A tankless heat exchanger runs water through a narrow coil at high temperature, which is close to ideal conditions for laying down scale. Here that happens in a year, not five.
The symptoms
Lukewarm water that swings hot and cold, and a unit that short-cycles at low flow.
Most warranties require it
Annual descaling is written into the terms on nearly every brand we fit. Skip it and a failed exchanger is yours to pay for.