How to Flush a Tankless Water Heater
To flush a tankless water heater, you shut off its power, gas and water, connect a small pump and hoses to the service valves, and circulate white vinegar or a descaling solution through the heat exchanger, then rinse it with clean water. Rinnai's procedure uses 4 gallons of undiluted white vinegar and can take up to 45 minutes, and both Rinnai and Navien say to do it at least once a year.
"Flush" is the common word, but the job is really a descale. A tankless heater has no tank full of sediment. What it collects is mineral scale inside the heat exchanger, and Rinnai warns that those deposits can erode the walls of the heating chamber.
Do tankless water heaters need to be flushed?
Yes. Navien recommends descaling at least once a year, and more often with high mineral content, a higher temperature setting or heavy hot water use. HomeGuide says hard water areas should flush tankless units more than once a year. A.O. Smith adds a practical trigger: descale when the flow rate starts to drop.
Skip it and scale builds up in the heat exchanger. The unit runs hotter, flow drops, and eventually you get error codes or a failed heat exchanger.
What you need
- A descaling kit, or the parts separately: a small submersible pump, two hoses, and a bucket. Navien lists a 5-gallon bucket and double-ended washing machine hoses.
- Descaling solution or white vinegar. Rinnai's procedure calls for 4 gallons of undiluted white vinegar and says not to use other cleaning solutions. Navien allows an approved chemical or vinegar. Follow your own manual.
- Your heater's manual, for the valve layout and restart steps.
- A small brush for the inlet filter.
The kits sold under various brand names are all the same idea: a pump, two hoses and a bucket. Buy whichever fits your valves.
Step-by-step descale
This combines the Rinnai and A.O. Smith procedures. Where your manual differs, the manual wins.
Shut off power and gas
Turn off the power to the unit and close the gas supply, or switch off the breaker on an electric model.
Isolate the heater
Close the cold (blue) and hot (red) isolation valves. A.O. Smith says they're closed when the handles sit perpendicular to the pipes. Some models need a service mode or bypass setting first, so check the manual.
Relieve pressure and attach hoses
Slowly remove the caps from the purge ports on the hot and cold valves. Keep the rubber seals inside the caps. Connect one hose from the pump's outlet to one purge port, and run a second hose from the other port back into the bucket. Your manual shows which port the pump should feed.
Add the solution
Pour the vinegar or descaling solution into the bucket and set the pump in it.
Open the purge ports and run the pump
The solution circulates through the heat exchanger and back into the bucket. A.O. Smith says circulation typically takes about 45 minutes, and Rinnai says the flush may take up to 45 minutes.
Rinse with clean water
Turn off the pump and remove the inlet hose. With the outlet hose still in the bucket (empty it first), close the hot valve and open the cold valve to push fresh water through. A.O. Smith says at least 5 minutes and Navien says a minimum of 10 minutes. Do the longer one.
Clean the inlet filter
Unscrew the small screen on the cold-water inlet and clean it with a brush, water spray or compressed air.
Put it back together
Close the purge ports, remove the hoses, and replace the caps snug but not overtightened.
Restore service
Open the hot and cold valves, turn the gas back on, and restore power. Run a hot tap until the air clears and it stops sputtering, which Rinnai says can take 2 or 3 minutes.
Vinegar vs. descaler
Both dissolve scale. Rinnai's flushing page specifies undiluted white vinegar and says not to use other cleaning solutions, since household water runs through the unit. A.O. Smith's procedure uses the cleaning solution that comes with a descaling kit. Navien accepts an approved chemical or vinegar.
The rule is simple: use what your manufacturer's manual names. Vinegar is cheap and you can pour it down the drain afterward. A commercial descaler may work faster on heavy scale, but check that it's approved for your unit.
Gas vs. electric tankless maintenance
The descale is the same for both. Gas units have more on the list: Navien names the intake air filter and the condensate trap as maintenance items. Electric tankless heaters have no burner or venting, but they still scale up on hard water. Shut off every breaker that feeds the unit before you start, since many whole-house electric models are on more than one circuit.
Navien recommends annual maintenance either way.
Mistakes to avoid
- Losing the purge cap seals. Without them the caps drip forever.
- Running the pump dry. Keep the pump submerged in the bucket.
- Forgetting to flip a bypass or service setting back. The unit won't fire normally.
- Hooking hoses to the wrong ports. Follow your manual's layout for which port the pump feeds.
- No isolation valves at all. Some older installs don't have them. That makes descaling a plumbing job. Have a plumber add a valve kit once, and every flush after is simple.
When to call a pro
Navien recommends hiring a professional for descaling and warns that a botched service that damages the heater could void the warranty. HomeGuide prices a professional tankless flush at $150 to $350. If you have no isolation valves, the unit is throwing error codes, or you're not comfortable with the gas shutoff, pay for it.
FAQ
How often should you flush a tankless water heater?
At least once a year, according to Rinnai and Navien. With hard water, plan on more often. A drop in flow is a sign it's due.
Can I flush a tankless water heater with vinegar?
Yes. Rinnai's procedure uses 4 gallons of undiluted white vinegar, and Navien accepts vinegar too. Check your own manual before you buy anything.
How long does it take to descale a tankless water heater?
Plan on about an hour. Circulation runs about 45 minutes, plus setup and a rinse of at least 5 to 10 minutes.
What happens if you don't flush a tankless water heater?
Scale builds up in the heat exchanger. Flow drops, efficiency falls, and Rinnai warns the deposits can erode the heating chamber.
Sources
- Rinnai13-Step Tankless Water Heater Flushing Procedure (opens in a new tab)
- A. O. SmithHow to Drain and Flush a Water Heater (opens in a new tab)
- NavienHow to Descale Tankless Water Heaters (opens in a new tab)
- NavienTankless Water Heater Maintenance (opens in a new tab)
- A. O. SmithTankless Water Heater Buying Guide (opens in a new tab)
- HomeGuideWater Heater Repair Cost (opens in a new tab)