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Discussion Starter #1 (Edited)
looking to re-configure the ducting for my HVAC. Right now - it is a mess due to several issues with routing under the house. Flex duct is sagging/pinched, ducts have critter chewed holes, and the plenum box is a joke. And, not one of the contractors who have come out are all too excited about crawling under there to work.


That said, I have two choices as i see it. One is to re-route the downflow output through 45° elbows to come back up into the house through the floor. From there, newly constructed ducting (framed in) up to the ceiling - and then run the length of the ceiling for distribution to each room.


The other choice is a re-stack/re-configuration of the exchange unit itself so that the blower unit is "inverted" to be upflow. Then from inside the exchange unit closet - new plenum up to the ceiling then through the wall to the same ceiling distribution construction. Of course, a "stand" would have to be added to the bottom of the re-stack to handle return.


And yes, i know I could keep the existing configuration - but the primary pieces of the ducting are beyond repair. In that, the $$$ goes way out of my reach to replace it all - be it fiberboard or metal duct (rectangular). And, I have ZERO confidence in the cheap plastic flexduct. None. Zippo. nada. Hate the stuff.


I guess I am asking which of two is the better route. I won't be doing any of the work myself. An HVAC contractor will be involved. Any advice?
 

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The first scenario sounds the best too me.

Make the elbows as large as possible so they add the least amount of resistance to the airlflow.

You cannot change the fan mounting or flip it in any unit. Illegal in furnaces as the high temp limit controls won't work and it won;t meet the UL and other codes or for insurance company reasons.

Same if you have electric elements in a airhandler. If the unit is a certified multi-position airhandler or furnace then you can flip it and use it any way you want.
 

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eeewwwwwww been there - done that !!
in FL, the building inspector said I could build my own plenum box
and run my own flex duct as long as it all meets "code".
plenum boxes are fun to build yourself. just know the code
for your area. mine says no zip-ties on any connections.
LOTS and LOTS of foil tape, spray adhesive and gray mastic.
I made the attic plenum box, ran all the flex duct, and connected
all the new vents myself and was very satisfied with the results of my
very first project of that type.
running new 6" flex duct from the attic plenum box to the room vents
is not that difficult of a task - even for the inexperienced.
then when the A/C company came to install the new unit, all they had to do
was connect the air handler to the part from the attic (chase??).
now is the time to fully address any concerns with the closet that you
intend to put the air handler in. air leaks, sound insulation, painting, etc.
find a HVAC supply company that will sell you the duct board and all the
supplies that you need for the project.
 

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What is the make/model # of your furnace and a/c coil ? Many furnaces and coils are multi-poise meaning that they can be configured as upflow, downflow and/or horizontal.
 

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Discussion Starter #5 (Edited)
"Illegal" and "insurance" - enough scare for me!



Option one it is! That said, my thoughts went to the 45's...and maybe eliminating them all together by utilizing a plenum box with the 'in' and 'out' on the same side. Shouldn't be too difficult to strap it to the floor joists...and the cost for sheet metal wouldn't be staggering to prevent any critter encroachment. It could definitely be mounted diagonally across floor joists to line up with a new floor opening (about 3 feet min in length). Would be kind of a big plenum box...but it would take the 45's out of the equation. Or am I missing something...?
 

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Discussion Starter #6 (Edited)
What is the make/model # of your furnace and a/c coil ? Many furnaces and coils are multi-poise meaning that they can be configured as upflow, downflow and/or horizontal.

it is a Goodman ADPF304216. The literature I can find says it is a "dedicated downflow" or "one piece air handler"
 

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Ideally you want a elbow so it "turns" the air. Not just dump it straight down as then you get back pressure/resistance.

In large ducts for commercial units and some resi units they put "turning vanes" inside the return duct to actually direct the air towards the fan. Not in the supply but you get the idea. You want a round or curved duct if you are going to make a 90 deg turn. It can work with a square box plenum but those are big and you only have a 90 deg turn vs the 180 deg turn I imagine you are planning?
 

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Discussion Starter #8
the plenum would serve to make a 180° turn. Howevber...I see the point in 'turning' the air. There is plenty of room to use 45's to make the 180° - and also to make the subsequent 90° turn to run horizontally across the ceiling.



FWIW - my part in all of this will be framing/drywall/finish-out of the new duct runs through the house. with the new duct and vents running at the ceiling I'll have to do some patching where the existing floor vents are - which is no problem.
 

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Discussion Starter #9
I'm continuing to dig around...and i see there is an insulated ductint with a semi-rigid aluminum core.



http://www.novaflex.com/productcart/pc/T-L-T-Insulated-Aluminum-Flex-Duct-60p565.htm


I would think it would be easier to route - and any bends cold be stretched out to afford smoother transitions. I would suspect that the ripples in the core would be somewhat restrictive - but in the long run it may be overall less restrictive due to the smoothing of the bends???


I can always wrap it in a 1/4" hardware fabric to prevent critters from making a meal of it.
 

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I'm not sure if the installers around there would do metal duct, but that's how I'd do it. I'd get the elbows made, then run a square supply up the wall. It's easier to frame in, and will last forever. You come then elbow at the top and run down your attic and get much better distribution. I'd probably get all insulated duct.

The aluminum flex is way better then the plastic stuff and when stretched out, isn't so restrictive. (The metal duct would be even less restrictive.)

Cheers!
 

