beenthere: Thank you this is indeed helpful. I will go back in about a week and they guaranteed up front in writing that they will change the equipment out if I am not satisfied so I need to gather ALL my facts. Therefore I would like to use this next week to become more educated.

The old furnace was natural gas at 162K BTU input. Perhaps 40% efficient would have been 64K useful heat output as a guess. The thermo calc sheet provided by the installer said 60K but that was based on the original 1800 SQ ft public records not the 2300 it is now with the closed in a ducted porch.

So if I understand you correctly I should expect (with right sized furnace) the low fire to run much longer than 33% of the time? The furnace I have now (105K) spec is 73K low 97K high. next one down is the 90 at 63K low 84K high, then the next down is 75K which is 52K low and 70K high and lastly the 60K which is 42K low and 56K high.

So it seems that the 75K or the 60K would be the ones in the running, not the 90K

I do understand that a different A/C and blower considerations also are important but I want to set that aside for the moment and focus on the heat. They will also change out the A/C.

So based on what I have provided does it seem reasonable that either the 60K or 75K units would be the areas of possibility?