O manual vai dizer algo como
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.
(Do manual do usuário do 2400E NG da Bosch )
ou talvez algo como
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.
(De Manual de Utilização Bosch GWH 2400 ES NG )
então
Under what situations should a tankless hot water heater be descaled?
Quando o acúmulo de minerais é evidente na inspeção anual.
Does it depend upon how hard your water is?
Sim
If descaling should be done periodically, how can I determine the right frequency for my situation?
por inspeção.