Plumbing – What could cause the tankless water heater to make a knocking sound

The manual will say something like

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.

(From Bosch 2400E NG user manual)

or maybe something like

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.

(From Bosch GWH 2400 ES NG User Manual)

so

Under what situations should a tankless hot water heater be descaled?

When mineral build up is evident on annual inspection.

Does it depend upon how hard your water is?

Yes

If descaling should be done periodically, how can I determine the right frequency for my situation?

By inspection.

We have a basic Bosch tankless, NG 117 kBTU/h, with a standing pilot light, no a/c power required, all mechanical controls, discontinued model I think, been in service 12 years. It worked fine until I changed to low flow shower heads (1.4 gpm). I took out the flow restrictors in the shower heads and the water heater worked again. So that is my experience.

Your Noritz NR981-OD-NG is larger and with more features so there are more things that can go wrong. If you remove your shower head and get full flow of hot water, does the water get hot? Or if you turn on all the hot taps in the house, do you get hot or even warm water?

Is the variable burner at fault? Is it stuck on 16 kBTU/h? Is the gas pressure and flow adequate for the heater? Can you tell from the gas meter if the heater is pulling the amount of gas required for maximum 200 kBTU/h heating?