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I was observing the operation of a gas boiler in a 6 family building. The boiler must be 10 years old. I am not sure whether there is a lining in the flue, or if there is one it is a proper material.. I noticed the vent damper closing when the fire was turned off by thermostat. It seemed to me that the flue did not have enough time to vent flue gases after firing stopped as the damper closed right away. Would it be more practical to have a delay in the flue damper closure to allow the escape of harmful exhaust that can condense on the chimney walls and cause damage to chimney?