Well after they changed out the Compressor (outside 4-ton York unit) and the inside coil, there is no difference.
Upstairs temps are always 10-20 degrees more than outside even when running the AC all day.
I still think the return vents on the second floor are just not what they should be. I am looking for a reputable AC company to come out an take a look to get a second opinion/estimate to fix this. I also think there are hidden areas of duct leakage. Maybe I should have a zoning system installed. Another thing is that now when the unit run and shuts off there is a loud bang coming from the duct work in the basement. What the heck.
I also noticed all the return vent connections in the basement were just appalling and had super leakage. I had them seal those up, but why did I have to find it and not them? duh
The banging noise could be caused by the return duct being undersized. Now hat its sealed, your hearing the duct noise due to too much static. Drawing the duct in while running, and poping back when it shuts off.
Your cooling issues are most likely all duct issues.
Call the builder, HVAC installer back for a 9th time and tell them to fix your return ducting from the second floor down to your furnace in the basement as this is your problem. Did you tell them on the first visit that you are not getting any return air movement ??? Did you tell them about the tissue test you did ? My money is on the return ducts from the second floor, there's obiviously a disconnect somewhere along the return ducting and more than likely someone is going to have to do some drywall repair and you should'nt care who it is that going to do it !!! You will not have air movement if you only have air coming in and nothing being sucked out of the upstairs very little if any, but you will have plenty of cold air dropping down your stair well to your downstairs return. You might want to have one of them install a centrally located retunr duct in your landing/hallway in the upstairs and let that return draw the air from your second floor, have them put one in at the floor level and one at the ceiling and I bet once that is installed properly your problems will be solved, and make sure they seal the ducting on the return too. Oh and tell them thanks for the new coil nd condenser units but it's a air flow issue not a cooling problem, LOL !!!
I have similar situation, one zone - two story and my temp diff is 4*
Number 1 - All the posts about attic insulation and sealing are 100% correct. Of course, you're new construction so we can assume you attic insulation to be at least R-30 and maybe R-38.
Number 2 - While talking to the facilities gut at work who's been working HVAC commercial and residential, he made a statement which makes perfect sense. Heating AND Cooling are about MOVING HEAT. So, when you talk AC don't think pumping cold air into room, THINK removing the hot air to replace it with cold air. In other words, RETURN, RETURN, RETURN...
If your returns won't hold a tissue, you have return issues.
Step-1 Make absolutely sure all return duct joints are sealed tight, especially when they use basement floor joist cavities and they connect to return trunks. On my house both these connections were gapped and I was pulling return air from basement and first floor only.
Step-2 Quick Test, while unit running place paper over 1st floor returns and see if you can get ANY pull on the upstairs returns. DON'T leave the paper in place, you run risk of destroying blower motor. If you don't get pull, then air is leaking in from somewhere else.
Step-3 You mention a 12" flex going from attic to basement. This flex will be nearly twice the length of 1st floor return and will by nature have less airflow with the 1st floor being favored. 12" gives an "effective" area of around 100 sq " (Reduced from 113 since flex not as good as rigid). Now (very roughly), a 4 ton at 1600 CFM probably wants around a 20x25 filter would suggest approx. 400 sq" of return duct. Your upstairs is only providing 1/4 of that and given its longer length and fact that its flex and you leaks you might be only pulling an 1/8th of you return air from upstairs. At 1600 CFM thats 200 CFM over 7 vents. IMHO (based on rough calc), thats why you don't feel any airflow.
I'm no pro, just an electrical engineer who dabbles in everything, so please, any pro please feel free to correct me where I'm wrong....
I agree. From her posts about closing dampers and vents on 1st floor, replacing compressors, (and surprised we didn't get a blower CFM increase attempt) it seemed like she and her contractor were trying to "push in more cold air". Wanted her and her contractor to start thinking more about pulling out the hot air thats there and then the cold air will come on its own (maybe).... I spent years in the same situation always thinking its about "pushing in more air". By blocking off 1/3 of 1st floor return vent and forcing more pull from upstairs, I shaved 2* off my delta....