I have a 20 year old CM 150,000 btu furnace (all 4 furnace pros I've had out have said it was way oversized) and a 17 year old 3 ton Miller AC in my 45 year old ranch with 1700 sq. ft. living space and 1500 sq. ft. basement in a Detroit suburb. I've been educating myself, talked to 4 HVAC pros and would rather spend more $$ up front to lower annual heating/cooling costs as I expect natural gas to keep rising. The furnace I think I'm going with is the Amana AMV90704CX, which has an AFUE of 95.5% and a 14 or 16 SEER AC, such as the ASX140361A. The company I think I'm going with is small; owner, secretary, 4 techs and they've been in business almost 30 years. He seems honest and knows his stuff and a couple of the techs have been with him 10 and 20 years. I have asked a lot of questions and have heard satisfactory answers for the most part but I have a couple concerns and would like some advice. I've read gradual curves are "required" on the pvc intake and exhaust but this guy said he uses sharp corners but uses 3 inch pvc where 2 inch is called for, as I believe my probably new furnace does. The other concern is he doesn't "strictly" use the Manual-J method of calculating the required sizes. He said he looks at home age, design, insulation, windows and uses experience. None of the 4 pros I've talked use seem to use the full Manual-J method but some modified version. Should these two concerns bother me? What other questions could I ask to insure I'm getting a good installer as I've read brand is not nearly as important as the installation. Thank you so much for such a great resource!