Bosch tankless -- cold water starts fan, what gives?

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 works.
If anyone has experienced this problem and figured out how to fix it, I'd be grateful to hear from you. Theories welcomed too.
Dave
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I don't quite have a theory, but I do want to pass along something I heard from a fellow who installed one of these. Apparently some of his older faucets provided a slight cross connect between the cold and hot water supplies, even when off. This confused the tankless unit when it received back pressure on the hot water outlet. His problems went away when he replaced the relevant fixtures.
Now I don't particularly see how this phenomenon could cause your problem, but it is something you could check for. If you have an isolation valve on the hot water outlet, just shut it off and see if running the cold water still causes the fan to come on. Or try shutting off all the supply valves at all the points of use of hot water. If either of these help, then you can narrow it down to a particular fixture by opening the supply valves one at a time.
Cheers, Wayne
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