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Hello, your HVAC man is correct on all counts, You have your choice, you can either add 4 inches or more insulation in the attic, or you can replace the 2 ton system with a 3 ton system, and increase the total air flow through the ducting by 50%... that will mean leaving the 6" ducting in tact on the smaller rooms replacing the 6" with 8" duct on at least 3 of the other rooms. The return air duct will have to be enlarged as well. Your HVAC man sounds entirely capable of deciding on the duct sizes.
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Hello again, if its been running for 10 years, the duct sizes is most likely OK, at 1100 square feet in a poorly insulated 1800's home you will typically need 3 tons, 2 tons for that space in a well insulated modern home would get you by in most areas except very hot and humid areas, then you would need 2.5 tons.
In an older home, especially upstairs, its going to take 3 tons unless you insulate heavily.