I live in Tennessee and my older sister lives in Sacramento, CA. She had her HVAC serviced for the fall and was told the heat exchanger is cracked. Her house was built in 1938, stucco, 1400 square feet. The unit she has now is 15 years old and is the typical roof top model, Trane. The HVAC company has recommended a "Day and Night", 3 ton unit, 14 seer. The cost is about $. Is she getting ripped off? What brand(s) would your recommend?