Thanks for the info. The reason I asked was I installed my ground source about 7 years ago and I have always thought the fan sounded loud. (A Lot of air noise) It is a two stage system which only uses two fan speeds also, and I don't believe it senses air pressure in any way. It did however have a small plenum opening, so I wondered if it may have made a difference if I had kept it this size for the 18". I think it may have more to do with putting a heat pump on old duct work. I have had no problems with it and the elec. bills are very low in the winter, just a bit noisey.
Well job looks very nice. One small thing though, the variable speed blowers require a min. Of 18 inches of the plenum height to be the same size as the opening on the top of the unit. The reason why is because the blower needs to have the resistance of the pressure of the duct as it try's to find the proper speed. The transition ( while nicely done) will not allow this to happen and you will experience harsh " huffing " noises and uneven airflow. This can cause the loss of capacity and motor damage. Other than that nice job.
Perhaps they use a different blower than geocomfort. I have also had the same problem on the climate masters. Last one I found was done with duct board and it sounded like someone was moving furniture around upstairs as it ran.