How-to
Flushing scale out of a tankless water heater
Once a year, shut an electric tankless heater's valves, circulate 4 gallons of undiluted white vinegar through its service ports, then rinse and restart it as its manual says.
Satisfy+ Editorial · Published May 18, 2026 · 2 min read · 1.5 hr to do · Under $100
At a glance
- How often
- At least once a year
- Flush with
- 4 gallons of undiluted white vinegar
- Never
- Chemical cleaners
- Time
- Up to 45 minutes of flushing
Minerals in the water build up inside a tankless water heater over time and can erode the inside of its heat exchanger. Flushing them out at least once a year keeps it working well. The flush runs through the heater's service valves, usually a blue cold-water valve, a red hot-water valve and small purge ports with T-shaped handles. Valve layouts and pumps differ by model, so keep your heater's manual open as you go. On a gas heater, the flush means shutting off and restarting the gas, which is a job for a plumber. On an electric heater, you can do it yourself.
What you need
- 4 gallons of undiluted white vinegar
- A bucket
- Hoses long enough to reach from the valves to the bucket, often supplied with the heater
- A small submersible pump, if your model's manual calls for one
- The heater's manual
Steps
- 1
Turn the heater off
Turn off the electric heater's power at its circuit breaker.
5 min
- 2
Close the three water valves
Close the cold-water valve, the hot-water valve and the main valve to the house, so no water flows in or out during the cleaning.
2 min
- 3
Remove the purge port caps
Take the caps off the purge ports on the cold and hot valves slowly. There may be pressure behind them, so check that the hot valve is fully closed first. Keep the rubber sealing discs in the caps.
3 min
- 4
Connect the hoses
Attach the hoses to the valves as your manual shows and run them to the bucket. Some models have you use a small pump to push the vinegar through.
5 min
- 5
Open the purge ports
Turn the T-shaped handles so they're perpendicular to the valves.
1 min
- 6
Flush with vinegar
Circulate 4 gallons of undiluted white vinegar through the heater as your manual directs. Never use a chemical cleaner. Your drinking and bathing water runs through the heater, so a chemical could be harmful.
45 min
- 7
Close up the purge ports
Close the purge ports, disconnect the hoses and put the caps back on firmly, without over-tightening them or damaging the sealing discs.
5 min
- 8
Restart the heater
Follow your manual to open the water valves and restart the heater. Then turn on a hot tap slowly and let it run until the sputtering stops and the water flows steadily, which can take 2 or 3 minutes.
5 min
Keep scale out of the tankless water heater
- Flush the electric tankless heater with white vinegar
- Book a plumber to flush the gas tankless heater
Hard water
The buildup is the mineral scale that hard water leaves. Hard water and the scale it leaves covers how hardness is measured and where scale shows up. For a tank water heater, see The yearly water heater flush and relief valve check.
FAQ
How often should a tankless water heater be flushed?
At least once a year, to clear the mineral buildup inside it.
What do you flush a tankless water heater with?
Undiluted white vinegar, about 4 gallons. Never use chemical cleaners, since your drinking and bathing water runs through the heater.
How long does flushing a tankless water heater take?
The vinegar flush itself can take up to 45 minutes.
Sources
Checked October 20, 2026
- Rinnai America Corporation (water heater manufacturer). 13-step tankless water heater flushing procedure (2026)
- Owner
- Plumbing
- Water heater
- Either
