Background:

I have new construction (built Dec 2011) two story modern home (house is very tight) in Houston TX. The house has high end argon filled low E windows, regular bat insulation on walls and R30 open cell foam on roof, the roof design is an non vented attic, meaning that the ducting on the upper floor is in semi conditioned space.

The house has 2 carrier systems, one system on each of the floors. The layout of the house has living / kitchen open plan downstairs along a bedroom, and 3 bedrooms upstairs separated by a large loft / landing that is open to downstairs. The manual J for upstairs call for 19kbtu heat and 2.1 ton cooling.

On the upper floor we currently have a 58CVA110 with a CA16 48 4 ton single stage condenser and a 5 ton CNPV coil, the above is controlled with an infinity controller and has two zones (no bypass damper design), all damper are infinity modulating. The load analysis was incorrectly done, and now the contractor is replacing the current system with a 58CVA90 with a CA16 36 3 ton single stage condenser along with a 4 ton CNPV coil.

Our issue was that the old system was oversized for the ducting that was installed, we had high noise and when we used the infinity controller to adjust zone airflow limits we started to come up against high static and airflow limited shutdowns. The contractor changed the setting on the infinity controller for the condenser from 4 ton to 3 1/2 ton and this helped some but it could not hide the fact that the equipment was oversized and the ducts undersized.

The duct setup upstairs was such that we ended splitting 4 main supply ducts to make a 60 and 40% zone.

The 4 supplies are:
- 2 x 12" flex duct
- 1 x 10" flex duct
- 2 x 8" flex duct

We split the two zones upstairs to 1 zone with 1x12" + 1x8" + 1x10", the 2nd zone has 1x12" and 1x8"

- Now along with resizing to the smaller furnace and condenser the contractor is proposing that they add a remote sensor at the open loft landing area (contains 1 x 8" + 1x10" supplies) and set it as its own zone in unnoccupied mode. This way they suggest that when the 2 separate bedroom zones are calling for cooling that area will be used as a dump for excess airflow. Bedroom zone 1 would have 1x12"+1x8" and bedroom zone 2 1x12".

What do the pros in the forum think about this application of the infinity system? Do you think that we would still get the high airflow noise and static when the zone initially calls and until the damper to the unoccupied zone opens to relieve?

I have been suggesting that the only real solution (aside from gutting the ducts and redoing...) is to have a 3 ton two stage condenser. I am being met with resistance and told that the next solution would be to take out infinity controllers and do a bypass damper (not thrilled with this option)

Thanking you in advance for your advice and time