers.
  • Always check your owner's manual for specific instructions regarding your unit. 

Gas Fueled Tankless Water Heaters

Some manufacturers provide Isolation Valves with Service Ports for their units. Other manufacturers leave the decision up to the plumber who is installing the tankless water heater. If your tankless water heater does not have Isolation Valves with Service Ports we strongly recommend contacting a plumber to have them installed. 

Necessary Supplies

  • Adjustable Wrench.
  • Needle Nose Pliers.
  • Screw Driver.
  • Non-contact voltage tester available from Amazon.
  • 4-gallons of virgin, food grade white vinegar.

This flush kit is available from Amazon and includes a 5-gallon bucket, hoses and submersible pump . . . all you need is the vinegar!

Or, the parts are available individually from Amazon.

6-ft washing machine hoses.

Submersible pump.​

These isolation valves with service ports come with a hot water pressure relief valve and they are available thru Amazon.

Cold and Hot Water Shut-offs - Yellow handles.

Cold & Hot  Water Service Ports - blue and red handles. 

  • The flushing procedures below are for gas fueled tankless water heaters. If you have an electric unit, follow the instructions outlined for electric tankless water heaters. 
  • The amount of time recommended to circulate the vinegar varies between manufacturers, but as a general rule of thumb, longer is better.
  • We highly recommend that you have Isolation Valves with Service Ports installed. If you do not, we recommend that you stop and call a qualified professional. 
  • These procedures should be used as a guide. You should always check your owners manual for instructions for your specific unit. 

Flushing Procedures for Gas Tankless Units

  1. Disconnect the electrical power to the tankless water heater. 
  2. Remove the access panel and test the wires leading into the terminal with a non-contact voltage tester to verify that all power has been disconnected.
  3. CLOSE the Hot and Cold Shut-off Valves. This will keep the cold water from entering your unit and prevent water from entering your home's plumbing system. 
  4. Relieve the pressure by opening the Hot Water Pressure Relief Valve on the Hot Water Line. Use caution as any water released may be very hot. 
  5. Connect one end of  Hose #1 to the Cold Water Service Port and connect the other end of Hose #1 to a Submersible Pump.
  6. Place the Submersible Pump into a 5-gallon bucket.
  7. Connect one end of Hose #2 to the Hot Water Service Port. Place the unattached end of Hose #2 into the bucket.
  8. Fill the 5-gallon bucket containing the submersible pump with 4-gallons of virgin, food grade, white vinegar. Do NOT dilute the vinegar.  
  9. OPEN the Hot and Cold Service Ports
  10. Turn ON the Submersible Pump and allow the vinegar to circulate thru the tankless water heater for at least 60 minutes. This process will break down and flush out the lime scale build-up. The pump should operate at a rate of 4-gallons per minute, if your pump circulates at a slower rate, allow the pump to run longer.
  11. Turn the Submersible Pump OFF and dispose of the vinegar.

Rinse the Vinegar from the Unit after Flushing

  1. ​Remove the unattached end of Hose #2 from the bucket and place it in a drain or outside.
  2. Remove Hose #1 and CLOSE the Cold Water Service Port and OPEN the Cold Water Shut-off Valve. Do not open the Hot Water Shut-off Valve.
  3. Once the Cold Water Service Valve is opened, fresh water will begin to flush out any remaining vinegar within the unit thru Hose #2.
  4. Allow the water to run for 10 minutes, then CLOSE the Cold Water Shut-off Valve. This will stop the water from flowing thru the unit.
  5. Once the water stops draining, remove Hose #2 and  CLOSE the Hot Water Service Port.

Clean the Inlet Filter

  1. Remove the Cold Water Inlet Filter.  (All tankless water heaters have a Cold Water Inlet Filter. A few manufacturers also install a Hot Water In-Line Filter which should also be cleaned at this time.) 
  2. Flush the Cold Water Inlet Filter with tap water to remove any sediment that has collected within the housing and screen. 
  3. Securely replace the Cold Water Inlet Filter and OPEN the Cold Water Shut-off Valve
  4. OPEN the Hot Water Line Shut-off Valve.
  5. Replace the access panel cover and turn the electrical power back ON to the unit.

