To properly compare the difference between 80 and 90 percent FAU's all things must be considered. It takes considerably longer for a condensing gas furnace to warm a given structure. A condensing gas furnace does just that, condenses the gasses of combustion...as the temerature is cooler in this type of heat exchanger the gasses liquify...you then drain those same acidic liquids into your homes existing drain pipes. If they are cast iron, will they be damaged? I've asked for conclusive answers to this question(I'm a licensed heating and A/C contractor)and have not been given any sufficient proof by any manufacturer that over a ten - twenty year period no damage will occur. I have been only verbally assured. As a contractor I will not lend myself to the possible future liability caused by either acidic damage to the existing piping or possible environmental hazards to the fish living downstream. Additionally the typical service call for high tech equipment is usually more than double that for standard 80% equipment. (trick parts have trick problems) Diagnosis is much more complex and more parts mean more failure potential. The zeal to be all efficient is somewhat skewed. A proper heating system should be quiet, trouble free and pretty much hands off. When you walk outside in the morning you should then realize "geez, it's cold this morning". A house fire is 100% efficient but not very desriable.