I do have a gas furnace, but the regular servicing, even if just once per year, enables you to find problems the season before, and the labor to replace parts is already included. Buying new anything is kind of silly unless the unit is pushing 20 years. If you have all metal ducts, just clean the heck out of it. I do have plastic flexipipe for my downstairs that I'll probably replace one day, but thats it!