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I might just mention about ducting "outside the conditioned space". Any ductwork that is outside conditioned space needs to be insulated. Attic, crawl space, etc. You can use metal ducting but make sure to install the best insulation you can get.

Personally I used 100% flex with Aluminum liner and in the attic, I buried everything with new blown insulation. I was able to quantify results with vent temps just by burying the attic flex. You will lose a LOT of BTUs without insulation and risk condensate in some areas.

I did make sure as part of this installation to mouse guard every possible opening in my structure. I have not had a rodent in years.
 

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Discussion Starter #12 (Edited)
I am trying to figure out how to scream in text! lol!

Just had another contractor come out and size up the project. At first, all he wanted to do was replace the mess that is already down under. I finally had to get a bit ..uh...firm and tell him 'you are not listening to what I want'.

From there, it was like he was imagining every thing and anything to make this some contraption fit for a palatial estate. Obviously, he has a boat payment coming up! Once i convinced him we would not be doing anything other than a straight line system - he settled down.

But - his plan is using 16" square duct. The guy before was 14". And of course, one guy pegged it at 15". If i apply the Goldilocks formula (porridge too hot, porridge too cold, porridge just right) then 15" would win out. But - this ain't porridge.

My understanding of the ducting is that it is mostly science - which then parlays into standard HVAC sizing. I certainly don't want air vents that hiss at me - nor do I want to set 20" box fans around to aid in cooling/heating.

The outflow port on the exchange unit is 12" diameter. The unit is rated at 1600CFM (high) and 1425 (medium). From the Engineer's Toolbox, I see that a 10" square duct will carry the same as a 12" round @ 1425 (which really makes no sense to me). So how the heck did he get to 16"?

I can't tell what is 'best' for performance vs what is 'best' for profit. And i am nearing that point where they all get the finger and i wade through this myself. I built my own workshop (14x32), ran all the electrical, set a new panel, installed the lights fans, did all the insulation and drywall, etc. Surely running duct can't be that much more of a challenge!


lastly - I'd like to express my thanks to all who have provided their expertise and experience. I appreciate your leading me out of my HVAC ignorance. Thank you!
 

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Discussion Starter #13 (Edited)
I might just mention about ducting "outside the conditioned space". Any ductwork that is outside conditioned space needs to be insulated. Attic, crawl space, etc. You can use metal ducting but make sure to install the best insulation you can get.

Personally I used 100% flex with Aluminum liner and in the attic, I buried everything with new blown insulation. I was able to quantify results with vent temps just by burying the attic flex. You will lose a LOT of BTUs without insulation and risk condensate in some areas.

I did make sure as part of this installation to mouse guard every possible opening in my structure. I have not had a rodent in years.

My thoughts were to definitely insulate - and then wrap that with 1/4" hardware fabric to keep any critters at bay. The perimeter 'skirt' is also being replaced so that should block any adventurous critters.


In regards to the aluminum flex - I was thinking it would be lighter and much easier to install than 10'ft run of rigid square duct. Can it be cut to allow smaller ducting to the vents?
 

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Njuneer
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I can move a LOT of air through a small pipe when I push it hard! AKA PSI!

You need to keep pressure and velocity in check for both noise and blower efficiency.

I have a single 18" return on my 1000cfm system. My system does NOT howl like most new cracker box homes.
 

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For 1600 cfm, you'd need at least 15x16, inside dimensions. (insulation takes some space.) An equivalent size would be 8x30 or 10x24.

What size is your AC?

Engineering toolbox is nice if you know the engineering behind it. Otherwise it can be a bit vague, and easy to misinterpret the charts. For ductwork, we design around 0.1"wc/100ft loss.

Cheers!
 

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In regards to the aluminum flex - I was thinking it would be lighter and much easier to install than 10'ft run of rigid square duct. Can it be cut to allow smaller ducting to the vents?
The stuff I am referring to probably won't be as rodent proof as you want. It is the basic insulated duct, just with an Al foil liner inside. Yes, you can cut and do anything with it. People just do dumb stuff with it like really tight radii. Not cool. Most of my runs were designed to be straight. Virtually no radii and easily replaceable.

I ran a stack into the attic with dedicated takoffs for each supply line. Once it was all plumbed, the blankets came in to insulate it all.

I like header systems. Just the engineer in me to want things simple and replaceable.
 

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Discussion Starter #19 (Edited)
It might then be best to go with full metal elbows down under, come up through the floor, alum-flex to the ceiling, a 90° right turn - and then it is a straight line of 30 ft aluminum-flex duct. I can frame/drywall it all in pretty easily as the entire thing will be inside the conditioned space (no attic). A simple drop soffit of 2x2's should suffice (I hope) to hide it all. A little texture, a little paint...


The largest size I saw of the aluminum flex was 16". I figure full outside diameter (with insulation) would be ~18" --- which will definitely fit in the soffit areas. Is 16" diameter sufficient for proper/adequate air distribution?


(total of 9 vents: 1 @ 4" duct, 3 @ 6", and 5 @ 6/8")
 

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Discussion Starter #20
I just hit a mental roadblock. I'm not clear how i would fit 16" duct through 14.5" between joists.


The supply port is 12" diameter. My floor joists are 16" OC - 14.5" between joists. So - from the 12" port do i use 12" elbows down under to make the 180 turn to go up through the floor...then transition to 16" diam duct for the rest. Can I screw two start collars back to back (one 12, one 16), then drop the 12 side through the floor cutout (to attach to elbows)...and 16 from there for rest of the run?
 
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