How to Flush Your Gas Tankless​ Water Heater

How to Clean Your Inlet Filter

  • BEST PRACTICE: Clean the Cold Water Inlet Filter in between flushings if your water supply is prone to sediment. Some areas have more sediment in the water supply than others. This simple task only takes a few minutes but it is very important to the health of your unit. Think of the Cold Water Inlet Filter as the last line of defense in protecting your tankless.

Electric Fueled Tankless Water Heaters

Although electric tankless water heaters require less maintenance than gas units, they do still require periodic maintenance and inspections. Just like the gas fueled, models, hard water is a concern because of the lime scale build-up that accumulates over time.

Most manufacturers recommend annual flushing, but it should be noted that ultimately, the flushing frequency needs to be determined by the hardness of your water. The harder the water, the more frequently you should flush.

  • IMPORTANT: Checking your owner's manual is critical with an electric unit since there can be significant design variations between manufacturers. If not flushed properly the tankless water heater could be damaged and require service/repair.

Why You Should Check Your Owner's Manual


Some manufacturers, like the EcoSmart, offer the option to install Isolation Valves with Service Ports that allow you to flush your unit the same way you would a gas tankless.  Installing the ports will require a larger investment upfront, but overall the task will be simpler since you will no longer need to remove the electric heat elements. 

If the Isolation Valves with Service Ports are NOT installed on an EcoSmart, the unit should be flushed using the procedures outlined below. 


Other manufacturers, such as Bosch, use a design with smaller water flow channels which will require an entirely different flushing method. If these units are flushed using the gas flushing procedures, the narrow passages and small dip tubes will become clogged as the lime scale is flushed from the system. A professional will likely need to repair the unit.  

​Bosch offers an O-ring kit that can be purchased. Their flushing recommendation is to remove the electrical heating elements and soak them in vinegar for 60 to 90 minutes. After soaking a brush can be used to help flick any lime scale from the elements. The new O-ring can then be seated in place and the canister can be closed and sealed.

  • The flushing procedures below are for electric tankless water heaters. If you have a gas unit, follow the instructions outlined above for gas tankless water heaters. 
  • If your manufacturer offers the option to install Isolation Valves with Service Ports, and you have them in place, follow the flushing steps above for gas units.
  • If you are unsure of how to flush your specific unit, you may want to consider hiring a professional and taking notes. 
  • These procedures should be used as a guide. You should always check your owners manual for instructions for your specific unit. 

Flushing Procedures for Electric Tankless 

  1. Turn OFF the electrical power to the unit at the breaker.
  2. CLOSE the Cold Water Shut-off Valve. This will prevent water from entering the unit.
  3. Drain the water from the unit by opening the faucets inside your home.
  4. CLOSE the Hot Water Shut-off Valve.
  5. Remove the cover and double check that no electricity is reaching the unit by using a non-contact voltage tester

Remove the Electrical Heating Elements

  1. ​Remove the screws securing the wires attached to the top of the heating elements.
  2. Use a wrench to loosen the hexagon brass top of the heating element. Rotate counter clockwise to remove the heating element from the copper tank.
  3. Inspect the heating elements. If any are cracked they should be replaced.
  4. Place the heating elements inside the cooper chambers of the tankless unit.
EcoSmart 27 Electric Tankless

EcoSmart 27 Electric Tankless Water Heater

Fill with Vinegar

  1. ​Fill the chambers (with the heating elements inside) with virgin, food grade, white vinegar. 
  2. Allow the vinegar to soak inside the chambers for 90 to 120 minutes.
  3. Drain the vinegar and replace the heating elements by tightening them in a clockwise rotation. 
  4. Using a screwdriver, secure the wires to the top of the heating elements.

Flush Vinegar From the Unit

  1. OPEN the Cold Water Shut-off Valve and allow the water to fill the tank.
  2. While the tank is filling, check for leaks.
  3. OPEN the Hot Water Shut-off Valve.
  4. OPEN several hot water faucets in your home and allow the water to run for about 5 minutes. This will help flush the vinegar and remove any air pockets within the lines. 

Clean the Inlet Filter.​

  • Remove the Cold Water Inlet Filter on the cold water line. Flush out any sediment and debris from the filter housing and securely replace the filter.