A Complete Tankless Water Heater Cleaning and Maintenance Guide

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Tips for servicing your tankless water heater
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Over time, a tankless water heater can accumulate minerals that can build up on and erode the walls inside your tank's heating chamber. To properly maintain and clean your tankless water heater, you must flush and remove the mineral deposits from your tank at least once per year (also known as "descaling"). We talked to professional plumber Rob Calame for tips on how you can maintain your tankless water heater to ensure its optimal performance and efficiency.

Maintenance Schedule for a Tankless Water Heater

  • Clean and inspect the system : Once a month
  • Clean the water filter : Every 6 months
  • Clean the air filter : Every 6 months
  • Test the pressure relief valve : Once a year
  • Descale and flush the system : Once a year
Section 1 of 5:

Descaling & Flushing a Tankless Water Heater

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  1. Tap water contains minerals like calcium and magnesium. Over time, those minerals get deposited on the insides of your pipes, where they can build up and block the flow of water. [1] To prevent that, Calame recommends descaling the system once a year.
    • If you have very hard water, you may need to descale your water heater every 6 months. Otherwise, once a year is probably fine. [2]
  2. Calame says that before you do any work on a water heater, it’s important to turn off the power. If you have an electric heater, unplug it or turn off the power at the breaker box. [3] If you have a gas water heater, turn off the gas supply, or turn the gas valve to the “pilot” position. [4]
    • According to Calame, the instructions for turning the gas on and off are usually labeled on the water heater.
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  3. Tankless water heaters often have service valves that allow you to flush the system. Check your owner’s manual for the exact location of the valves, or look for two levers—one with a blue handle for cold water, and one with a red handle for hot water. Look for twist-off caps near these levers, then unscrew them. [5]
    • A little water may spill out when you remove the caps—keep towels handy just in case.
    • Your tankless water heater may also have a third valve. If it does, open the cap on this valve as well.
    • Some water heaters are installed without service valves. Unfortunately, if you don’t have service valves, you’ll need to consult a professional for advice on how to flush your tankless hot water heater.
  4. Service valves typically have a 3 ⁄ 4  in (1.9 cm) diameter and male threads, so choose plumbing hoses with 3 ⁄ 4  in (1.9 cm) female ends. Attach a hose to each service valve—one to the hot side and one to the cold side. [6] If you have a third valve, attach a hose to it also.
    • Tighten the hoses securely by hand.
    • Put the other ends of the hoses into a clean 5 US gal (19 L) bucket. [7]
  5. Start with the cold water lever and pull it upwards about 90°. Then, do the same thing for the hot water. [8]
    • Water will probably start coming out of the hoses, so be sure they’re securely in the bucket.
    • If there are small T-shaped levers on top of the service valves, slowly turn each of those to open them after the water has drained out of the system. These are pressure valves, and they will help release any pressure that’s built up in the system.
  6. Place a submersible water pump into the bucket with the ends of your hoses. Then, screw the end of the cold water hose onto the pump’s output valve. [9]
    • If your pump doesn’t have a 3 ⁄ 4  in (1.9 cm) male threaded end, attach a 3 ⁄ 4  in (1.9 cm) plumbing adapter to the outlet so you can attach the hose.
    • Leave the hot water hose (and the third hose if you have one) in the bucket—but be sure the end of the hose is pointed downward near the bottom of the bucket to avoid splashes or spills.
  7. Add enough vinegar or descaling solution to cover the bottom of the pump—about 2–3 in (5.1–7.6 cm) in the bottom of the container. When you start the pump, the level of the solution will drop, so add more if you need to. Don’t let the level go below the bottom of the pump—letting the pump run dry can damage the motor. [10]
    • You’ll probably need about 2 US gal (7.6 L) of solution.
    • Check the owner’s manual for your tankless water heater. Sometimes, they’ll specify that you need to use a specific cleaning solution. [11]
  8. Plug the pump into a nearby power outlet to turn it on. You should see vinegar or descaler coming out of the hot water pipe within a few seconds. [12]
    • As the pump runs, the cleaning solution may get cloudy, and you may start to see debris floating in it. This is normal, and it means you’re successfully descaling your tankless water heater.
  9. After you take the cold water hose off of the pump, leave it in the bucket until any remaining vinegar or descaler has fully drained out of the system. This may take several minutes. Once it’s drained, unscrew the pipes from the hot and cold service valves and screw the service caps back on. [13]
    • If your tankless water heater has a water filter, remove it and rinse it out while the heater is drained, then put it back in. Check your owner’s manual for tips on how to remove the filter.
    • Either tighten the service caps by hand or very gently use a pair of pliers. Don’t overtighten them, or you could damage the service valves. [14]
  10. Slowly push the red-handled hot water lever down a quarter of a turn. This will open the hot water lines to the rest of your house again. Then, do the same for the cold water lever. [15]
    • Opening the cold water service lever will allow water to start flowing through the system again.
  11. There may be residual vinegar or descaler inside your water heating system. Run the hot water in an indoor sink for about a minute or until the water is clean and fresh again. [16]
    • Because the power to the tankless water heater isn’t back on yet, the water coming out of the hot water tap will be cold.
    • When you’re finished, you can turn the gas and power back on again. [17]
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Section 2 of 5:

