In general, you should flush a tankless water heater once a year (more often if you have hard water or poor water quality. Not only does this clear away of calcium and other hard water mineral buildups but it can increase system efficiency and longevity. To properly flush a tankless water hear we’ve created this guide to get the job done quickly, simply, and safely.

Gather Plumbing Supplies

Here are some of the things you will need to complete flush a tankless water heater:

  • Tools: A screwdriver, adjustable wrench, and needle nose pliers.
  • Hose: About 6 feet of washing machine hose.
  • Electrical Tester: A non-contact voltage tester.
  • Pump: You’ll need a submersible model.
  • Container: 5-gallon bucket to hold excess water.
  • Vinegar: Virgin, food grade vinegar or commercial deliming solution (4 gallons).


Preparing to Flush a Tankless Water Heater

  1. If you have a gas tankless water heater, shut off the gas supply. For electric water heaters, disconnect electrical power by turning off the unit and switching off the breaker for the circuit.
  2. Open the access panel and use the non-contact electrical tester to verify there’s no electrical current flowing to the unit.
  3. Next, locate the hot and cold shut-off valves. These should be closed to stop water from flowing into the heater. On the hot water line, there is a pressure relief valve. Open it to release any pressure, being careful to avoid contact with hot water.
  4. Now, you can connect hoses to the cold-water port and hot-water port. The submersible pump should be placed in the five-gallon bucket.
  5. Next, attach the cold-water hose to the pump and place the other end of the hot-water hose inside the bucket. Fill the bucket with the white vinegar that you have set aside.

Flushing Your Tankless Water Heater

  1. To begin flushing, open the hot- and cold-water ports, turn the pump on, and allow the vinegar to circulate for about an hour. Any mineral scale in the copper lines should break down and sediment should be washed out.
  2. Dump the vinegar from the bucket and close the cold-water service port.
  3. Remove the hose and open the cold water shut-off valve to flush any remaining vinegar.
  4. Let it run for about 10 minutes before closing the valve.
Once the water finishes draining:
  1. Remove the second hose and close the hot water port, then remove the cold-water inlet filter and, if your tankless water heater has one, the hot water in-line filter.
  2. Wash sediment out at the sink.
  3. Replace the filters, reinstall the access panel, and restore the gas/electrical supply depending on the unit.


Note: When you need to flush a tankless water heater that’s electric, you’ll also need to remove the heating elements.

Tankless Water Heater Parts

To Remove Heating Elements:

  1. Remove the screws holding the wires to them and loosen the hexagonal brass top of the heating element by turning it counterclockwise.
  2. After inspecting the heating elements for damage, place them in the copper chambers and rinse the chamber with vinegar for 90 minutes to two hours.
  3. After flushing, drain the vinegar and replace the heating elements and wires.
  4. Open the water shut-off valves and turn on a few hot water faucets; run them for about five minutes to rid the system of vinegar and air pockets.
  5. Turn off the taps and close the cold-water valve before removing/cleaning the cold-water inlet filter.
  6. Reinstall the filter, close the access panel, and restore power to the unit.


Contact Home X Plumbing & Rooter

If you need assistance when you flush a tankless water heater, we can help. We’re also trusted to provide water heater repair in Anaheim and Southern California. To request service, contact us on the web or call 714-515-7588.

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