The 78 year old house I recently bouhgt was converted to a gas forced-air furnace in 1998. the exhaust vent for the furnace has a venter motor and the vent is routed over to and up through an existing brick chimney that extends beyond the roof of the house.
It has been raining lately - not too heavy or too windy - and I noticed water leaking out of the insulation around of the exhaust vent. I removed it and discovered a rusted out vent pipe, which I quickly replaced - not wanting to have any CO issues. The water seemed to be coming in from inside the vent pipe - it didn't appear to be leaking from the hole in the chimney that it passes into.
The vent extends to the top of the chimney and the rain cap is in place. Of course, water could be getting in the chimney from another location and coming in through the 90 deg elbow at the bottom of the chimney where the vent pipe enters.
Before I break out the extension ladder and start climbing to the roof, I thought it might simply be condensation. I live in a humid part of the country and the furnace is in the basement - usually the most humid area in the house. The furnace is in our laundry room, adding to the humidity.
During the summer when the furnace was not in use, we had some very strong thunderstorms but no water issues. Now that the furnace is in use, I have discovered the water and we have only had light rain but nothing serious. The other thing that makes me think of condensation is the very small amount of water - less than a cup of water over a couple of days. However, when the pipe was insulated, it held most of that water and it was enough to rust out the vent pipe over time.
The water is clearly getting into the vent pipe somewhere after it enters the brick chimney - it is not from the A/C chiller (hasn't been on for weeks and is far from the vent) or the humidifier (water is not even connected).
So, any experience with condensation in vent pipes or should I get out my ladder and flashlight?