Answer: 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.