Tankless Water Heater Descaling Guide

A tankless water heater does not store water, so it does not collect sediment in a tank the way a storage heater does. It still faces the same mineral problem in a different place. Scale builds up inside the heat exchanger, and clearing it is called descaling or flushing. This guide ties the why, the how often, and the process overview to manufacturer sources.

Why Tankless Units Need Descaling

The reason descaling exists is the heat exchanger. Navien states that mineral deposits accumulate in the heat exchanger, causing erosion and clogs that reduce efficiency, reduce flow, and lower overall performance. Because a tankless unit heats water on demand by passing it through that exchanger, any mineral coating sits directly in the path of heat transfer and water flow.

Rinnai describes the same buildup, noting that over time a tankless water heater may accumulate mineral build up which can erode the walls inside the tank's heating chamber. Both manufacturers frame the deposit as something that both degrades performance and physically wears on the internal surfaces.

This is why a tankless unit is not maintenance-free even without a storage tank. The mineral load in the incoming water still has to go somewhere, and in a tankless system it concentrates inside the heat exchanger. Descaling is the routine that removes it before it thickens enough to restrict flow or force longer heating.

How Often to Descale

The baseline interval across manufacturers is roughly annual. Navien recommends descaling a tankless water heater at least once a year. Rinnai gives the same starting point, recommending flushing at least once a year to maintain performance and prevent mineral buildup. Rheem, in its hard water guidance, points to descaling tankless systems every 6 to 12 months.

Rheem also notes in its tankless maintenance material that the heat exchanger should be flushed, with intervals that will vary based on water condition. That qualifier is important. The once-a-year figure is a floor for typical conditions, not a fixed rule that ignores local water quality.

The practical reading is that once a year is the default anchor, and the 6 to 12 month range from Rheem shows how the interval tightens when conditions call for it. A maintenance plan should start from the annual baseline and then adjust based on the water the unit actually receives.

Hard Water and Usage Drive Frequency

The variable that moves the interval is water condition and how the unit is used. Navien states plainly that the higher the level of mineral content in the water, the higher the set water temperature, and the more hot water the home uses, the more often descaling is needed. Three factors push the frequency up together: hardness, temperature, and volume.

A.O. Smith echoes the water-quality side from its maintenance guidance, noting that in a hard water area a unit may need to be flushed more often because hard water contains a higher concentration of minerals, increasing the risk of sediment accumulation. Rheem similarly frames hard water areas as calling for more frequent maintenance rather than the standard interval.

This is why a single number does not fit every home. A household with soft water and modest usage may sit at the once-a-year floor, while a household with hard water, a high temperature setting, and heavy hot water demand moves toward the shorter end of the range. The Navien factors give a clear way to reason about which situation applies before committing to an interval.

The Descaling Process Overview

The descaling method the manufacturers describe is a circulation flush. Navien describes circulating an approved solution through the system and then flushing with fresh water afterward, using either an approved tankless water heater descaling solution or household white vinegar. Navien specifies allowing the vinegar or descaling solution to circulate through the water heater for at least 45 minutes, and always flushing with fresh water for a minimum of 10 minutes afterward.

Rinnai gives compatible figures, directing use of 4 gallons, or 15.1 liters, of undiluted white vinegar to clean the unit, and stating never to use chemical solutions of any kind. Rinnai notes the procedure may take up to 45 minutes. The two manufacturers agree on the vinegar option and the roughly 45 minute circulation window, while differing on branded solutions, so following the specific unit's manual matters.

The overview to hold is a sequence. Isolate the unit, circulate the descaling solution or vinegar through the heat exchanger for around 45 minutes to dissolve the mineral deposits, then flush with fresh water before returning the unit to service. This guide describes the shape of the process from the manufacturer sources and is not a substitute for the step-by-step instructions in the specific unit's manual.

Preventing Scale Between Descalings

Descaling clears existing scale, and reducing water hardness slows how fast it returns. Rheem describes installing a water softener as the best long-term solution and the most effective way to protect a water heater, which for a tankless unit means less mineral load reaching the heat exchanger between service intervals. A softener does not replace descaling, but it can lengthen the practical interval by lowering the incoming mineral content.

The performance stakes are the reason prevention matters. Navien ties the mineral deposits directly to reduced efficiency, reduced flow, and lower overall performance, so slowing their return protects the same qualities that descaling restores. Rheem's tankless guidance reinforces that flush intervals track water condition, which is exactly the variable a softener changes.

The combined approach is consistent with the manufacturer sources. Descale on the schedule that fits the water, from the annual floor toward every six months in harder water, and reduce hardness at the source where practical so the heat exchanger stays cleaner for longer between flushes.