Ideas about strange behavior of Bosch 250SX Tankless?

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.
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On Tue, 11 Apr 2006 21:56:13 -0500, Dave S

the choices are obvious Dave. you get what you pay for.
pay your attorney to ensure Bosch warrantees their product
or do nothing and live with the imperfections.
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the Bosch units are more expensive than the tank type, about the same price as other tankless units by Rheem. Based on this experience, I would not recommend one, even with the current federal rebates, energy savings. *** Free account sponsored by SecureIX.com *** *** Encrypt your Internet usage with a free VPN account from http://www.SecureIX.com ***
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ramoomau had written this in response to http://www.thestuccocompany.com/hvac/Ideas-about-strange-behavior-of-Bosch-250SX-Tankless-31195-.htm : Put a check valve on the cold line that feeds the tankless. Then when somebody turns on cold water elsewhere, there won't be a pressure drop and reverse flow inside the tankless that triggers the unit. I see this often.
------------------------------------- Dave S wrote:

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On Apr 12, 12:31 am, ramoomau_at_yahoo_dot snipped-for-privacy@foo.com (ramoomau) wrote:

The water heater is probably not defective. I do not know how the Bosch tankless water heater works, but it sounds like there may be two sensors. One senses a drop in the pressure of the hot water, which turns on the gas and exhaust fan. When the hot water is turned off the first sensor turns off the gas and would turn off the fan but a second sensor senses the rise in pressure and actuates a timer that runs the fan for a few seconds.
The second sensor may be more sensitive than it should be. Or the pressure drop when the cold water is run may be more than usual. A check valve will probably fix it. And be easier to do than installing larger pipe for the cold water so the pressure drop is not is big.
Note I am not a plumber or HVAC person. So this is just a guess. Could easily be just one sensor and a microprocessor. But the solution is likely to be a check valve.
Dan
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