ductwork, mitered corners, what can be done to help the air movement!!!

I have a two story home, with 6 x 18 steel ducts used as a main supply trunk. The outlet to each room is off of this main trunk. I have really poor air flow in all the rooms, even with all vents open, or balanced, or all registers removed, anything!! the problem as far as I can tell is that my 22 foot long, as the crow flies, 6x18 inch ducts have four 90 degree mitered corner bends each (upper and lower floors) . I am no engineer, but it seems to me that square comers are bad news when trying to move air!!!!
as to the particulars. 5 ton hp (this should be 2200 cfm, as per mfg)
1200 sq inches of return grill, so I don't think I am starving the return
two 18 inch flex ducts for the return, total flex run, less then 10 feet.
2500 sq foot home
14 supply registers, 3 6 x18, 6 6x18 and 5 6x8
all metal duct, including the feed to the 6x18 ducts.
6x18 does NOT step down from outlet to outlet, thus the end of the line has very little throw!!
all duct is buried in walls, zig zaging through out the load bearing areas etc. NO way to change the 6x18, unless I take down miles of drywall and plumbing
so my question is, how can I or what can I do to optimize the mitered corners?? Some internet investigation found "turning corner vanes" but are there other ideas???
on a side note, originally the 6x18 duct was fed with about 23 feet of 16 inch flex, total air flow out of the supply vents was about 700 cfm. I took out the flex and replaced it with round metal and the cfm went to about 900!!! I also sealed all the ducts I could access with mastic or tape. additionally, I have fooled around with stepping down the 6x18 duct as it nears the ends and have gotten better throw.
thanks in advance, for any ideas!!!
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Not being there, I may or may not have the picture. My first couple thought. The supply duct may be under sized for that CFM. And four bends aren't helping. You need someone who knows duct work and heat pumps.
Since you can't do much with the 6 x 18. I'm wondering if a higher fan speed might help. Or cleaning the blower wheel, or cleaning the coils inside the unit.
Have you had a local (to you) tech out to see the system, or are you strictly DIY?
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ok.. here is the whole shebang.....
rudd 5 ton heat pump outlet in the bottom of the rudd goes into an 18 inch flex, about 9 inches long, pulled very very tight!! system is a one speed, one zone system. rated at 2200 cfm
the 18 inch feeds the plenum which has one 18 inch hole at the top and then 22 inches latter, has two 16 inch holes at the bottom the first 16 inch feeder duct runs about 2 feet into the 6x18 distribution duct via a "T", I have installed a diverter to help the air move right and left. the 6.18 then has two 90 degree mitered corner, further down the line, and feeds a total of 5 6x8, 2 6x14 and 1 6x18 registers.
The second 16 inch pipe from the plenum is metal duct. this duct makes a 90 (adjustable elbow), and runs down inside a wall. This duct also makes a "T" into the downstairs 6 x 18 and has a diverter in it, so the air goes right and left. there are 4 90 degree mitered corner in this 6 x 18 that feed 1 6x8, 4 6x14, and 2 6x18 resisters.
So hope that helps to paint a better picture.
My original intent is and was to provide balanced air flow, reduce short cycling and to reduce air loss due to leaks and friction. So far, the entire system is in much better shape now, but those dang 90's that I can remove are keeping me up at night!!!
Thanks in advance!!!!!!!!!!

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I would venture to say that unless you have 20ft ceilings, your oversized, and the existing system is probably pretty noisy too, not to mention an energy hog. You don't give any info on the existing system... age?? type of system?? model numbers??
Bigger is *NOT* better. First thing you need is a complete room by room Manual J heat load/loss calculation, as well as a Manual D duct calculation. Then have a system and ductwork designed accordingly. Your comfort level will be greatly increased... no more than 1 1/2 degrees difference between any 2 rooms, the system will be very quiet, and you should see a very noticable decrease in your utility bills. Currently the federal energy tax credit (up to $1500) expires Dec 31.... something else to consider.
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I am reading up on what to do!!!

