Why do furnaces need to be replaced in this regularly-occurring amount of time?(http://www.homeenergycenter.com/8furnace_signs.html)
Is there something about the physics of the high heat and components involved that makes this an absolute necessity? It's obviously very expensive and something to try to avoid if possible.
I've also seen some websites say around a 30-year lifetime for some furnaces. Which brands last the longest and do any last indefinitely?