Handboken säger något som
Flushing the heat exchanger with a descaling solution if mineral build up is evident. Scale build up will shorten the life of the water heater, descale heat exchanger thoroughly and repeat annually depending on mineral content of ground water.
(Från Bosch 2400E NG användarmanual )
eller kanske något som
Periodic descaling may be necessary in areas with high mineral content in the water. Scale buildup in the heat exchanger may result in lower flow rates, error codes of A7 and E9 and boiling sounds (knocking and banging) in the heat exchanger.
(Från Bosch GWH 2400 ES NG användarmanual )
så
Under what situations should a tankless hot water heater be descaled?
När mineral uppbyggnad är uppenbart vid årlig inspektion.
Does it depend upon how hard your water is?
Ja
If descaling should be done periodically, how can I determine the right frequency for my situation?
Genom inspektion.