Help Needed for a New Gas Pack and Duck Work
My 13 years old Carrier gas pack has just died out. We live in Raleigh NC where heating is generally needed for 3 winter months while cooling is needed over 5-6 summer months. We got quotes from 4 contractors and three proposal calls the Trane XL16c/XL14c unit, one proposal recommends the Carrier Performance Series 48VL-A.
A few years ago, we added in a room of 210 sqft, the load calculation indicates that we need to up the HVAC from 2.5 T to 3T, therefore, the supply and return ducks need to increase from 16" to 18". Since the new room is situated at the far end corner of the house, it is always warmer/cooler than the rest of the house. In the current duck design, there are two supply boxes, the primary box (16" duck delivers air to 1300 sqft) is fed from the gas pack and the second box is fed from the primary box (12" duck delivers air to 600 sqft).To improve the heating and cooling of this room, a couple of proposals call for an additional supply box that gets fed from the gas pack with manual damper to distribute air directly to the two existing supply boxes. I think this design may improve and give the secondary supply box a little more juice and eliminate the delay. Is it worth it the effort? Cost wise, since the primary supply box needs to be replaced to accommodate the 18" feed, it may not be too expensive to go with the new design.
My questions are:
1. Do we really need to increase the supply and return flex ducks to 18" given the fact that the unit may operates in the 1st stage most of the time. It is my understanding that running in the 1st stage, the unit only uses 2T, so there may not be adequate pressure to push the air thru the 18" duck.
2. In the new design, the main duck splits into 14" and 12" branches, again running in 1st stage, there may not be adequate pressure to push the air thru the 2 branches to cool or heat all rooms especially the new room.
3. From the above facts, should I stay with the plain single stage 3T gas pack?
Please help as I'm trying to understand the merits of the proposals in order to make the right decision.