my ducts are scaring me!

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Hope someone can give me some guidance on this one!
I removed the dropped ceiling in part of the downstairs of the 1981 house we just bought in Austin, Tx, and am having to reroute all of the ducts in the first floor to get them up in the joists. That's not a problem - I'm just replacing the old gray flex-pipe with new pipe of the same diameter.
But upon pulling out the old stuff, I saw that ALL of the ductwork - from the 12" cold air return to the two 10" main feeders from the attic unit to the duct board distribution triangles and ducts to the individual vents, had a think black powdery crust lining at least the bottom surfaces of the ducts and duct board register boxes (boots?). The crust is dry, dark gray/black, and will flake off if you scrape it. It's kinda furry, but it's dry. We did 2 mold tests, and they were not out of the ordinary for this kind of house. It's kinda like just 30 years of dust and dirt accumulating in the ducts, and crusting up. Is this normal?
Anyway, to be on the safe side of getting as much crud out of the system as possible, I decided to replace the 12" cold air return flex pipe and two 10" main supply ducts from the attic, as well as the duct board distribution triangles and smaller ducts that I had to change out anyway.
My question is: when I pull the big ducts off their collars on the unit in the attic, am I going to find a big, gross mess of crust in the boxes that connect to the unit in the attic? Will I need to replace those as well? What about the rest of the unit itself?! And what is this junk anyway?! I can't take on replacing the attic unit now - just no time or money left. And it works fine. Am I going to cause bigger problems by disturbing what is up there?
The thing is, if I'm going to replace these ducts, I have to now because I have the 1st floor ceilings and the chase all opened up. After I close them up, I'll never be able to replace them. Is it okay to put the new ducts on the old plenum and intake at the unit in the attic, then maybe replace that some other time?
Should I just close it all up and pretend I never opened it and saw the insides in the first place?
I appreciate any help!
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This is a multi-part message in MIME format. --------------070002090301020707090704 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit
snipped-for-privacy@gmail.com wrote:

What size furnace & Air Conditioner (if you have one) do you have? I would even go with a 14" Return Air duct for a one & a half ton A/C. Most home A/Cs are 2-ton or considerably larger.
Get the ducts cleaned.
Here is info on sizing ductwork for efficient operation: http://www.udarrell.com/proper_cfm_btuh_duct_sizing_air_conditioning_systems.html
udarrell

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Ductboard & Flex are going to be almost impossible to clean. Replacement with galvanized sheet metal is the best cure, but is an expensive option. Insulate the outside of the metal duct. The fiberglass duct board & the plastic flex duct are great magnets for dirt, dust, soot, etc... This is what you're seeing in your existing system. If you burn a lot of candles, live next to a plowed field or dirt road, have lots of pets, etc... you have a better idea of what the "stuff" is.
I'd check the sizes as well, as a 12"rnd return & 2-10"rnd supplies only add up to about 600 cfm SA & 500 RA (@.08/100'), that's about 1-1/2 ton or 1000 sf in my area... I'll have to check my Flex Ductulator (Hart & Cooley) but most show about a 60-75% cfm capacity of galv. SM based (TRANE) ductulators.
good luck geothermaljones

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Really helpful stuff, all - thanks!
The house is about 2000 square feet, 2 story, and the ducts and return in question are all downstairs serving about 950sq feet. There is also a return with a 12" flex duct in the ceiling of the upstairs hall (does that make it a warm air return?) so I guess that one handles the other 1000 sq feet... Don't know how big the system is, but the external unit is an Amana RCE48A2D.
I'd love to put in the galvanized sheet metal, but that's beyond my skill and budget. Sounds like the stuff in the ducts is from years of accumulation, but I guess I never thought so much would be in there past the filters! I'll also have to replace the upstairs flex at some point - it's in the attic and starting to come apart.
But I just took a look at the attachments in the attic; looks like I'm not going ot replace the big ducts; way huge amounts of pain there, including having to rebuild the ductboard plenums on both ends and reattach all teh upstairs stuff, which means I'd have to replace it all now too, since it's fragile. One of you guys could probably do the whole thing in a couple hours , but it would kill me!
But all this crud in the system - is it a problem waiting to happen in the attic unit?
Thanks!
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In article
snipped-for-privacy@gmail.com wrote:

4 ton with less than 1/4 of the required duct. You need a load calc done on your house to see what's what.
Who installed this mess and when?
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Sid wrote:

Just another example of lunatic work, - its everywhere. - udarrell
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"Sid" wrote

An HVAC professional, of course. (He was probably in a hurry to get back to berating DIYers on usenet.)
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Then he needs his license yanked. Most stuff this bad is someone hiring someone "on the side" to do it. Usually someone who hasn't a clue as to sizing.
This isn't a DIY newsgroup. Are you one of the berated DIYers?
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"Sid" wrote

Isn't much of an HVAC one, either...
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Actually it is an HVAC newsgroup.... its for professionals, its not for home owners to be told how to fix their 30 year old POS furnace for free.
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"Noon-Air" wrote

The level of professionalism demonstrated in here boggles the mind.
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I'm sure it does with the wannabe's, DIYers, EE"S and homemoaners that can't grasp that this isn't a newsgroup for them.
Know whut I mean Vern?
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"Sid" wrote

Yes, and if you want to form a chartered or moderated group, feel free.
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Actually it *IS* a chartered group.
BTW... Welcome to my killfile with the rest of the idiots and shit disturbers.
*click*
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"Noon-Air" wrote

Imaginary charters don't really carry much authority, do they?

Sorry about disturbing you.
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residental A/C Duct trunk lines, in the attic, are (seems to be) always insulated on the outside. why not on t insulated inside like comercial buildings?. is it because the the ductwork is above a suspended cieling and this space is a semi codition space?
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AS IN CO HABITATE ( KEEP SEX OUT OF THE RETURN PLENUM )

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$$$$$$$$$
But, who'd want fiberglass insulation in their ductwork anyway????????
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We formed this one. No one sent you an invitation. Take your own advice and don't let the door hit you.
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"Sid" wrote

Where is participation in this group declared to be by invitation only? And how do you propose to enforce that?
Look, the only beef I have with this group is that what could be a useful discussion forum is instead a pointless cesspool where amateurs can freely enter and they, regardless of whether their questions are reasonable or not, are routinely advised to "Fuck off and call a tech." Show me a serious technical discussion in here and I'll show you the thousand hackneyed flame posts that you had to wade through to find it.
If everyone here agrees that the group should be closed to homeowner questions, it would be simple enough for the core users to agree on a faq or charter and post it at regular intervals. Guileless OT posters could then be referred to the faq and the insults could be saved for the guileful ones. The current situation leaves the naive outsider who happens upon this group with the impression that the typical HVAC tech in here is an ill-mannered supercilious prick who wouldn't light your pipe if his house was on fire. If that's the result y'aller after, you're doing a great job.
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