New Duct Work - Wall Stack 12in" x 3.5in" x 5ft' ???

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Billy123
Posts: 41
Joined: Tue Sep 03, 2013 6:44 pm
Location: Spotsylvania Va.

Tue Sep 24, 2013 5:05 am

I have used wall stack in the past on another mobile home. It turned out really nice as a replacement for the main trunk line. So here I go again and I wanted to shoot this past you guys before I spend the money on it this weekend.

My intentions again are the same but on a bigger scale. This time around I plan to use it in my 14x70. Do you guys see any reason that I should not use this product as a replacement for the main trunk line?

Also should I insulate this new truck line at all? I do plan to run my new pex along side of the new duct.

Thanks for your time.
Billy


DaleM
Posts: 385
Joined: Wed Apr 07, 2010 10:07 am

Wed Sep 25, 2013 5:45 pm

I don't think you will get enough air flow with such a small duct. Most I have replaced started out 24" wide by 6" tall, then reduced down to 18" wide by 6" tall. The really old homes, 60s and 70s, had small trunk ductwork but those furnaces moved very little HOT air, The newer units move 3 times more air but it's warm air.

Billy123
Posts: 41
Joined: Tue Sep 03, 2013 6:44 pm
Location: Spotsylvania Va.

Wed Oct 02, 2013 4:48 am

Hey Dale, Thanks for the reply. I had a chance last weekend to cut a piece of the old duct work out. I then put a ruler on it and took some pictures. I didn't do a great job either so I will need to get more pictures of this crap. Anyway here is what I have...

http://www.mrrepairs.net/mh/ductwork/

I cancelled the duct work I had on order till I get this figured out with what is best for my mh. Based on the crappy pictures I do have the "wall stack" I had on order would have been smaller that is for sure. Not by much tho.

This duct work in my MH is the cheapest stuff I have ever seen. New coke/pepsi cans are made of thicker material. lol

Based on the pictures i have the top of the duct is 12 inches wide and almost 4.5 inches high. Both side's of the duct have a bend in them down to create a triangle when both side are crimped. At the most it adds an inch to the width of the duct on both sides but tapered. If you know what I mean.

So it seems the hard duct I was going to order is 1 inch smaller in height and maybe two inches narrower. But that extra two inches is a triangle on each side.

I have no idea how to figure the cfm of that stuff because its not square. So a few questions...

1) How do I figure the correct cfm of the duct work needed?
2) Does anyone have any suggestions of what duct to buy for my trunk line?
3) Will I be ok if I use the smaller "wall stack" like I was going to do?

Wall stack only comes in two sizes... 10"x3.5"x5' & 12"x3.5"x5' so if the 12"x3.5"x5' is to small I need to find something completely different to use.

Thanks in advance for any of your time.
Billy

Billy123
Posts: 41
Joined: Tue Sep 03, 2013 6:44 pm
Location: Spotsylvania Va.

Wed Oct 02, 2013 5:22 am

I just realized why they put such thin ductwork in mobile homes. It's so the heat can transfer better to the plumbing. You know to keep the plumbing from freezing up. It goes to stand that with thicker & better ductwork that the heat inside wont transfer as much outside of the duct and inside the belly liner.

Just a thought...

So i am thinking I will go with 14x4 or 12x5 wide/thick. I can't do anything about how thick the material is. It going to be a lot thicker but I guess that's the price I pay for replacing the stuff to begin with.

HouseMedic
Posts: 342
Joined: Fri Apr 06, 2007 9:14 am
Location: Delaware
Contact:

Wed Oct 02, 2013 6:20 am

I have a 73 pre-fab two part house that is made part like a mobile home and part like a stick built home and the duck work is 3 1/2" x 14 1/2. I have to rebuild all of mine also but have not figured out what would be the same equivalent.

Ron


Billy123
Posts: 41
Joined: Tue Sep 03, 2013 6:44 pm
Location: Spotsylvania Va.

Wed Oct 02, 2013 7:43 pm

Hey Ron I made a mistake above in the sizes of the duct work. I hope the correct size for mine this time would be 12x6 or 14x4 w/h... That's as close as I will get & think either would work well. I will get better pictures of it this weekend when I go down to finish the clean up of all the joists. The porter cable has made short work of all the glue on them.

My mobile duct is not laid between the joist long ways. Its strapped underneath the joists on this mobile thank god. The last mobile I had to put the 12x3 1/2 in. Because the joists ran the length of the mobile and it was tucked up between them. What a mess that was but in the end the duct worked well. I think I did pull out the same size as yours above. It was a choice I had to make and again plenty of air came out of the new duct during the winter months to keep us warm.

Billy123
Posts: 41
Joined: Tue Sep 03, 2013 6:44 pm
Location: Spotsylvania Va.

Wed Oct 02, 2013 7:57 pm

I uploaded a duct sizing chart to my website.
http://www.mrrepairs.net/mh/ductwork/

I would need to know how many cfm my furnace blower pumps out from the label on it. Then its just a matter sizing the the duct to it. Based on what I have read there are other factors like friction and such but in a nut shell all that is needed is the cfm of the furnace.

DaleM
Posts: 385
Joined: Wed Apr 07, 2010 10:07 am

Thu Oct 03, 2013 10:54 pm

Billy123 what make and model furnace do you have and is there air conditioning on it as well?

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