I just moved into an 800 square foot condominium with a Rheem downflow gas furnace and A/C combination. My question is whether I should get some quotes to upgrade to a newer 90+ furnace, or stick with what I have for a few more years. The Rheem will be 20 years old this year, but is in great condition, and has been impeccably maintained by a professional (I know this because my Grandfather lived here before me). The A/C on the other hand definitely needs a new condensing unit, and probably a coil. Also, does anyone know about what the efficiency of my current furnace may be? It is natural draft with HSI.