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yep... My wife agrees... meds are the answer.. LOL
I have had 6 different contractors out to look at the issues. Each one of them had a different answer, from live with it to pull the drywall, replace the ducts and a new HP in. SO 6 different contractors, 6 different answers. NOT a single one of them did a manual J or D... they all said that they have been in the business long enough to know how to do the work so they dont need to.... so am I frustrated???? yes very. is it keeping me up at night... Nope!!!!!!!! LOL

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You are obviously perseverating on your ducts with square corners even though there may or may not be anything wrong. You need to calm down and quit worrying so much about your ductwork. If you want some piece of mind, get an experienced technician to come out and look at your system, check it over, and provide any needed service and/or repairs.
Your psychiatrist can easily help you with your anxiety problems with some zanax or maybe some Zoloft.
When you talk to the tech, don't tell him what to fix or what you think is wrong. Just ask him to evaluate the system and make needed repairs.
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People who don't know how to do the load calculations often use a rule of thumb of 500 square feet per ton. 2500 square feet in your house, 5 tons.
It might even work, depending on the climate (Maine or Florida?), number of windows, amount of insulation, etc.
So your equipment might or might not be oversized.
But 700 cfm for 14 registers? That's only 50 cfm each. Seems too low.
How did you measure this? It isn't easy to get that measurement right.
How does it hold temperature? Does it cycle? or run all the time never getting it cool?
What about humidity? Are you getting any control at all?
I think there's some missing math in your air flow. I'm assuming you borrowed a flow hood and took some data. Does the sum of the supply registers add up to the return grill? Can you punch a couple holes in the duct and do a traverse?
2500 square feet, two floors, on a single zone system, on one thermostat, all sounds like a bad idea. But that's just me.
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answers below

***** climate is phoenix az ie very hot. number of windows 14, 1/2 of them dual pane. Insulation is r13 walls, r 30 or more in attic. Attic is very well ventilated and only has about a 20 degree difference from outside, on really hot days.

The flow was measured with an anemometer, set for fps, then the duct area (with and without grill) was figured out and the cfm was mathmatically figured out. And the numbers were checked with a hand held cfm anemometer, they were very close, but not exact.

the system is short cycling just about all the time. hot or cold!!!

humidity??? whats that??? LOL phoenix has a normal year round humidity of around 12 to 14 percent. Its a dry heat!!! In fact, I was just up on the roof and the condensate was so dry it has dust in it and the P-trap was dry!!!

The return and supply add up to about the same... there is a 20 cfm difference, return sucks more then the supply puts out. I have sealed up all reachable areas of the ducts with either mastic or tape, I have sealed the attic ducts and transitions etc, but I can not get to the first floor ducts. From what I can tell, all the seams are standing seams. The seams overlap and get pinched together. Apparently my 90 year old grandmother did the overlap pinching work, as there was not one I found that was even close to tight!!! come to think of it granny could have done a better job!!!!!!!
Not to sure what a taverse is?????

I agree that this is a really bad design. but that's what I have. I am trying to make the best of it. Thus my question about turning corners to bring the tesp down. Its currently about 1.4, when measured at the package unit. It was closer to 2.5 before I made the change from flex to tin!!!! and I also doubled my return filter area!!!
One tstat , 2 feet under the return air!!! LOL .... the tstat got moved!!
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So in that climate, with that construction, not likely to be hugely oversized. Possibly pretty close.

I've never seen duct flow measured that way, if I'm understanding you correctly. Duct flow is usually measured by inserting a static-pitot tube through a hole in the duct. Traverse means you take measurements at several points as you cross (traverse) the duct. Register flow is usually measured with a flow hood.
If your registers add up to close to your return, then maybe you aren't far off. But I'm not so sure about your numbers. an airhandler rated at 2200 shouldn't be putting out 700 without massive leaks and whistles. Unless you have a fire damper closed maybe.
Maybe I'm misunderstanding what you did. I would want to know supply air flow right off the airhandler. If that is really 2200, and if you really get only 700 out of the registers, you have 1500 worth of leaks in the system. If it is 700, and you get 700 out of the register, then you have an equivalent 1500 cfm worth of resistance. Two sharp corners won't do that.

An oversized system will short cycle. But a 700 cfm system in AZ would be massively undersized. This doesn't make sense. Do the rooms get cold before it cycles?
to

I don't know what tesp is. I strongly suspect that smoothing your abrupt corners will gain you only a slight increase in flow. I would be surprised at more than a few percent, when what you need is double.

Stat by the return is pretty standard, it's going to pick up the temperature of the air going back to the unit, which hopefully is representative of air in the space.
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