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bigburb 06-11-2008 08:20 PM

Replacing 30 yr old duct... couple questions
On a hot day in Houston my house will barely get below 80 if we are lucky. Among the problems are just flat out old ducts. Besides replacing the ducts, I was thinking about converting my main trunk line to rigid or metal duct, i've heard it flows better.

My question is: How would this throw off the balance of my system? I ask because I've got about 4 lines coming straight off the plenum to various rooms, then a trunk line goes about 25 feet to the other side of my house to the kitchen, formals, breakfast area, and laundry room. Would a more efficient trunk on one side reduce flow on the other side?

Also, the splitters look like they are made from ductboard taped together, I've got 2 splitters with 4 and 5 ducts coming off each one and they leak awfully. What is a good replacement for these? Is aluminum tape appropriate for sealing ducts?

geo fan 06-11-2008 09:29 PM

fiber board ducts if un lined release fiberglass into the air and is illegal in13 states the ones that care whats in the air the best install is whats called a reducing plenum depending on the size of the system you start with 12 or 14 inch round ducts insulated with 3 inch wrap every 2 or 3 branch lines you reduce the trunk by 2 inches each branch should be 6 ,7,8 r-8 flex attatched with a sadle collar with volume dampers
this method is the right way it will maintan equal static pressure and volcity and allow you to balance the system perfectly
no flex run should be over 15 feet
every reducer should have a short peice of flex of the smaller size to prevent the reducers from breaking
all of these connections should be made with screws and zip ties silver tape at all insulation seams

bigburb 06-12-2008 06:22 AM

Wow you know your stuff. If not ductboard, what would I build the reducer plenums out of?

geo fan 06-12-2008 06:10 PM

use pre fab round metel duct its cheaper then square duct and the install is faster less time in that 130+ attic

bohica24 06-09-2009 06:32 PM

30 Year Old Duct work sweating...causing drywall issues
The old duct work is sweating in a couple of spots causing drywall issues in a couple of spots on the ceiling. I've been in the attic, extremely tight and access is limited to address the areas giving me headaches, and added insulation in some areas. On the coast in Texas with this humidity causes many problems.
I'm considering replacing all of the metal/'insulated' duct with flex. Any guidance or suggestions would be appreciated.

bigburb 06-09-2009 06:48 PM

I've heard you should have runs longer than 25', though I was not able to avoid it and my house cools just fine. Have you tried an insulating jacket on your metal duct?

bohica24 06-10-2009 09:09 PM

Insulating Jacket
Are you saying that the runs should be 25ft. or longer in this type of climate, does that reduce sweating? The runs are already insulated, but in a couple of places they sweat anyway. More wrap, jacket?? I'd rather not replace all the ductwork if there is a better solution.

bigburb 06-11-2009 05:26 AM

I meant to say shouldn't have runs longer than 25', this causes too much resistance in the line to effectively deliver air, or cool air at that. Like I said, it can be done, I did it, its just not recommended if you can avoid it.

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