ok 92% 93 94 99.... percent is not what I'm gettin at here so please don't go there.... What I am thinking here, I want another 400 opinions but here goes: Seems to me with tubular heat exchangers, The clamshell heat exchanger has a LOT more surface area, so when a furnace calls for heat and starts blowing, it would seem to me that a furnace would have to run a little longer trying to get heat off the tubular into the house than with a clamshell.... hmmmm In other words a tubular would cool down a little faster, making the furnace have to run a little longer to get the same heat??? I haven't noticed a difference but how do ya know...... I guess what I'm gettin at here is 92% is fine but thats not going to cover a longer run time.... total fuel deal..... Anyway without a lot of insults....what do you guys make of this thinkin?....