Brainteaser for the last 8 yrs, help needed

[Piston]

I have a three flue chimney, one for furnace, one for fireplace and one for woodstove (basement). When i use my woodstove in the basement and get it going good i have no problems, but when the stack cools down i'm getting a large amount of water dripping into my cleanout. it almost acts like its pulling in condensation and turning to water as the heat reduces near 300-350 degrees.
to correct the problem, i have tried a different stove, caps on the chimney, no caps on the chimney, extending the chimney (with pipe) as well as kiln dried wood. the chimney is clean, i also was told the pine tree near my chimney could be interfering with draft so i cut that down. i can't see any cracks in the flue, it looks good the whole way up or down.
i realize now that my only alternative is to put a liner in the chimney since i don't have one in. I will do this eventually but due to the shape of the flue it won't be easy to weave in past the angle. I really just want to know WHY this is happening because i built the house in 1986 and it has always worked fine up until one winter. i have been using the same stove, the same wood and everything since i built the house, i can't seem to think of anything that has changed or any reason why this happens. it is enough water to puddle up in the cleanout and has even ruined my rug in the finished basement.
also, its a 6 1/2" clay flue.
Do you think that by putting in a 6" flue all my troubles will magically disappear? also, do you have any idea as to WHAT is causing this???? Its been driving me crazy forever now.
I appreciate your help and have learned a lot from reading through all the posts at night, so far im still searching.

 

[edthedawg]

22 yr old clay flue, and the water pools up only when the basement stove cools down? Does the affected cleanout serve all 3 flues? They all tie together in the basement cleanout, which I presume is just a few feet below the thimble for the basement stove?

What about if you use the upstairs fireplace? Any stove in there? or just a wide open fireplace? Does that stack produce any kind of condensation issue?

Have you done anything to change the windows or insulation in the house? Trying to think of anything that could be affecting the way moisture in general is handled within the house (laundry, dishwashers, bathrooms, windows, insulation, etc...)

 

[oconnor]

My guess is that the condensation is happening in the other two flues, not the wood flue. When the wood flue heats up, the other two start to warm up, and as a result, they draw moist air from inside the house envelope thru them. This is possibly where the condensation comes from.

What else in the house changed the year you noted the issue? (Ed already asked that) Your house is a system of systems. A new oil furnace for example?

Do whatever you can to limit non combustion airflow into the flues (An Outside air kit on the furnace will draw dryer outside air in the event the flue is drafting due to heat induced by the wood flue, for example). As well, it may be worth applying a sealer to the brick flue surfaces outside so that it absorbs less moisture from the environment - then it will be dryer, and can absorb more moisture introduced by moist draft, and will be a less likely source of the moisture itself.

Also, what is the relative humidity in the house?