Noritz N-069M-OD question (on demand water heater)

I just had one of these installed. My impression is that the fan (and burner) would go on and stay on for a while after you turned off the hot water. Today, I was outside and heard the fan go on for a few minutes. There was no hot water being used and hadn't been for quite some time (at least an hour). Is this normal? Why? TIA.
Dan
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Actually, my understanding is that the burner shuts off as soon as the hot water shuts off. The heat exchanger only holds 0.3 gallons, so it is only safe to heat the water when the system is open at a tap. The fan runs a little longer to completely vent any combustion products.
The unit is in standby mode while the fan continues running; if you reopen a tap, then it can fire up more quickly as the fan is already at speed. After a short while, if the tap stay closed, then the fan runs for a while on high--my assumption is that this is to drive off the excess heat stored in the copper heat exchanger, as otherwise the small amount of water in the unit could get too hot.

WAG--the unit is in the sun, and the heat exchanger got hot enough from the sun to kick in the fan to cool it down. Not very likely, I guess. I'm not aware of my Noritz N-063S-OD doing what you describe. Noritz technical support is very helpful, you could try calling them.
Cheers, Wayne
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On Thu, 08 Jun 2006 23:51:48 GMT, Wayne Whitney
: :> I just had one of these installed. My impression is that the fan :> (and burner) would go on and stay on for a while after you turned :> off the hot water. : :Actually, my understanding is that the burner shuts off as soon as the :hot water shuts off. The heat exchanger only holds 0.3 gallons, so it :is only safe to heat the water when the system is open at a tap. The :fan runs a little longer to completely vent any combustion products. : :The unit is in standby mode while the fan continues running; if you :reopen a tap, then it can fire up more quickly as the fan is already :at speed. After a short while, if the tap stay closed, then the fan :runs for a while on high--my assumption is that this is to drive off :the excess heat stored in the copper heat exchanger, as otherwise the :small amount of water in the unit could get too hot. : :> Today, I was outside and heard the fan go on for a few minutes. :> There was no hot water being used and hadn't been for quite some :> time (at least an hour). Is this normal? Why? : :WAG--the unit is in the sun, and the heat exchanger got hot enough :from the sun to kick in the fan to cool it down. Not very likely, I :guess. I'm not aware of my Noritz N-063S-OD doing what you describe. :Noritz technical support is very helpful, you could try calling them. : :Cheers, Wayne
I did call Noritz tech support and was told that it does happen on occasion, but somewhat rarely with some installations. In my case it appears to be because I have old galvanized piping and it's corroded to the extent that there is some backdraft in the cold water piping when the cold water is turned OFF. This teases the Noritz into thinking demand is imminent and the fan kicks on (but it doesn't fire up the gas).
Other than repiping my house, the solution is to install a "check valve" on the pipe to prevent the water from flowing back into the unit. I called the installation company and they are going to send out a guy next week to do just that. Otherwise, the fan kicks on for approximately a minute quite unnecesarily every time I use any cold water in the house, which is a whole lot.
Dan
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