The air supply for combustion is colder, usually during a winter day.But any of the following problems can cause the vapor to condensate on the inside of the flue and drip back down: Normally, the hot combustion gases heat the flue enough so that the water remains in vapor form as it rises through it and out into the atmosphere.
A water heater produces about a half gallon of water per hour of continuous operation. Water vapor is one of the main byproducts of the combustion of natural gas. Corrosion of the water heater jacket just below the draft hood (flared bottom of flue above it), like in the photo above, is another sign of condensation. You may see it puddled on top of the water heater or under it. Although water dripping down the flue can be caused by a missing or damaged rain cap at the roof, it is more likely that the wetness is caused by condensation of combustion gases on the interior surface of the flue.
