Descaling a tankless water heater
Tankless heaters need regular flushing to clear mineral scale from the heat exchanger.
Updated · 3 min read
The quick version
Close the unit's isolation valves, connect a small pump and hoses, and circulate a descaling solution through the heat exchanger, then flush with clean water and clean the inlet screen. How often depends on your water hardness and use.
Sections
Why tankless units scale up
A tankless heater pushes water through a narrow heat exchanger at high heat. Dissolved calcium and magnesium drop out as scale there, which the USGS explains happens more as water is heated.
The Department of Energy says tankless units last about 20 years. Keeping scale off the exchanger helps them get there.
San Gabriel water hardness
Most San Gabriel homes are served by San Gabriel County Water District, which reports a 2025 average of 97.4 ppm and a range of 17 to 200 ppm in its water quality report. Some blocks get water from another supplier.
That is moderately hard. Owners usually descale more often than in soft-water cities, and the unit's manual sets the minimum.
Signs it is due
- Hot water swings between hot and lukewarm
- The unit shows a scale or flow error code
- Less hot water at the tap than before
- Whistling or rumbling from the unit
What the service looks like
Most installs have service valves with hose ports. We close the main valves, open the ports, and run a pump that circulates descaler through the unit.
Questions we hear
Can I descale with vinegar?
Many makers allow white vinegar or a food-grade descaler. Check the manual for what yours specifies.
Does a water softener remove the need to descale?
It reduces scale a lot. Many makers still recommend periodic flushing and filter cleaning.
Will descaling fix an error code?
It fixes codes caused by scale or restricted flow. Other codes point to gas supply, venting or sensors.