Informational Guide

How To Flush A Tankless Water Heater

Tankless water heaters can be a large financial investment. Learning how to flush them properly can be a big return on your investment.

by Holly Curell

Tankless water heaters can be a large financial investment. So learning how to flush them properly can be a big return on your investment.

Properly flushing your tankless water heater can make sure that you get the maximum efficiency with your installation. Read on to learn more about how to install a tankless water heater 

Tankless water heaters can fill with calcium and other mineral debris from the water. To keep your machine efficient and in good working order, you should try to flush it once every year to a year and a half. Flushing will clear out the filter inside the machine so it can properly filter water again.  

Among other things, flushing your water heater can: 

  • Reduces Clogging 
  • Reduces Scale Build-up 
  • Reduces Noise 
  • Improves Heating speed  
  • Prolong Equipment Lifespan 
  • Significantly Cut Costs 

Failing to flush your heater on a regular basis can reduce efficiency and lead to problems down the road.  

Do Tankless Water Heaters Need To Be Flushed

What Happens If You Don’t Flush Your Tankless Water Heater? 

Most water contains mineral deposits in it. These mineral deposits can settle in your tankless water heater filters. The more minerals in these filters, the less efficiently the heater can do its job.

Tankless water heaters that do not get flushed build up considerable residue on their filters, reducing the efficiency of the machine. Failing to flush your tankless water heater on a regular basis can lead to:

  • Less Hot Water in Your Pipes 
  • Higher Energy Bills 
  • Inefficient Heating 
  • Leaking Water Heater 
  • Water Damage 
  • Broken Filters 

A broken water heater can be a costly replacement, so you should get in the habit of regularly flushing your system.  

How Often Should You Flush Your Tankless Water Heater? 

You should flush your tankless water heater at least once every 12 to 18 months. Any longer than that, and mineral residue on your filters can severely impact filter performance.  

Depending on the water content in your location and the frequency of heater use, you may have to change your filter earlier than once every 12 months. Common signs that you need to change filters include: 

  • No hot water 
  • Strange odors 
  • Loud noises from heater unit 
  • Rusty or discolored water 
  • Lowered water temperature 

Failing to flush your heater can lead to long-term problems and can permanently damage the unit.  

Preparing a Tankless Water Heater for Flushing (Safety Precautions & Tools) 

Before starting working with your tankless water heater, here are a few safety precautions and tools to keep in mind.  

  • Tankless water heaters have very hot water, so it can be dangerous to handle them. The water is scorching hot, especially if it has been just running recently.  
  • Do not use harsh chemicals to flush your tankless water heater system. Harsh chemicals like bleach can permanently damage internal components and may require you to install a new water heater.  
  • Make sure that you have your water heater manual on hand to consult and design issues.  

Here are the tools you will need:

  • Wrench 
  • Pliers 
  • Screwdriver 
  • Washing machine hose 
  • Voltage tester 
  • 4 gallons of vinegar 
  • Submersible pump 

How to Flush a Tankless Water Heater (Step-By-Step Guide) 

1. Gas-Powered Water Heater 

  1. 1
    Locate the electrical power to the tankless heater and turn it off. Test the wires into the terminal using the voltage tester to make sure that it is turned off.
  2. 2
    Close the hot and cold water shut-off valves. Closing these valves will ensure that cold water will not enter the unit and your home’s plumbing system.
  3. 3
    Open the how water pressure release valve. Be very careful here as any water released will be very hot.
  4. 4
    Connect a hose to the cold water service port and connect the other end of the hose to the submersible pump. 
  5. 5
    Place the submersible pump into a bucket and attach a hose to the hot water service port. Next, place the end of the second hose into the bucket with the pump.
  6. 6
    Fill the bucket and pump with at least 4 gallons of food-grade white vinegar. You should not have to dilute the vinegar.
  7. 7
    Open both the hot and cold service ports.
  8. 8
    Let the submersible pump run for about 2 hours. This will allow the vinegar to work through the system and dissolve any buildup.
  9. 9
    Turn the pump off and get rid of the vinegar in the bucket.
  10. 10
    Remove the ends of the second hose and place it in a drain or outside. Remove the hose attached to the cold water port, shut the port, and open the cold water shut-off valve.
  11. 11
    Let the water run through to flush out any remaining vinegar in the system. Close the cold water shut-off valve after about 10 minutes.
  12. 12
    Remove the second hose and close the hot water service port. Open both the hot water and cold water shut-off valves.  

