sizing a system properly is not as easy as one would think
it would take a year to explain all the variables involved in making the proper choice for the HVAC system in your house
before a system is "sized up" all duct issues must be repaired
the bottom line is;
a 3 ton 20i is not the perfect system for every house
a 3 ton infinity is not the perfect system for every house
a 3 ton rheem comm. system is not the perfect system for every house
a 3 ton 16i is not the perfect system for every house
a 3 ton lennox comm. system is not the perfect system for every house
etc. etc. etc.
in general, with some of the systems listed above, each system gives different btu outputs for low and hi speed
due to all of the "sizing variables" it becomes tricky when choosing equipment
the op's problem has nothing to do with the way the Trane 20i is operating
either the 20i was not the correct unit for the application, or they sized it wrong, or the system needs to be "gone over"
.