Air Conditioning - Where can I buy fiberglass duct board?

Ive looked at lowes and home depot for some fiberglass duct board so that I can build some splitters up in the attic. I cant find the stuff anywhere not to mention they dont have the 9" collar I need. Where can I find this stuff at locally in Houston Texas? Thanks for any info!

Ive looked at lowes and home depot for some fiberglass duct board so that I can build some splitters up in the attic. I cant find the stuff anywhere not to mention they dont have the 9" collar I need. Where can I find this stuff at locally in Houston Texas? Thanks for any info!


Are you thinking of adding registers to the existing system?

If you are then keep in mind when the system was designed and put together it was for what it is doing.

As for the duct board I have not a clue where to buy it (other then a supply house) In NY where I am we only use metal duct.

Well when an a/c guy saw my setup he noticed that it looked like they took two smaller tubes and shoved it inside a larger tube and just taped it up. He said that is a bad way of doing things. I started getting my tube size numbers and noticed it felt rigid.. turns out there is a metal splitting tube inside it and not just two small hoses shoved inside a larger hose. I guess this is fine and there is no real need for duct board at this point. I had just planned on building my own.

Well when an a/c guy saw my setup he noticed that it looked like they took two smaller tubes and shoved it inside a larger tube and just taped it up. He said that is a bad way of doing things. I started getting my tube size numbers and noticed it felt rigid.. turns out there is a metal splitting tube inside it and not just two small hoses shoved inside a larger hose. I guess this is fine and there is no real need for duct board at this point. I had just planned on building my own.


Ok cool

The first thing to do is get a "air duct calculator" this will tell you how many cfm's are comming from the large duct and you will be able to convert it to square duct.

Rule of thumb on round duct
6" 100 cfm's
7" 150 cfm's
8" 200 cfm's

1 ton of airconditioning is 400 cfm's
1 1/2 tons 600 cfm's
2 tons 800 cfm's
2 1/2 tons 1000 cfm's
3 tons 1200 cfm's
3 1/2 tons 1400 cfm's
4 tons 1600 cfm's
5 tons 2000 cfm's

When I design a system I will go by (on the top of my ductulator) the recomended residentail setting of .10 for friction.


Hope this helps.

I have a 4 ton unit.... I noticed I have some 10" and 12" lines coming out that branch off.. 9" the branches off to a 5" and a 7". I have a 10" that branches off to two 8" that then branches to two 6"... i see two like that.. is this normal?

I have a 4 ton unit.... I noticed I have some 10" and 12" lines coming out that branch off.. 9" the branches off to a 5" and a 7". I have a 10" that branches off to two 8" that then branches to two 6"... i see two like that.. is this normal?

The 5 and 7 is 200 cfms and the 9 is 200 cfms.


With a 4 ton unit you have 16oo cfm's

This sounds like a (what we call in NY) a spider type duct work.
It does work but, it could be better.

If you are going to replace the duct then the first thing I would have would be a canvas collar (to reduce noise) then it would be a 15 x 16 duct. (this is after the transision from the unit) This is the start of the trunk line.

I have a 10" that branches off to two 8" that then branches to two 6"... i see two like that.. is this normal?

Sure is look at the cfm for pipe size Rob gave.

Also the start duct off unit . A tight job is 60sq " per ton and a good job is about 80 sq" per ton. Or on your 4 ton a return duct at the unit should be about 320sq"

ED;)

What do I use to seal up leakin collars at the main supply duct/cabinet? I have a few I think I need to remove the duct and push the tabs back but will probably still let some air out. Do they make any kind of gasket? I have some Iron Grip but that gets real messy real fast. Thanks for all the info.

Not sure just what you have there. But some codes call for the flex duct inside slid over the take off and a duct tie put on it. Then the outside slid over it and a duct tie its like a draw band put on it.Then the mess . It has tobe sealed with a Duct Sealant. i think this is what you want to seal the leaks you have there.

ED;)

well im lost.... what i have is a metal cabinet that has colars for each of the flex duct. The flex duct is slid over the colars. Some of these colars have come loose like the metal tabs on the inside need to be pushed back to hold it in place. This is created a leak. Is there something I can use between the collar and were it sits against the metal cabinet? I should probably takes pics and point this stuff out.. im sure yall can pick apart this install for me.

If they are real loose you just may have to open one of the ducts and reach in and 'tighten' the adapters. You might be able to reach them all from one duct.

Then, go to the Lowe's A/C section and find a quart of white paint on duct sealer. You just brush it on around the joints. It's very thick and will seal the joints.

Is this the same as Iron Grip? I have a whole tub of that.. its thick as well.

You can try what you have But will it take temp????????

ED;)

Your the a/c guys.. you tell me lol.. its Iron Grip.. its the stuff made for a/c systems.. im in a warehouse looking at some right now they used on some metal to metal connections in building im in. The a/c guy who gave it to me told me to use it all over the taped areas to seal and prevent them from coming up later on. I think its hard cast Irongrip. Wasnt sure if its ok to use or not.. also for connecting the ducts.. what tape do i use? Should I use Zip ties instead? or both? Sorry for all the questions.. this is my first home and ive blown almost all my money on repairs... roof is next :(

Hey if they gave it to you use it. i dont know that name is all. Yes on the ties code here calls for one on the inside part of the flex and another on the outside of the flex pipe . Then get you hand full of the yuk and put it on around where the flex hit the trunk line or Y box

ED;)

ya know.. I dont even know what a/c duct looks like on the inside... is what your saying I stick the inner blackish tube around the collar.. put a strap on... then slide hte insulation/aluminum color part back up over that and zip tie it again?

ya know.. I dont even know what a/c duct looks like on the inside... is what your saying I stick the inner blackish tube around the collar.. put a strap on... then slide hte insulation/aluminum color part back up over that and zip tie it again?


With flex tubing (your duct) it has an inside part that looks like a streched out spring, and the outside that is isulation covered with plastic.

After you reseal the metal flange comming off the main duct, then put the flex on (pull back the insulation and plastic to expose the spring) then attach that to the flange and seal it. Then pull the insulation and plastic over that and seal that.

Ok this is what I figured. Is there special duct seal tape I need to use? What I have now it looks like the used some black tape and then drove a screw into the duct that screwed into the collar.. is this a bad way of doing it?

Ok this is what I figured. Is there special duct seal tape I need to use? What I have now it looks like the used some black tape and then drove a screw into the duct that screwed into the collar.. is this a bad way of doing it?


Just use duct tape and make the tape tight to seal the flex to the collar. Then pull the insulation over the collar and tape that tight.


I have done it this way for years and never had a problem.
Make sure you have a little slick in the flex as it does tend to shrink some.

Fl code calls for the two duct ties one on the inside lining and one on the outside of the flex pipe. Then the end of the pipe sealed to the trunk line with the Mastic duct sealant. Have seen many jobs where just duct tape will come off from the heat in the attic or on the unit.

ED