I'm concerned that first stage efficiency on 90% furnaces is less than 90%. On my own 90% furnace I have compared second stage increase in temperature rise and the cfm increase against the first stage. I am not a pro, but my tests indicate that the furnace is actually delivering 53% more heat on second stage while consuming only 47% more BTUs.(68,000 to 100,000).This plausible only if the first stage is performing less than 90%, more like 78-80%.
Since manufacturers state that 90% furnaces run on first stage 80% of the time,I called the furnace manufacturer's technical department and inquired about efficiency on first stage. The answer was that they don't know! Wanna bet? Their concern is meeting 90% efficiency on second stage only under laboratory conditions.
So, based on my time consuming, but not necessarily laboratory level testing; are professionally sized 90% single stage furnaces that are always on high stage the most efficient?