I am in the process of getting bids on a new HVAC system in my home. I've gotten bids from four contractors and different answers from all. In addition to getting a newer, more efficient system, I am interested in better cooling my living room which is an addition. The original house is 1600 sq feet and there is 300 sq ft addition that was built on. It has three external walls, two of which are mostly windows each with a glass door. There is no attic on this room. THere is one supply register at the end of the room (the end that opens to the rest of the houe).
I live in Austin, TX where it is very hot, a lot (especially this summer). I've asked the contractors providing me the new HVAC bids how we can better cool this room (in the afternoon it is about 8 degrees warmer than where the thermostat is). Two of the contractors had interesting ideas that were somewhat in conflict (and the other two said there was no solution).
One contractor recommended changing the flex duct that goes to the addition to hard pipe duct. He said that this would enable more cool air to pump into the room because of less restriction on the air flow. This made sense to me. He said that we could push about 40% more CFM with the hard pipe. (By the way, expanding the existing duct is not really an option due to there being a very small space in the attic where the addition connects to the original house).
Another contractor recommends installing balancing where each duct from the plenum would have a damper (that's my term, not his) installed so that the air flow could be controlled, thus pushing more of it to the duct that supplys the hot room. This same contractor said that changing the flex duct to hard pipe would not provide any significant increased cooling for the room (although 40% CFM seemed significant to me).
One more piece of info, I've tried to push more cool air to the hot room myself by closing or partially closing various supply registers in rooms that are cool enough or not frequently used. I've heard that this could be a bad idea. How is this any different than putting dampers on the ducts and controlling the airflow this way?
So, in conclusion, my questions are:
1) Can hard pipe really increase CFM 40% over flex duct? The duct is about 15 feet long. And, would this be significant enough to better cool a room?
2) Using the theory that it is not good for your system if you try to balance it by closing supply registers, why would it be okay to do effectively the same thing by balancing via dampers installed in the ducts in the attic?
3) What questions should I be asking my prospecitve contractors?