A "decorative" residential fireplace typically won't do much to provide real heat. If you want something that will make a difference it is usually going to be a sealed unit or something a little more purpose-built. Still, when you compare the burner in the average small fireplace to what you have in a typical furnace, it becomes clear pretty quickly that a fireplace is not likely to be a heat source of much merit, and even if it did have a decent BTU output, it's unlikely to be evenly distributed around the house.

tl;dr - a typical residential fireplace isn't an alternative to a furnace.