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In case anyone is still checking this thread, here's an update. All the work I did (adding 18 inch flex to the middle of the trunk mainly) did not really do much good. Basically no difference really. Today I added dampers to all the ducts that run to the top floor and mostly closed them all, for...

@PoorUB what was the main thing that worked for you to balance out your system? This does give me hope!

Update: I replaced the existing 18 inch flex duct with an R8 version (the old one was R4.2). It's still 25 FT as I cannot get around the attic without that length, I tried! Also I added a 12 inch flex supply (R8) (attached PDF shows where I put it) So far there is no perceptible difference in...

I've decided to first shorten the long 18 inch flex and take out that big 90 degree bend. While doing this I will rebuild that box that is on the end of the air handler to enlarge it a bit.

Air handler in attic above second floor All flex except for trunk About the blower motor, do you mean the "fins" on the motor? They do appear clean, but I'm going to check again to make sure I can eat dinner on them :-) I'm tempted to remove the register vents downstairs to improve air flow...

Also, don't laugh but are those little vent booster fans useful at all? I'm still looking at where I might be able to add a vent in the first floor, but I'm not sure if there is a place I can run another duct down there!

Where should I add the 14 in flex supply on the trunk? I could put it right next to the 18 in flex if that is best.

@PoorUB No the trunk just has 7 inch lines that run directly down to the first floor. I'm roughly mapping out the system today and will post it when I have a better idea of where everything is.

@PoorUB I have 9 registers upstairs and 8 downstairs. Obviously adding one downstairs is more challenging since I would need to find an”empty” wall to run it through. I’ll see what I can find! This advice about expanding my trunk. The middle section is 10x20 with the ends down to 10x18 Does...

It's Thanksgiving and I must say thank you all for the amazing feedback and ideas. I'm down to two options: 1. Get rid of the giant 18 inch flex and connect the air handler right into the trunk. 2. Install a 20 inch (or larger?) flex in place of the 18 inch, attempting to shorten it if at all...

@fitter30 My returns amount to about 372 sq in and my heat pump is 5 tons so it seems I'm short. Should I be adding more return upstairs or down? I did install a 20x24 4 inch deep filter unit (Honeywell) last year and that has helped a bit with the filter cleaning. The old filter was a 20x16 1...

There are two 13x6 returns on the first floor, both in the front of the house, nothing in the main living room or kitchen. Upstairs there is the main return which is 12x18. That's it for returns. It's worth noting that the living room downstairs has a vaulted ceiling which extends to the...

Great advice here @metlmunchr . I'll see if I can move the fan speed switch to the high speed side. I like that idea. There are no dampers in the trunk lines bummer. Also the upstairs and downstairs ducts are scattered down the trunk line so I cannot just slightly close the "upstairs damper"...

There are two 13x6 returns on the first floor, both in the front of the house, nothing in the main living room or kitchen. Upstairs there is the main return which is 12x18. That's it for returns. It's worth noting that the living room downstairs has a vaulted ceiling which extends to the...

5 tons. It is connected in the middle and I suspect that is why they ran all that flex but wouldn’t it be better to connect directly into the trunk (literally right next to the air handler) rather than run that giant flex to the middle? Also note that the downstairs runs (the ones that go...

Thanks. I have done that with a slight improvement. I’ve heard that I should not close them entirely so I left them slightly open.

Thanks for the replies everyone, it's all very helpful. I had no idea that length was to reduce fan noise. Now I have to decide if I want to remove it and risk the house being louder!

Thanks for the info PoorUB. Do you mean I will hear the fan in the rooms (via the vents). The rigid duct trunk is 10x20 inches (at either end reduced to 10x18 inches). When you say I will not gain anything if it is sized properly do you mean it will not help to connect directly into the rigid...

I want to remove the 15 foot 18 inch trunk and connect with plenum right to the trunk. Is there a reason why this will not work and will it improve my air flow/heating/cooling? I cannot see why this long flex is there in the first place??