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feasability of replacing AC flex ducting by myself?

41K views 47 replies 6 participants last post by  rdc 
#1 · (Edited)
so apparently the ducting in my attic is substantially leaky... to such a degree that i can actually feel cool air flowing down the attic and out the soffits that i was in process of replacing. from what i can tell several joints branching from a main rectangular duct are leaking, i suspect that they are responsible for the majority of that cold air i felt. i was aware of the poor condition of the ducting prior to this point... but honestly i had not thought it was as bad at this.

what concerns me however; having spent the last few days reading and researching proper residential hvac installations, is that the attic ducting is just garbage. nothing is strapped, the ducting runs over each other... theres alot of slack in the smaller ducts, and there are dozens of fairly bad kinks that ive been able to observe. i havent seen any mastic used anywhere, just tape and zip ties.

i had an HVAC company(apparently they only install trane products) come out last year to adress some minor issues. while they were there i asked if they could service the coil in the attic, at the time i was under the impression they needed cleaning somewhat regularly, it was my understanding at the time that it had not been cleaned in like 5 years. anyway they didnt seem to want to clean it, they actually said it appeared to be in OK condition along with the rest of the ac system. i asked if could give me a quote for just servicing the ducting. he hem'd and hawed for a while and i got the general impression that he didnt want to do it, but thought i personally should do something with it.

my question is simply... can a grown fat man about 300lbs 6'3" feasibly replace a small single story homes entire poorly constructed AC ducting with something far more appropriate for a 14 cent/kwh electrical service? its a fairly small single story cheaply built tract home built in the 70's , i think there are only 19 registers?
im mostly worried about what i should use to replace the main trunk line with and how to get it back into the attic, it looks like the existing one is just sheet metal? is that even possible? it seem like it would sweat badly..which is didn't appear to be doing. i cant actually get to it without hands and knees crawling and probably cutting some drywall. i understand that there is something called duct board that sounds like a suitable replacement, but can i work with it without specific tools? i have at a table saw and all the typical power tools, but nothing specific for sheet metal working.

anyway back to the original question. should i even bother trying? should i just fix what i can get to? if i can actually just blow out all the old duct work, is there anyway to do it in phases such that i can basically keep the ac functioning while i replace sections of the duct.
im going to buy the manual D and J books regardless so hopefully what ever i end up doing is done correctly.

thanks in advance for what ever advice you can give me.

edit: bad spelling, grammar etc.
 
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#39 · (Edited)
some folks like ductboard, some like flex duct, some like sheet metal. When I re-did my ducts, I went with sheet metal. The cost was not bad and install was easy with a pair of tin snips, self tapping screws, mastic and tape. I can tell you that the electrical bills have been down 25 percent, since the change. Please read up on flex duct installation and read up on ductboard. There are some specific things you have to do for ductboard sealing when you expose the raw fiberglass when cutting. as far as which is best, I will say that there are no issues with steel ducts.. no air contamination (ie fiberglass and mold breeding ground, they are cleanable and the last longer. Just my thoughts and what I have been told.
I also forgot to add that a central line duct, is normally much better, in terms of air flow then a plenum box. I also feel that WYE's, pants WYE's and even Tri-WYE's are better then a plenum box with takeoffs.... again, just my two cents work... remember to watch that static pressure also!!!!!
 
#40 ·
Bob, I'd love to have all sheet metal but its a lot of work to piece it together and then insulate it. I am using SM elbows for my corners, flex for the straight runs, pulled tight and supported. This new flex is smooth when its pulled tight. Seems like a good compromise. I thank Fred at askweldin for that tip.

Originally I was just going to replace my old flex duct and Ys with new. Then I got a look inside the plenum. My flex duct is filthy and the duct liner in the plenum is too. Now I'm wondering if I can just remove that duct liner and wrap this plenum from the outside, or whether I have to get a new one to do that. Has anyone ever refurbished an old filthy plenum like this one?
 

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#42 · (Edited)
I got a huge reduction from opening up the blower side of the condenser and cleaning that - there was about a 1/2 millimeter layer of matted stuff covering a large part of the condenser grill. So make sure you check that. Who knows who may have run your AC without a filter? That saved me $100/mo in electricity.

