How to Flush a Tankless Water Heater

Turns out you’re supposed to do this every year or so if you have hard water.

The Technique

Below is my best attempt to capture the step-by-step process from the video. Watch the video first and use the list to jog your memory – no guarantees I captured everything or that I’m describing it well enough to get it right without seeing his video.


  • You’ll need pliers, a submersible pump, bucket, two washing machine hoses, and a gallon of vinegar.
  • Make sure your tankless hot water heater has isolation valves on the bottom.
  • Unplug and turn off the gas.
  • Turn off the regular flow to and from the unit: turn off the valve for the cold water coming in and the hot water leaving the unit.
  • Attach a hose to the hot and cold flush fittings (garden hose thread)
  • Attach the cold side to the outflow from your submersible pump. Put the pump in your bucket.
  • Attach a hose to the hot side. The other end of this hose goes back in the bucket.


  • Now open the valves that let liquid flow through the flush fittings.
  • Pour the gallon of vinegar into the bucket
  • Turn on the pump. If it doesn’t flow, you might need to plug the heater back in. It will probably beep to say there’s no gas.
  • Let the vinegar circulate for 45 minutes or so.
  • Shut off the pump.


  • Close off the cold bypass and disconnect the cold hose.
  • Now you can run regular service water through by opening the regular cold water feed. Don’t open the hot yet. This does a flush of the unit, with the water exiting the unit into the bucket.
  • Once you’ve flushed the tankless heater, you need to flush your pump. If you leave vinegar in the pump and hoses they will get ruined. Run about 10 gallons of water through it.
  • Disconnect all hoses, replace caps on hose end fittings on flush valve, close bypass valves, and open the hot water valve to the house. Turn the gas back on and plug in the unit.