I ran into a furnace last week and took a snapshot of the label 100K input, 80k bonnet capacity. The bonnet can handle 80k but has a 100K firing rate? What technically is the "bonnet" of a furnace anyways? I've only seen this term used on the really old stuff.
It's the way furnace manufacturer's used to rate furnaces, but is not an actual efficiency rating. Ratings were done with an 85% temperature rise and stack temperatures were allowed to be tested at 550 degrees. All furnace inputs were given a nominal output bonnet rating of 80%, based on the high temperature differentuals and the high flue gas temperatures. Bonnet capacities had nothing to do with efficiency ratings as they do in the methods of testing furnaces today.