You need isolation valves, a submersible pump, two hoses, a bucket and 2–4 gallons of descaler or undiluted white vinegar (if your manual allows it).
Circulate the solution through the unit for 45–60 minutes, then rinse with fresh water for about 5 minutes and clean the inlet filter.
Flush once a year, or every 6 months if you have hard water. Always follow your model’s manual.
What you need
To descale tankless water heater units, sometimes called descaling tankless water heater maintenance, you need just a few tools. If you’d rather not, book a tankless water heater flush service from a local plumber.
A tankless water heater flush kit: small submersible pump and two hoses with washer fittings. See our flush kits.
A 3–5 gallon bucket.
2–4 gallons of descaler or undiluted white vinegar if your manual allows it.
Towels, gloves and eye protection.
Your unit’s manual.
Your water heater must have isolation (service) valves on the hot and cold lines.
Step-by-step: how to flush a tankless water heater
These steps also answer how to descale a tankless water heater, how to flush tankless water heater models from most brands, and how to clean a tankless water heater heat exchanger.
Turn off power to the unit at the outlet or breaker. For gas units, also close the gas supply valve to the heater.
Close the isolation valves. Turn the main handles on the hot and cold service valves to closed.
Relieve pressure. Open a hot tap in the house briefly, or carefully open the pressure relief valve as your manual describes, with a towel ready.
Connect the hoses. Remove the caps from the service ports. Connect one hose from the pump outlet to the cold-side service port. Connect the second hose to the hot-side service port and put its free end in the bucket.
Add the solution. Pour descaler or vinegar into the bucket and place the pump in it.
Open the service ports on both valves so solution can flow through the heat exchanger.
Run the pump for 45–60 minutes, or as long as the manual or descaler label says. The solution flows in the cold side, through the unit and back into the bucket.
Rinse. Turn off the pump. Remove the cold-side hose from the pump and dispose of the used solution as the label directs. Close the cold service port, put the hot hose into an empty bucket or drain, and open the cold isolation valve to run fresh water through the unit for about 5 minutes.
Clean the inlet filter. Close the cold isolation valve, remove and rinse the inlet filter screen, and reinstall it.
Restore service. Close the service ports, remove hoses, replace caps, open both isolation valves, run a hot tap until the air clears, then restore gas and power.
Check for leaks and reset any maintenance indicator per the manual.
Flushing a Rinnai tankless water heater
The process is the same. Many Rinnai models show an LC code when scale is detected. After flushing, the code can usually be cleared by turning power off and on or using the controller’s reset procedure. If the code returns quickly, the unit may need a second flush. See Rinnai error codes.
Flushing a Navien tankless water heater
Navien units use the same isolation-valve method. Many Navien manuals also call for cleaning the cold water inlet filter and, on recirculation models, checking the recirculation filter. Follow the model-specific steps in your manual.
Cleaning Navien tankless water heater units also means rinsing the inlet filter at every flush.
Any time the unit shows a scale or maintenance code.
Should you hire a pro?
If your unit has no isolation valves, shows error codes after flushing, or you’re uncomfortable working near gas and electrical connections, hire a licensed plumber. A professional flush service typically costs $150–$300. See the maintenance guide.
Get a flush kit
Pump, hoses and descaler that fit Rinnai, Navien, Rheem, Noritz and more.
How often should I flush my tankless water heater?
At least once a year. With hard water (above about 7 grains per gallon, or 120 ppm), every 6 months or as your manual recommends. A scale warning code also means it’s time.
Can I flush a tankless water heater with vinegar?
Many manufacturers allow undiluted white vinegar. Others specify a particular descaler. Check your manual; using the wrong chemical can damage parts or affect the warranty.
How long should I flush a tankless water heater?
Circulate descaler for 45–60 minutes in most cases, or as directed by the manual or descaler label, then rinse with clean water for about 5 minutes.
Can I flush a tankless water heater without isolation valves?
Not practically. Isolation valves with service ports are needed to connect the hoses. A plumber can add an isolation valve kit.
Can I flush a tankless water heater without a pump?
Some people use a gravity method, but it is far less effective. A small submersible pump is inexpensive and circulates the solution properly.
This guide is general information for U.S. homeowners, not professional advice. Gas, electrical and venting work must follow the manufacturer’s instructions and local codes, and in most areas requires a licensed plumber, electrician or gas fitter and a permit. Specifications and prices change; always confirm details on the product page and in the installation manual.
