For what its worth, I'll add my two cents worth. I was literally burned by a down draft problem at our former house even though the chimney was above the roof peak and therefore met the 3/10 rule in our area. What happened was that a downdraft forced the flame from the furnace outside the bottom edge of the cabinet and apparently there was some flamable material close enough that it caught fire. We had just left on a vacation but fortunately our daughter came home the night we left. The dog woke her up about 4 am and she immediately smelled the smoke and called the fire department. They arrived in short order and limited most of the damage to the basement. There was a lot of smoke damage and the dry cleaning bill alone was over $10,000.
So my message is to make sure that there are no flamable materials anywhere near your furnace, hot water heater, etc. that could be ignited by a downdraft. After the fire I made a small wall out of concrete blocks around the furnace and kept that area meticulously clean. I also put a water sprinkler head over the furnace. Probably overkill but it eased my mind. In our current house the burners for both the hot water heater and furnace are quite high off the floor so there isn't the same danger. But I still keep the area around them clean and free of flamable materials.
