Bought this house last year. Furnace works fine, but it is old and inefficient, so my wife and I are looking at replacing, but we also want get a sense of whether the whole system is designed correctly first.
When we use the fireplace, it does a pretty good job of heating the main areas of the house, and the furnace doesn't kick on all day. However, cold air leaks from the registers...well, more than leaks: it comes out hard enough to lift a tissue laid across the register. This, needless to say, this creates cold drafts. The fireplace has it's own makeup air, so it doesn't pull air from the living space. So I don't think it is the direct source of the problem, in that sense. The return is placed in the highest spot in the house, and the furnace is in the unheated garage. Warm air, presumably, enters the return and cools on it's drop to the furnace, and this is "pushing" the cold air out the registers. I haven't found any leaks, and everything is insulated. Of course, I don't have testing equipment. I've just looked for obvious leaks in the flex duct connections.
So, is this a normal condition, or does this indicate a design flaw?