Tankless Water Heater Descaling: How Often and Why

01 What is tankless water heater descaling?

Descaling a tankless water heater is the process of flushing a specialized cleaning solution through the unit to dissolve calcium and magnesium deposits. This maintenance clears the heat exchanger so the system can heat water efficiently.

Tankless units heat water on demand by passing it through a narrow, highly conductive heat exchanger. As cold water heats up rapidly, naturally occurring minerals like calcium and magnesium separate from the water and stick to the metal surfaces inside the unit. Over time, this creates a hardened layer of scale. Descaling, which is also commonly known as flushing, uses a submersible pump to circulate a mild acid through the isolated water heater.

Plumbers typically use food-grade white vinegar or a specific manufacturer-approved descaling solution. This liquid is pumped continuously through the heat exchanger for about 45 to 60 minutes. The continuous flow and acidity dissolve the mineral crust without damaging the internal copper or stainless steel components. Once the scale is fully dissolved, the system is flushed with clean water to remove any remaining debris and cleaning solution. Finally, the inlet filter is cleaned before the unit is reconnected to the home water supply and powered back on.

02 How often should you descale a tankless unit?

Flushing frequency depends entirely on your local water quality and the specific manufacturer instructions for your model. Homes with hard water may need annual descaling, while those with water softeners might wait longer.

There is no single maintenance schedule that applies to every tankless water heater. The San Gabriel Valley area often has hard water, meaning the municipal water supply contains high levels of dissolved minerals. When hard water runs through a tankless unit, scale builds up much faster than it would in areas with softer water. Manufacturers design their units with specific maintenance intervals in mind, and following these guidelines is important for keeping the equipment in good working order and maintaining warranty coverage.

If your home has a whole-house water softening system, the mineral content entering the water heater is significantly reduced, which can extend the time between necessary flushes. However, even with treated water, periodic inspection and cleaning remain necessary to ensure the flow sensor, water filter, and heat exchanger are clear of debris. Relying on a fixed calendar date is less effective than monitoring your specific water conditions and system performance.

• Check the manufacturer manual for your specific model baseline recommendations.
• Test your home water hardness to gauge how quickly scale might form.
• Monitor the unit for built-in maintenance alerts or error codes indicating restricted flow.

03 What happens if you skip descaling maintenance?

Skipping descaling allows mineral scale to insulate the heat exchanger, forcing the unit to burn more fuel to heat the water. Eventually, this buildup restricts water flow, triggers error codes, and can cause premature component failure.

The heat exchanger relies on direct contact with the water to transfer heat quickly and efficiently. When a layer of calcium forms on the metal, it acts as an insulator. The burner must run longer and hotter to achieve the same water temperature at the faucet, which increases energy consumption and places extra thermal stress on the internal components. As the scale thickens over months or years, it physically narrows the water pathways inside the unit.

You might notice a drop in hot water pressure at your faucets or showerheads, or temperature fluctuations during a shower. Modern tankless water heaters are equipped with sensitive internal components that monitor flow rates and internal temperatures. If the scale buildup becomes too severe, these sensors will trigger an error code and shut the system down entirely to prevent overheating or permanent damage. Ignoring the maintenance can lead to costly repairs, such as replacing a cracked heat exchanger, or the need for a complete unit replacement long before the expected lifespan of the equipment is reached.

04 Manufacturer guidelines and local water reality

Equipment manufacturers explicitly state that flushing and descaling frequency must be adjusted based on local water quality. Ignoring these guidelines can impact the performance and longevity of your specific model.

Different brands and models have varying tolerances for scale buildup. For example, manufacturer guidelines note that their maintenance intervals are directly tied to the hardness of the water supply. Because water quality varies significantly from city to city, a blanket recommendation of once a year is not accurate for every household. In areas like Pasadena, Glendale, or Burbank, the municipal water supply carries varying levels of minerals depending on the specific water source.

A professional assessment of your specific unit, combined with an understanding of your local water conditions, determines the correct maintenance schedule. Relying on the manufacturer instructions ensures the cleaning process uses the right solutions and flow rates to protect the heat exchanger. Using the wrong chemicals or pumping at the wrong pressure can void warranties and damage the internal flow sensors.

05 Professional maintenance and next steps

Professional plumbers have the correct pumps, hoses, and approved descaling solutions to flush your tankless water heater safely. Contact All Systems Pros to schedule maintenance or discuss replacement options if your unit is failing.

While some homeowners attempt to flush their own units, professional maintenance ensures the process is done thoroughly without risking damage to the sensitive internal sensors, gas valves, or electronic boards. A professional service call includes isolating the unit from the main water supply using the isolation valves, cleaning the inlet water filter screen, circulating the descaling solution for the required time, and verifying that the system returns to normal operation without error codes. Plumbers also inspect the venting and water connections during this process to ensure safe operation.

If your tankless water heater is consistently shutting down, displaying error codes, or failing to provide adequate hot water despite recent maintenance, it may be time to evaluate the system for deeper repairs. All Systems Pros provides plumbing services across the San Gabriel Valley. We offer free estimates exclusively for installations and replacements if a new unit is required. Contact us to schedule a service call for your plumbing needs.