House is 25 years old and since we're getting ready to retire, decided to replace the furnace and switch evap to refrigerated air. The existing furnace is 130,000 BTUH. The house is brick with 6 inch walls. Outside (realtor dimensions) measures 1850 sq ft, inside rooms total only 1511. R19 in walls and ceiling, and new low E double pane windows. 4 contractors looked at the old furnace and based estimate solely on that, adjusting down a little for 80% AFUE vs old 60%, and all quote 4 ton AC. Last guy though does a sort of Manual J (I guess) and tells me I only require 38,000 BTUH and 3 ton AC. So then I am concerned that the other guys are way oversized and then what good is a 2 stage variable speed system? But again, what if the last guy is wrong and the new system can't handle the job? So I try to get a mechanical systems engineer to come out and actually spec the requirement. NO LUCK. Nobody seems to want a small few hundred dollar job. So I buy HVAC Calc (Homeowners) and it says 2 tons AC and around 50,000 BTUH. The floor edge insulation jacks it up - seems to just be 1/2 inch fiber board expansion joint. So I asked the HVAC contractors to estimate 75,000 BTU furnace with 3 ton AC and they all - ALL - tell me that they know their business and that software crap on the internet is worthless and yadda yadda and one even says I will have to sign a waiver if I want that size system. None of them looked at the insulation or windows or the floor edge or my heating bills. I am discouraged.