I don't have any other results yet as I'm in the middle of revising my design, now that I've seen the plenum. But I'm targeting 20% reduction of cooling bills in the summer. I will also benefit from everything being clean.

Regarding trunk duct: I read bobinphx's message as being against trunk and in favor of Ys like you already have. Pick a duct traveling from your air handler going to a Y or a register and remove it and have a look inside the plenum. If its clean then you can just replace the duct. Your earlier pic of the duct looks clean, a good sign. My mesh is clogged with dust. BTW, triangle is not a plenum, its a distribution box. You can buy those prefab cheaply at the AC place I mentioned. Ask for ductboard triangle box.

I will also blow cellulose into my attic. I have already sealed it and built dams around the bottom of my rafters. But have to redo duct first. I will be lifting it up in the air also. I am adding a bit of floor joists on top of the attic rafters to build a raised floor in a few spaces so I can still get around after the insulation is added. Make sure you use wide strap to hang the duct, and support enough so it doesn't sag. Strap available at AC supply. I will hang the strap with wire and eyehooks screwed into the rafters.

Radiant barrier - I'm installing some because its cheap enough DIY and easy enough to install and it looks good when you sell the house. I dont expect dramatic results from it, but it does have some effect. The idea is that your roof absorbs sun's heat and radiates that into your attic, heating up insulation which then spreads into interior. Barrier will reflect the radiated heat back to the roof. For installation options see attic foil dot com. I'm using their stuff and it works well.
 
#46 ·
Hey thanks for the tips, do you mind elaborating on how your are going to build a raised floor? id like to build something like that in my crawl space if i can. otherwise you would have to crawl through the new cellulose i imagine. also what are these rafter dams you are talking about? are you talking about these?



i dont know about your system but my air handler is currently sitting on the top of the raters, im not terribly comfortable lifting it up, so i thought id build some sort of dam around it? ever seen something like that?

re: radiant barrier,
i understand the concept of reflecting radiated heat, but i guess im not convinded hot wood emits enough heat to necessitate a reflective surface. i could be way off though, as is frequently the case. ill have to take a look at that website when i get a chance.

i would think that creating a ventilated envelope of air between he roof sheathing and the attic would be better? it seems like ive seen something like that done with polystyrene foam boards that took air up the soffit vents and out the attic vent.
regardless i think ill just roll it all back up, and reinstall it later, no sense in not using it i guess.

thanks.
 
#43 ·
Filter on a condenser? Inside is the evaporator coil. On this side of the system, the inside of your home in which enveloped air is being (re) circulated and conditioned, is where a return filter would be associated with the system, not on the outside condenser.

You can also take a hose to your (outside) condenser, rinse off the coils as best you can. Best to turn it off first.

Now you'll be becoming close to having proper cfm across the coils which means close to a properly running system. Close, not perfect, but close(r).
 
#44 ·
Btw RDC, I'd scrap that plenum and get a brand new one. It's actually less headache and much less time consuming and possibley even cheaper. Granted you'd have to install it yourself but it's not a big deal to do so. If you need some tools to do it with, I'm in Houston as well.

I can't charge you as that is against the site rules, to offer my services for monetary gain, but if you do enough of your own work than when I have the time I wouldn't mind assisting.

Btw, that would mean my tools never leave my line of site, you do the work in front of me type deal. :thumbsup:

..if you need it.
 
#48 ·
Queequeg152, that picture is correct for what I did for ventilation, stapling those foam channels up there, and I also put cardboard beneath them to block the insulation from falling into the soffit.

Re lifting the air handler, no, I'm not going to do that. I will build a short wall around it with plywood. My floor will be 2x8s spaced 12-16 inches apart, held in place by 2x4s nailed to the rafters and cross-braced. I will put plywood on top after I blow the insulation. I will also build a short wall around the attic opening. My water heater is up there too so I have to be able to get around.

The radiant barrier I'm using is nailed to the bottom of the rafters, so it works as you described, creating an air channel where soffit air can flow up to the ridge vent. You can also just lay that sheet on top of your insulation. I'm not familair with the Lo/Mit that Doc Holliday is recommending.
 
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