Other Regular Maintenance Tasks

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  1. About once a month or so, wipe down the outside of the tankless water heater with a damp cloth. This will help prevent dirt and other buildup from getting into the system and causing damage. As you do this, check around the tank for any drips, leaks, or signs of wear. [18]
    • Don’t ignore water leaks around your water heater . Over time, these issues can get worse, leading to problems like mold, mildew, and a risk of electrical fires. [19]
    • How often to do it : At least once a month
  2. Place a bucket under the drain pipe running from your pressure relief valve (PRV). Turn the lever on the valve until water starts to pour out of the drain pipe, then close it again. If water doesn’t flow, or if it doesn’t stop flowing, have a professional service your water heater. [20]
    • Check your owner’s manual for guidance on where to find your pressure relief valve.
    • How often to do it : About once a year
  3. Check your owner’s manual to see how often you should have a professional plumber come out to check your tankless water heating system. This will increase your chances of catching problems early—which could save you a lot of time and trouble in the long run. [21]
    • How often to do it : Follow the manufacturer’s guidance
  4. If your tankless water heater has a water filter, check it periodically to see whether it needs cleaning. Turn off the water to your tankless water heater and drain the system by opening a tap inside. Locate the filter and remove it based on the instructions in the owner’s manual. Clean the filter housing with soapy water, replace the filter with a new one, and replace the filter. [22]
    • Fill a container with water before you turn off the water to the house so you’ll be able to clean the filter. [23]
    • How often to do it : Every 6 months or according to manufacturer’s instructions
  5. Not all tankless water heaters have an air filter, but if yours does, you should inspect it regularly. Check your owner’s manual for instructions on how to remove the air filter. You may be able to clean some filters and re-use them, but others will need to be replaced periodically. [24]
    • Only use the replacement air filters specified for your system.
    • How often to do it : About every 6 months
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Section 3 of 5:

How much does flushing and descaling cost?

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  1. You’ll need a submersible pump, two 3/4 “ plumbing hoses, a 3/4 “ adapter for your pump if it doesn’t come with one, a bucket of water, and 1.5-2 gallons of descaling solution or white vinegar. You can either purchase these items separately or in a tankless water heater cleaning kit .
    • After the first time you flush the tank, you’ll already have all of the supplies—so the next time you need to flush the system, you’ll only need to purchase descaling solution or vinegar.
  2. It will cost a little more to have a professional plumber come out and flush your tankless water heater, especially considering you only have to buy the pump and other supplies once. However, a professional will thoroughly inspect your system during the service, so they may be able to catch potential problems early. [25]
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Section 4 of 5:

Do I really need to flush my tankless water heater?

