If anyone has seen the following mystery problem in a natural gas water tankless heater before and knows the fix, I'd be grateful to hear it.
When cold water is run, the unit's exhaust fan goes on briefly after the cold water stops. This fan is supposed to run a few seconds after the hot water is turned off to exhaust gases from the burner unit. The fan is not supposed to run when cold water alone is run.
A Bosch-recommended plumber installed the unit. Bosch told the installer it had not heard of this problem before.
The installer added a "water hammer arrester" on the cold pipe leading into the unit. This did not fix the problem.
Bosch has offered to send out a new water heater. We are trying to avoid this costly step as we don't know if it is defective. It heats water just fine.
Dave
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