HEARD OF THIS BEFORE? A Bosch-recommended plumber installed a natural
gas 250SX tankless water heater in my home. Except for the problem
described below, the unit works fine.
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 lingering gases from the burner unit.
The fan is not supposed to run at all when cold water alone is run.
Bosch told the installer it had not heard of this problem before and did
not have advice on dealing with it.
As a first attempt to fix it, the installer put a "water hammer
arrester" on the cold pipe leading into the unit. The theory was that
water hammering in the pipes was mimicking the shutoff of hot water,
causing the fan to start. Unfortunately, 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 Bosch does not cover the labor to replace a
defective unit. And we don't know if it is defective, since the heater
If anyone has experienced this problem and figured out how to fix it,
I'd be grateful to hear from you. Theories welcomed too.
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