My previous comments may have led to the impression that I dislike the Infinity. In fact, there is much that I do like. When it is on low (which is most of the time) it is almost inaudible, definitely quieter than the refrigerator in the kitchen. The Infinity thermostat is extremely flexible and easy, even fun, to use. The system maintains setpoint temperature to within one degree or less at all times. It's only on those infrequent winter nights when it goes on high that I am unhappy.
With regard to some questions, sizing was done as follows. The old Lennox was a 105,000 BTUH input. I had observed that on the coldest nights recorded here it ran about two-thirds of the time. Guessing that it was 65% efficient, I came up with 68,000 BTUH output. Going down a bit, I chose the Infinity 60,000 BTUH input with 57,000 maximum output. This sizing has in fact, I think, been satisfactory, with the high mode output being needed only in the coldest weather.
The ductwork system is all galvanized in what I think is called a spider configuration. There are only nine outlets. All the final ducts are 6" diameter. They join by Ys to 8". Some of them join to Ys with an 8" to a 9". The 9" and 8" ducts go directly to a bonnet on top of the furnace.
Probably the main cause of my distress is that do to an accident of the floor plan the furnace is located directly under the master bedroom with a short 8 foot run to the bonnet. It's like having an ear trumpet right on the furnace. Maybe I should run a padded duct once around the furnace room before feeding it to the master bedroom.