The maker of your heater sets this number, and the big three do not quite agree. All of them land near once a year, then adjust for how hard your water is. A tankless heater has no tank to collect sediment, so “flushing” here means descaling: pumping a mild acid through the heat exchanger to dissolve the minerals that hard water leaves behind.
What the manufacturers say
- Rinnai ties it to hardness. The table in the installation manual for its Sensei RX models says soft water, up to 4 grains per gallon, needs no flushing. From 4 to 12 grains, a home heater should be flushed once a year, and a softener or Rinnai’s scale treatment is recommended. Above 12 grains a softener is required.
- Navien recommends annual maintenance, and more often if you have hard water.
- Rheem says at least once a year, and about every six months in hard-water areas.
These are each company’s general guidance, and individual models vary. If your manual gives a different interval, use that one.
Hard water or soft
Hardness is the amount of calcium and magnesium in the water, given in grains per gallon or in milligrams per liter. Your water utility publishes a water quality report every year. Many list hardness, and the utility can tell you if theirs does not. On a well, test the water yourself with a kit or strips.
Two things push the interval shorter. One is hardness. The other is temperature: Rinnai notes that a high temperature setting speeds up scale. A busy household that runs a lot of hot water at a high setting should lean toward the short end.
A softener works the other way, which is why Rinnai recommends one from 4 grains up and requires one above 12.
What a flush involves
The heater needs isolation valves on its hot and cold lines, the kind with small service ports for hoses. Rinnai calls them highly recommended. Without them there is nowhere to connect the pump.
Rinnai’s own procedure gives a good idea of the job. You shut off the gas and the controller and close both water valves, then connect a small pump and hoses to the service ports. Four gallons of undiluted white vinegar go in a bucket, and the pump circulates it through the heater for at least an hour. Then fresh water runs through for five minutes to rinse it out. Last, you take out the small filter on the cold water inlet and clean it. Navien and Rheem both list the inlet filter as part of routine care too.
Not every manual names vinegar, so check which cleaning solution yours calls for before you buy one.
When to call a plumber
If there are no isolation valves, adding them means cutting into the water lines, and that is a plumber’s job.
The manufacturers also want a professional involved for more than the flush. Rinnai says its heaters should be inspected every year by a trained and qualified professional. Rheem says work on gas, burners or electrical parts is best left to a licensed professional. Navien warns that damage done during your own maintenance may void the warranty, and that using one of its certified technicians protects it. If the heater is still under warranty, read that section of the manual before deciding.
Skipping the flush has its own cost. Rinnai heaters show an LC code when scale is building, and Rinnai does not cover scale damage under its warranty.
Keeping track
Househand, our home maintenance app, includes a yearly water heater flush on its starting schedule, with a note that a tankless heater needs a descaling flush. In hard water, change the interval to six months. Each time you log the flush, you can add the cost, a note and a photo, and the history exports for the day you sell the house. It is free on the App Store and Google Play.
Download on the App Store ▶ Get it on Google Play
While you have the calendar out, the smoke alarms want a test every month. How often to test smoke alarms has the rest of that schedule.
Frequently asked questions
How often should you flush a tankless water heater?
Once a year is the usual answer. Navien and Rheem both say annual maintenance, and Rheem says about every six months in hard water. Rinnai's table goes by hardness: no flushing needed in soft water, once a year for a home with harder water.
What happens if you never flush a tankless water heater?
Scale builds up in the heat exchanger. Rinnai says that hurts performance and, left alone, damages the heat exchanger. Its units show an LC code when scale is building, and Rinnai doesn't cover scale damage under its warranty.
Can I use vinegar to flush a tankless water heater?
Yes, if your manual allows it. Rinnai's procedure uses four gallons of undiluted white vinegar circulated for an hour. Check that your own model's manual names vinegar before you use it.
Do I need isolation valves to flush a tankless water heater?
In practice, yes. They're the valves with service ports that let you shut the heater off from the house and hook up the pump hoses. If your heater doesn't have them, a plumber has to add them first.
Does a water softener mean I don't need to flush?
It stretches the interval a lot. In Rinnai's table, soft water needs no flushing at all. Rinnai still wants the heater inspected once a year, though, and the inlet filter still needs a rinse.