My home has significant problem with the balance of the A/C system. Blower is installed in attic and has 2 duct runs coming off. One run is a 14" line going to the front rooms and another is a 12" line going to the back. All the registers connected to the 14" line (total of 6 I think)blow very strong. Feels and sounds like a blizzard coming out. The registers being fed by the 12" line (4) are extremely weak. You can feel the cold air coming out if you put your hand directly under them, but it feels more like it's trickling out rather than being pushed out under pressure. I suspected there was a kink in the 12" line (all flex duct) and wanted to resolve.
I've had 2 service techs out to look at it with the following results:
#1) Inserted plugs made of galvanized sheet metal into all the registers on the 14" line. This did increase the flow to the registers on the 12" line but only marginally. The registers on the 14" line gave out very little air after this. He told me this was a short term measure until he could get a balancing team out.
#2) Now that all the vents are putting out very little air I have another tech from teh same company to address the root cause (which I still believe to be a kink in the 12" line). Tech holds his hand under the register and says he can feel air coming out. I explain what the first guy did and that I want to find the kink, fix it, and remove the plugs in the front line. He says there's no problem, that's normal, but maybe I should change my filter.
So, I've started looking myself and this is what I find:
SIZING: The main trunk sizes (14" and 12") seem correctly sized. 14" line goes to larger and more rooms than the 12".
KINKS: I've now cut several holes in the walls to inspect and find there are no kinks in the 12" line or any of the 4 6" lines that feed off it.
DISTRIBUTION: Something strange. The 12" line terminates at a galvanized steel box/plenum. Input to this box is not a 12" circle but a 7"x15" oval. Estimated cross sectional area of the oval to be about 70% of the 12" circle. The duct enters on the top of the box and splits the flow into 3 seperate runs. Two 6" runs and one 8" run which splits into two 6" runs later. All the connections to this plenum are on one side. Input is on top right and outputs are on right side. The left side of the box is dead space.
My question: I believe the fact that the 12" circular duct is getting squeezed into a smaller oval is bad and may account for some of the poor performance of the 12" duct run. But it doesn't seem enough to account fully. Could the design of this distribution box between the 12" main line and the feeder lines be the real killer? My limited understanding of air flow through ducts makes me believe the box may be creating a lot of turbulence as air flows through.