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  1. If you don’t flush your water heater, mineral deposits called scale will build up inside the pipes. Over time, these deposits will block the flow of water through the pipes, which will decrease the effectiveness of your water heater. Eventually, hot water might not come out at all. [26]
    • Regularly flushing a tankless hot water heater can help prevent more expensive repairs down the line.
Section 5 of 5:

When to Call a Pro

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  1. Leaks from a tankless water heater can be difficult to find and fix. It’s best to have a professional inspect your system—they’ll be able to identify the exact source of the problem and the most efficient way to fix it.
    • If you prefer a hands-off approach, you may also want to have a professional plumber handle your annual flushing and descaline.
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Community Q&A

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  • Question
    What do I do if there is water leaking and steam comes out when the hot water is in use?
    Community Answer
    Your heat exchanger has cracked and needs to be replaced. You can buy just that part, or replace the whole tankless heater. Replacing just the part is less expensive (for parts), but I replaced the whole unit because the water had leaked onto the electronics, and who knows when that would cause another failure.
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    Not Helpful 2 Helpful 8
  • Question
    Do I need to do this even if I have a water softener?
    Community Answer
    Yes, as it's important to properly maintain a tankless water heater. The heating elements get extremely hot and bake calcium and other deposits on the heating units quickly. Filtering even clean municipal water should be done. Neglecting to maintain a tankless heater results in a very short life span. They're a very sensitive piece of machinery and require you to maintain them. Install a filter before the heater and clean them twice a year for a long lifespan. It is required. You're cleaning baked on junk off the heating element more so than dirt or debris.
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    Not Helpful 9 Helpful 18
  • Question
    After flushing a Rinnai tankless water heater with white vinegar, can I store and reuse the vinegar next time?
    Community Answer
    No, the used vinegar has dissolved solids in it and is no longer "virgin." Discard the used vinegar and always use fresh vinegar for flushing.
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      Tips

      • Refer directly to the manufacturer's manual of your tankless water heater to verify that you have all the supplies and equipment needed to flush and drain your heater.
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      Warnings

      • Use caution if you have used hot water immediately before performing this procedure because the water may be excessively hot when handling the purge port valves.
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      Expert Interview

      Thanks for reading our article! If you’d like to learn more about heating systems, check out our in-depth interview with Rob Calame .

      References

      1. ↑ https://youtu.be/wx6o-27k8RA?t=27
      2. ↑ https://youtu.be/wx6o-27k8RA?t=472
      3. ↑ https://youtu.be/wx6o-27k8RA?t=155
      4. ↑ https://www.thisoldhouse.com/plumbing/21016402/how-to-maintain-a-water-heater
      5. ↑ https://youtu.be/wx6o-27k8RA?t=164
      6. ↑ https://youtu.be/wx6o-27k8RA?t=220
      7. ↑ https://youtu.be/wx6o-27k8RA?t=223
      8. ↑ https://youtu.be/wx6o-27k8RA?t=231
      9. ↑ https://youtu.be/wx6o-27k8RA?t=249
      1. ↑ https://youtu.be/wx6o-27k8RA?t=296
      2. ↑ https://youtu.be/wx6o-27k8RA?t=118
      3. ↑ https://youtu.be/wx6o-27k8RA?t=354
      4. ↑ https://youtu.be/wx6o-27k8RA?t=375
      5. ↑ https://youtu.be/wx6o-27k8RA?t=431
      6. ↑ https://youtu.be/wx6o-27k8RA?t=448
      7. ↑ https://youtu.be/wx6o-27k8RA?t=454
      8. ↑ https://youtu.be/wx6o-27k8RA?t=466
      9. ↑ https://www.thisoldhouse.com/plumbing/21019184/read-this-before-you-buy-a-tankless-water-heater
      10. ↑ https://www.thisoldhouse.com/plumbing/21016130/how-to-install-a-tankless-water-heater
      11. ↑ https://www.thisoldhouse.com/electrical/21016242/best-uses-for-an-electric-tankless-water-heater
      12. ↑ https://www.thisoldhouse.com/electrical/21016242/best-uses-for-an-electric-tankless-water-heater
      13. ↑ https://youtu.be/HHllP8HQGEg?t=111
      14. ↑ https://youtu.be/HHllP8HQGEg?t=181
      15. ↑ https://www.thisoldhouse.com/plumbing/21016130/how-to-install-a-tankless-water-heater
      16. ↑ https://homeguide.com/costs/cost-to-flush-tankless-water-heater
      17. ↑ https://youtu.be/wx6o-27k8RA?t=27

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