Thanks lostpilot28, I had a builder friend of mine come over to check it out. He said the Trane XR90 is a single speed unit that runs wide open at 120 ton BTU all the time; it's a basic contractor model for large homes greater than 5000 sq ft. However, the ducting design I have limits the intake and restricts air flow volume to through the burner causing it to run too hot and cracking the heat exchanger. Additionally the A/C cooling coil creates additional resistance to the air flow volume which exacerbates the temp situation. The Trane dealer was coming over to estimate the cost of adding more intake duct tomorrow. My builder buddy recommended his HVAC contractor who came over and recommended a York Affinity, a multispeed blower with automatic gas throttling and two stage heating, so it can sense the air flow volume and automatically optimize the performance to heat the house to 1/2 degree of the set temp. It too is a large 120 ton BTU unit but it can operate at lower air volume and also improve the AC performance. About $4100 installed, no change to ductwork. Rebuilding the old Trane was going to cost $1400 and five weeks so I scheduled the replacement for end of week. I am really excited about having good heat again.