At the time, I was the training manager for a Carrier distributorship, and to counter interest in its rival's product, Carrier Corp. produced a training program (which I taught) that outlined all the methods that companies could use to create higher-efficiency gas furnaces, and the problems each method would create. It's a shame that more furnace manufacturers didn't see this program, because a few of them used the methods that Carrier talked about and had the problems that Carrier foresaw, which resulted in product recalls, customer dissatisfaction, and huge losses - and there are still problems that people are experiencing and may yet be unaware of. What are these problems?
All furnace manufacturers have known of methods to increase furnace efficiency since the early 1920s, because Carrier Corp. (for one) produced a line of very efficient gas furnaces (90+ efficiency) way back then. However, the idea was soon abandoned because they were just too costly to produce and the savings to customers didn't justify the extra expense. Gas furnaces needed some wasted energy to get chimneys and vents to draw properly (to heat them up), and anything that was much more than 80% efficient would have problems with acidic moisture condensation in the heat exchangers, which would cause them to rust out if the moisture wasn't dealt with properly.