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Discussion Starter · #1 ·
I live in south florida. My air handler is in a small closet (upflow), ducts are in the attic, and many are uninsulated. The ductwork has been hacked over the years, so I'm going for a clean install here. I have a 3 ton AC unit and am assuming 1200cfm. House is about 1800 sqft.

First, the duct plan. I plan on using rigid for the trunks (one 16" right, one 10" left) and mostly flex for the branches to keep costs down. Does anything look ridiculous? I have to follow directly under the ridge of the house because it is a low slope roof (and a flat roof on the bottom) and there is no other way - it has to be set up like that. But do the sizes seem ok? I based it all on some online calculators. (Note: from the plenum, I going to try to have a 16" and 10" takeoff instead of a 16x16x10 wye as in the drawing, if space allows.)

The big problem is the plenum. Right now, there is a 20x20 duct board plenum on top of the AHU in the small closet, and the plenum is maybe 12" tall. Then they put a 14" flex on the top of it that goes up into the attic and over to the trunk. I want to put a taller plenum on, but have a few questions. First, how does the plenum attach to the top of the AHU? And in a small closet (2'x2'), how do you get to the back of the plenum to attach it, etc? I actually have the roof off right now, so I can put in a taller plenum, maybe 2' or 3' total, if I can figure out how to attach/tape it to the top of the AHU in the back of the closet, without moving the AHU which is impossible for me.

Pictures will follow, it won't let me post them yet.
 

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Discussion Starter · #2 ·
Well, I'd like to post pics, but it won't let me yet, even though I have "1 post" as it says is required. So I guess y'all will have to copy paste:

Plan:

i.imgur.com/gTrX9Sy.png

Current mess:

i.imgur.com/BJ8V0KP.jpg

i.imgur.com/30Ih5hz.jpg

i.imgur.com/h7LcyWn.jpg
 

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Bead of caulk or the back. Or, an access hole in the front of the plenum so you can seal it from the inside.

A 16" and 10" trunk line is a bit large, and may be a bit low on velocity. How did you decide what size supply to each room.
 

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Discussion Starter · #4 ·
Bead of caulk or the back. Or, an access hole in the front of the plenum so you can seal it from the inside.

A 16" and 10" trunk line is a bit large, and may be a bit low on velocity. How did you decide what size supply to each room.
Thanks for replying.

So you just leave like a 6" circle in the front and tape it back in? How do these plenums get attached to the air handler - gravity and tape? So I tape the back and the sides from the inside? Huh...never thought of that. Could I put the back three sides on first and tape them from the inside, then tape the front on last? Or should I do what you said? How big of an access hole?

I used an online calculator - google "Resduct." I started with 1200 cfm, then distributed it evenly to each room based on size - so, from 80 cfm in the small bathroom up to 208 per duct in the great room. I ended up with 246 cfm to the 3 ducts on one side and 979 cfm to the 6 ducts on the other side (total 1225 cfm), and putting it in the calculator, it said 9" to the left at .08, 557 FPM, for a trunk, but I think I can only get 10" ducts. (It actually says 8.48" exact, but I'm told to upsize?) If I go down to 8", it says 220 cfm at 630fpm. Is that better? For 979 cfm at .08 to the right, it says 16" at 701 FPM for a trunk. Both those assume rigid.

How should I size them? What else would you change?

Images hopefully:

Plan:



Current mess:





 

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I'd just run a bead of caulk on the air handler for duct board. Set the plenum on the air handler. And only tape the front and sides(from outside) or what ever areas I could reach. Sheet metal, I would mastic the inside of the plenum.

While your velocity for 979CFM in a 16" is right. The FR would only be .046".

A 10x12 room with a south west exposure usually needs more air flow for cooling then a 10x12 room with a north east exposure.

You have a 10" flex going to a 12x12 bedroom, and a 10" flex going to the 13x16 master bedroom. Hopefully you will install balancing dampers.

A room by room heat loss/gain calc tells how much air/CFM a room needs.
 

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Discussion Starter · #6 ·
I'd just run a bead of caulk on the air handler for duct board. Set the plenum on the air handler. And only tape the front and sides(from outside) or what ever areas I could reach. Sheet metal, I would mastic the inside of the plenum.

While your velocity for 979CFM in a 16" is right. The FR would only be .046".

A 10x12 room with a south west exposure usually needs more air flow for cooling then a 10x12 room with a north east exposure.

