Thanks for all the replys. I've got three CO dectectors, one on each level so if it had gotton really bad, I would have at least been alerted. I rememebr a post by Yanche awhile ago about the threshold it takes for these damn things to sound. I'm going to go back and reread that today.
I'm going to take alittle advice from several of you. My first thought was the top mount spring loaded damper on the fireplace flue as Dallas suggested. By installing that and keeping the fireplace damper open, the flue will stay warmer and be less likley to experience a downdraft. Adding an outside air source to the coal boiler will help somewhat but there are alot of things in a home that cause negative pressure. Bath and kitchen exhaust fans, clothes dryers, radon mitigation system, etc. In many commercial buildings, there is an outside air source into the building which is slightly pressurized. That keeps the buildings interior under postive pressure. I might experiment with that. If I could find an attractive terra cotta extension for the coal boiler flue, I would do that if the other measures don't work. Here's the offending chimney. Luckily I walk on roofs in the rain for a living. Now you've got to wonder. Is he a sniper or a home inspector?