I had a similar problem a few months back, I use an Exhausto fan always set on low to help the flue draft (old house, sometimes suffers from stack effect, so we just leave it on low to draft properly, so we do not wake up dead one day). Anyway I was planning on replacing firebrick so I loaded it with the intent for it to go out by morning. I replaced the firebrick, cleaned the furnace out, got it all prepped to be lit, then my bro inlaw called. Long story short, my wife and I went out with bro inlaw and his GF for the day, gas furnace keeping the house at a nice 58F (my cats were warm enough
).
I get home, I realize I left the exhaust fan on and was like: "oh, what a waste of electricity! doh!" it is about 9pm, so I turn it off and think, "ohhh to hell with this, I'll light it in the morning." I get up, turn the fan on (or so I thought) and light it, go up stairs to grab a cup of coffee and the lower smoke alarm goes off. I am like, "ought-oh! did I forget to close the feed door?!?!?" I go down, smoke everywhere! I race for the basement door, open it grab the water hose while I am at it and analyze the situation, nope, no fire door open, back drafting badly! I close the dampers, including the intake on the draft fan, now it is still smoking from the feed door (never seals airtight, hotblasts suck like that) and around the stove pipe leading to the tripple wall which goes outside. My first thought is, "okay, must be massive stack effect issue and the exhaust fan is not working." so I leave the basement door open, open more basement windows. The smoke decreases basically to nothing and what is still coming out of the stove pipe is blown out the other windows quickly.
I finally find my multi-meter, the circuit is not drawing crap for the exhaust fan. Should be about an amp or so, nada, zero. I am thinking, "ohhh great, did a diode blow? Cap?" After doing some additional checking I come to the conlusion it is blocked. Getting to the top of my flue requires a bucket truck currently, so my bro inlaw brings one over, he doesn't believe me that it could be blocked. Sure enough, we get up there nice ice block froze around the fan blades causing the fan to stall out.
So yeah, you can get condensation in your flue and you also can clog your exhaust fans if you use them. This springs project (once my Son is born) is to wire up a voltage and current monitor for the circuit, so when we do light up, we can still the exhaust fan is running before ever striking a match. I also plan to add a backup battery supply to the exhaust fan and my hotblast blowers, that way if I lose juice we can still starm warm and safe. Actually my fire works well without the exhaust fan, but there was this one situation where we were burning coal that we got some massive backdrafting issues, it was cheap ass coal (read: free) so to be safer than sorry, I swore I'd always keep the flow going the right way
With the little one coming very soon (next week or so) anything that allows us to heat cheap and keeps us safe is a must have!
Sorry kind of a long story, but I am in that mood tonight
Tes