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Does the banging continue until the heater eventually shuts off or does it quit shortly after the heater burners start up? In either case you can repair the heater for much, much less than a new unit. The bypass valve inside the header may be the issue if it is stuck closed. You will also want to inspect the thermostatic control valve in the header and inspect the overall condition of the heat exchanger. It is likely aggressive water has damaged some or all of these parts causing the banging noise you hear. It is also possible there is an issue with the pump and/or filter not getting enough flow thru the heater but I have to assume you have looked at those already and have narrowed the issue down to the heater. If not, check the overall system flow. If it is reduced to low the water inside the heater gets to hot and begins to boil causing the noise you here. Speeding the flow back up will eliminate the issue.
If this answer does not fix your problem, please comment with additional details prior to rating the answer. Positive feedback is appreciated once your problem is solved!
Answers & Comments
Thank you for posting your question here on Fixya.com.
Does the banging continue until the heater eventually shuts off or does it quit shortly after the heater burners start up? In either case you can repair the heater for much, much less than a new unit. The bypass valve inside the header may be the issue if it is stuck closed. You will also want to inspect the thermostatic control valve in the header and inspect the overall condition of the heat exchanger. It is likely aggressive water has damaged some or all of these parts causing the banging noise you hear.
It is also possible there is an issue with the pump and/or filter not getting enough flow thru the heater but I have to assume you have looked at those already and have narrowed the issue down to the heater. If not, check the overall system flow. If it is reduced to low the water inside the heater gets to hot and begins to boil causing the noise you here. Speeding the flow back up will eliminate the issue.
If this answer does not fix your problem, please comment with additional details prior to rating the answer. Positive feedback is appreciated once your problem is solved!
John