If your furnace was installed before the 2000s, it may run at only 60 to 80% efficiency. That means up to 40 cents of every heating dollar goes up the chimney. A new high efficiency furnace changes that, but you still need to choose between single-stage, two-stage and modulating models. Our Calgary guide compares all three, links to signs your furnace is due for replacement, and helps you plan your upgrade before the coldest months arrive.