Thank you all for the feedback! To follow up on a few of the threads, no company provided a written load calc but after talking to all three, it sounds like it is oversized. My current unit is 75k BTU. The Lennox dealer did not provide any load calc information when asked and specified a 70k BTU unit. The Trane dealer talked through his hand written calculations and specified a 60k BTU unit. The Bryant dealer said his experience tells him that the unit may be oversized but he specified a 75k BTU unit. The only dealer to do a static pressure test was the Trane dealer and he indicated that the test indicated that there is a pressure differential indicating the A-coil may be dirty.
Based on the differing feedback, I am discouraged by the quality of each company except the Trane dealer who seems better than the rest. However, I'm not sure that is a good thing.
Sounds like the most important thing to do is get the sizing right. Couple questions about sizing it correctly:
1) Should a manual J be more official than "experience" and/or hand calcs?
Yes, a load calc is more accrate then an off the cuff "by experience guess". Load calcs were done by hand before computers and software were around.
2) Is there an easy "Do it yourself" J calc or free on-line calculator?
Yes.
http://hvaccomputer.com/talkref.asp
3) Does a manual D make sense to do on an existing house where I have no exposed ductwork except the 6x8 furnace room?
Thanks for the continued feedback!