Great that you had a CO alarm and that you caught it.
Something that I was just thinking....This forum is really good about not having condescending replies that beat people up for questions. Other forums I see newbies get beat up pretty good for asking basic questions, but that really doesn't happen here. The only responses (coal related threads) that I see people really get tough about on here is when it comes to CO issues. We really, really don't want any tragedies. now the polijive....that is another story....
I really enjoy the filter on the 'active' posting I learned about recently....I get to miss all that stuff!
As mentioned earlier i also recommend a manometer permanently hooked up.
You would have probably seen a drop in the draft number as that creosote choked part of the chimney got further clogged with the fly ash.
Once you have been burning for a while you get used to what the draft reading "should" be during different weather conditions and if the draft number isn't near that you start looking. My example is toward the beginning of one season I noticed I couldn't get a draft above .03 no matter what. I knew I should have been able to force it to at least .06 by running it hard and putting foil over the baro given the cold outside temp. After checking many things (including the zeroing of the manometer), I went outside and it turned out the cap on the bottom of my SS chimney had dropped off. I hadn't got it all the way on after cleaning the chimney at the beginning of the season. The low manometer reading tipped me off. I would have been ok until the temps warmed back up a little, if the daytime temps had got up into the 50's with it like that I may have had trouble with CO.