Chimney liners: When your home was built the existing masonry chimney was designed to vent your furnace and a water heater. The new high efficiency furnaces don't use the chimney for venting, they use plastic pipes, which go directly outside, so your existing chimney is now oversized for your water heater. A chimney liner protects your masonry chimney from damaged caused by condensation from the water heater's flue products and ensures proper venting of the water heater. Most newer low efficiency 80% furnaces required a chimney liner, due to condensate issues.
Zoning Systems: Electrically controlled dampers, regulate where the warm or cool air is distributed in your home, normally first and second floor. So with one furnace and one air conditioner, you will have two thermostat, which allow you to control the temperature on the first floor and the second floor, individually.