2. Electric Tankless Water Heater 

  1. 1
    Turn off the electrical power to the breaker and close the cold water shut-off valve.
  2. 2
    Open the faucets in your home to drain all water from the electric tank.
  3. 3
    Close the hot water valve and test the heater unit with the voltage tester to make sure all electricity is shut off.
  4. 4
    Remove the electrical heating elements on the heater. Remove the heating element from the tank using the wrench and pliers. Make sure to replace any pieces that have cracks or damage.
  5. 5
    Put the heating elements in the unit chambers and fill them with white vinegar.
  6. 6
    Let the chamber soak with vinegar for at least 2 hours. Do not dilute the vinegar by adding water.
  7. 7
    Drain the vinegar from your system and put the heating elements back into their rightful spots on the tankless unit. Make sure that the heating element wires are properly attached.
  8. 8
    Open the cold water shut-off valve and wait for the tank to fill. Once it does, open the hot water shut-off valve.
  9. 9
    Make sure to check the tankless unit for any leaks when it is filling up.
  10. 10
    Open some faucets in your home on hot water and let them run for around 5-10 minutes. This will flush the remaining vinegar through the system and out into your drains.
  11. 11
    Check the line periodically for air pockets that might have formed when you were draining the vinegar from the system.  

Tankless Water Heater Flush Kits 

Several brands sell tankless water heater flushing kits that are designed to help you with the process. Most of the time, you do not need to purchase a kit, but they can be useful because they come with the hoses and cleaning solutions that you will need to use.  

3. How To Flush Rinnai Hot Water Unit 

Flushing a Rinnai water heater is not hard. First, close the hot and cold water shut-off valves. Then connect a pond pump and garden hose to the serviced valve. Flush a mixture of about 4 gallons of vinegar through the system, then connect a hose to the second service valve. 

Place the end of the hose and the pump in the bucket. When done, remove the hoses and turn the shut-off valves back open.  

How To Flush A Tankless Water Heater

4. How To Flush Rheem Hot Water Unit 

Turn off the electricity and gas supply to your heater unit. Then, turn the cold water supply off by turning the handle clockwise. Then thread a garden hose through the drain valve into the heater tank.

The drain valve on most Rheem units is on the lower part of the tank near the gas valve. Place the end of the hose outside and turn a faucet of hot water on.  

Let the water drain from the heater and then open the cold water supply line. Let it run through the system out of the hose until the water is clear. Then, close the drain valve and let the tank fill with water. Remove the hose and reconnect the electrical and gas supply lines.  

5. How To Flush A Navien Hot Water Unit 

When it comes to flushing a Navien hot water unit this is what you need to do. Turn off the gas and electrical power lines. Then, remove the cover and unplug the unit.  

Remove the cold air intake filter, clean it, and return it to its place. Then drain the water heater by turning off the cold and hot water valves—hooking hoses to the service valves and emptying the unit into a bucket.

Take the dirt trap out of your system and run vinegar through the pump system with a sump pump and hose setup. Flush the system, turn the valves open, replace the cover, and reconnect the power and gas to the unit.  

6. How To Flush Noritz Hot Water Unit 

Turn the gas of the unit off and turn the hot and cold water valves off. Remove the drain caps on both sides, turn the drain valve on, and let the unit drain into a bucket. Connect a hose to the pump and place it in the bucket.

Connect the other end of the hose to the cold water outlet. Connect another hose to the hot water drain valve and put the end of the hose into a bucket.  

Flush the system with 4 gallons of vinegar for about 2 hours. Change the solution about halfway in. Remove the pump and cold water line hose, leaving the hot water line hose in. Open the cold water shut-off valve and let the system drain into the bucket. 

People also Ask (FAQs)

Where can I buy tankless water heater flush kits? 

You can buy tankless water heater flush kits from your local hardware stores like Lowes or Home Depot. You can also buy them from online retailers like Amazon.  

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

It costs very little if you DIY. A tankless water heater flush kit costs anywhere between $40-$130.  

What is the average life of a tankless water heater? 

The average tankless water heater can last for 20 years if taken care of properly. 

Can I use CLR to clean and flush my tankless water heater? 

Yes, you can use CLR in addition to distilled vinegar to clean your water heater unit.  


Water heaters are an essential part of the water system in your home. If you have a tankless unit, it is important that you regularly clean it out. Clearing your tankless unit filters can ensure that it runs effectively for a long time.  

Holly Curell is the editor extraordinaire for Plumbing Lab. Having grown up in Michigan, Holly has spent time living in New York, Virginia, & currently North Carolina, where she lives with her husband & family. Holly loves DIY & has years of experience with at-home plumbing problems that arise from having 3 kids & living in colder climates. When she's not writing about her plumbing knowledge, Holly enjoys reading, hiking & relaxing with family.