You have a 10" flex going to a 12x12 bedroom, and a 10" flex going to the 13x16 master bedroom. Hopefully you will install balancing dampers.

A room by room heat loss/gain calc tells how much air/CFM a room needs.
What is the best caulk to use?

So is a 16" and a 10" trunk good? Or should I change them up? 14" and 8"?

Is there a good site to go to with a calculator so that I can do a room by room heat loss/gain?

The problem is, say the 144 sqft needs a 9" duct and the 208 sqft needs a 10" duct - I only seem to have access to 8" and 10" ducts, so both get 10". Every ceiling register will have dampers on them though.

Right now, all bedrooms have 8" rigid going to them, but because of the convoluted path, the 144 sqft is hotter than the 208 sqft. And that is with a southeast exposure to the smaller, hotter room, and a west exposure for the larger room (but only has 1, not 2 or 3, exposed walls.)

I would like to do the calculations correctly, but I need a link to the right website to do them. Are there any out there?
 

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Not sure there is any website that has a free room by room load calc. HVAC CALC is a 50 dollar fee to use it, and is room by room. http://www.hvaccomputer.com/gtarget...XxnAr3zAY18whsLdUyS1WkxH-nXIO94gYEaAnVg8P8HAQ

You should be able to get 9" hard duct.

14" in place of 16, and 12" in place of 10 will probably work a bit better for you. So a 16" from the plenum, to a 16x14x12 tee or wye.

And silicone caulk will work. I get mine from HVAC supply houses, so its rated for HVAC work.
 

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Discussion Starter · #8 ·
Not sure there is any website that has a free room by room load calc. HVAC CALC is a 50 dollar fee to use it, and is room by room. http://www.hvaccomputer.com/gtarget...XxnAr3zAY18whsLdUyS1WkxH-nXIO94gYEaAnVg8P8HAQ

You should be able to get 9" hard duct.

14" in place of 16, and 12" in place of 10 will probably work a bit better for you. So a 16" from the plenum, to a 16x14x12 tee or wye.

And silicone caulk will work. I get mine from HVAC supply houses, so its rated for HVAC work.
I am limited as to what is available to me because only one supplier will sell to non-professionals down here, and they have limited selection and high prices. It looks like I'll have to order online, so I'll try to find a 9" duct, etc.

I have a mac so it looks like that software won't work without an emulator.

Instead of a 16x14x12 off the plenum, can I come off the back of the plenum with a 14" and off the side of the plenum with a 12"? (Why do I want to size down the side that goes to 6 branches and size up the side that goes to 3 branches, btw?)

When you're going to make a branch off a trunk, does a wye with a 45 or a T "flow" better, or are they the same? Is there a reason not to use a wye with a 45 other than maybe cost?
 

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A wye with a 45 flows better then a tee.

The FR of a 16" moving less then 1000 CFM is so low, hat you would have to throttle down the supply registers dampers so much, that they would become noisy. Increasing to 12" lowers the FR closer to what the 14" will be.

Its also always better to have balancing dampers the supply branch right at the trunk line, less noise in the rooms that way.
 

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Discussion Starter · #10 ·
A wye with a 45 flows better then a tee.

The FR of a 16" moving less then 1000 CFM is so low, hat you would have to throttle down the supply registers dampers so much, that they would become noisy. Increasing to 12" lowers the FR closer to what the 14" will be.

Its also always better to have balancing dampers the supply branch right at the trunk line, less noise in the rooms that way.
I'm so confused right now.

My calculator for the left side of the house tells me that for my total 246 CFM at .08 I want a 9" rigid trunk (I rounded to 10") at 557 FPM. For the right side it says 979cfm at .08 and 701 FPM is a 16" trunk. If I go to a 14" trunk on the right side, it tells me I'm at 916FPM and I am under the impression that is way too high. Plus, I need 3x more CFM to the right side of the house than the left, but I'm getting the trunks closer together in size? A 14" flows ~745cfm at ~700fpm and a 12" flows ~550cfm at ~700fpm, all at .08FR. So, if I size them at 14" and 12", aren't I starving the (larger) right side while oversupplying the left side? What am I missing here? You're telling me to go +3" trunk on one side and -2" trunk on the other side, and I'm completely missing why.
 

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14" round metal at a .08" FR is 950 CFM at 900FPM.

1,000 FPM for a main supply trunk is fine.

Use balancing dampers in the take offs at the main trunks. You will have plenty of air on your 14" trunk side.
 
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