VERY long story short: 2 zones: 1 for 1st floor and basement works fine. 1 for 2nd floor has had problems since day 1 in house 5 years ago. Although AC seems to work OK. When outside temps are below 32, we have freezing Heat Exchanger, Reversing Valve, Accumulator, and all the lines in between. FHP split runs constantly but does not satisfy stat. Have replaced everything except the line set, ductwork, and loop. Replaced FHP split twice. Ducts in attic have been inspected. Line sets have been cleaned and pressure tested. Loop fluid has been replaced. Manual J indicates 2.5 ton, but our FHP is 2 ton, which is what original HP was.