Comparing a modulating vs single-stage furnace? These two sit at opposite ends of the range. A single-stage furnace is simple and affordable but runs at 100% every time, causing temperature swings of a few degrees. A modulating furnace costs more but adjusts output in small steps, keeping your Calgary home within about half a degree of your setting. See how they compare on comfort, noise, efficiency and price, and whether a two-stage model is the better middle ground.