How Often Should You Flush A Tankless Water Heater?

Most manufacturers recommend flushing a tankless water heater once a year, and that number isn't arbitrary. A tankless unit heats water by passing it through a narrow heat exchanger rather than storing it in a tank, which means mineral scale from hard water builds up on a much smaller surface area than it would in a traditional tank. On Anderson's municipal supply, drawn from groundwater through Anderson Water Department's two well fields, that buildup happens faster than in a genuinely soft-water market.

Hardness And Usage Both Move The Number

The annual recommendation assumes moderate water hardness and typical household hot water use. Two things push that interval shorter: harder water and heavier use. A household running a tankless unit constantly, multiple bathrooms, a dishwasher, and laundry all drawing hot water through the same unit, sees faster mineral accumulation than a lighter-use household with the same water hardness.

If your unit has never been flushed and it's been more than a year since installation, that's the point to schedule it regardless of any other symptom, since scale buildup often develops before it causes an obvious performance problem.

What A Skipped Flush Actually Looks Like

An overdue tankless unit doesn't usually fail outright. It degrades gradually: water temperature fluctuates rather than holding steady, the flow rate drops so the shower that used to run strong now feels weaker, or the unit throws an error code specifically tied to flow or temperature sensing. Most modern units are designed to flag these faults rather than let the heat exchanger silently degrade.

None of these symptoms are exclusive to scale buildup, a failing igniter or a venting issue can look similar, which is part of why a proper diagnosis matters more than assuming and attempting a flush that isn't the actual fix.

The Actual Process

A proper flush isolates the unit using dedicated service valves, then circulates a descaling solution through the heat exchanger for roughly 45 minutes before flushing it clean with fresh water. Skipping the isolation step and just running vinegar through the cold line, a common DIY shortcut, doesn't fully clean the heat exchanger the way a proper closed-loop flush does. Our tankless water heater service covers this exact process, along with diagnosing an error code if one has already appeared.

Manufacturers like Navien and Rinnai specify this maintenance as a condition of keeping the warranty valid, not an optional suggestion under the manufacturer's own maintenance standard, which is worth checking in your specific unit's documentation before deciding to skip a year. If your household is dealing with genuinely hard water, pairing your unit with a water softener meaningfully extends the interval between required flushes. This comes up often enough in Pendleton, where a lot of newer construction includes tankless units from the start, that we treat softener sizing as part of the same conversation.

What You're Actually Weighing

A DIY flush kit costs less upfront and some homeowners are comfortable doing it themselves, provided they follow the manufacturer's specific isolation and circulation steps rather than an improvised version. The real trade-off is that a professional visit also includes an inspection of the inlet filter, venting, and any error history stored in the unit, which a straightforward flush alone doesn't check.

For a unit under warranty, using a documented professional service also creates a maintenance record, which matters if you ever need to make a warranty claim and the manufacturer asks for proof that required maintenance was actually performed on schedule.

Error Codes Worth Acting On Promptly

Most tankless units display a specific error code rather than simply shutting off, and a code tied to flow restriction or temperature sensing is worth scheduling service for promptly rather than resetting and hoping it doesn't recur. Resetting the unit clears the immediate fault but does nothing about the underlying scale buildup causing it, so the same code typically returns within days if the real cause isn't addressed.

Keeping your owner's manual, or at least a photo of the model and serial number, on hand makes any service call faster, since a technician can look up brand-specific error codes and known issues before even arriving.

Common Questions