Yes, smoke (at least the smell, not a visible smoke) comes down the other two fireplaces. During the lighting process, I light the fire properly and have no smoke coming from the lit fireplace during its use. The house and fireplaces are only 2 years old. The fireplaces were "professionally" installed by the fireplace contractor - so I would assume they are correctly sized. How do I tell? I plan to have someone come out (as soon as we recover from Christmas) to check the flues for obstructions and clean them. Am I crazy to think if the "design" were not correct, the fireplace contractor would have told us or our building contractor so? I've read about a chimney cap that is spring-loaded to open and close, but if two of the fireplaces are shared by one of the flues, that isn't going to work either. I am confounded by the problem.