Descaling
Flushing mineral scale out of a tankless water heater's heat exchanger to keep it efficient.
Descaling (also called flushing) is the routine maintenance a tankless water heater needs that a tank doesn't: you circulate a mild acid — usually white vinegar — through the heat exchanger for 45–60 minutes to dissolve the calcium and magnesium scale that [hard water](/glossary/hard-water) leaves behind. Scale acts like insulation inside the narrow passages of the heat exchanger, so a neglected tankless unit loses efficiency, throws error codes, drops its flow rate, and can fail years early. Most makers recommend descaling once a year (more often on very hard water, less on soft), and many warranties require proof of it. It's a roughly $5–10 DIY job with a vinegar jug and a small pump, or $100–250 if a plumber does it. See [how to maintain a tankless water heater](/guides/how-to-maintain-a-tankless-water-heater).