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Old 04-01-2013, 01:20 AM   #1
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1950s remodel-New Ducts or repair existing?


First, a little background. I continue to work on 1950 1 floor 2100 SQFT 3/2 ranch style home outside Austin, TX. So, obviously a/c is essential.
The short version is, should I tear out the existing trunk and branch metal ducting in the home and replace it with flexible ducting or try and repair the existing metal ducting? Condensor/compressor/gas furnace system itself is nearing 20 years old and will eventually be replaced when we find more money but still blows cold and great presently (although obviously not nearly as energy efficient as a new system).

Here is a bit more detail: The house was apparently originally built with some sort of heating/cooling system as there are copper pipes embeded in the concrete that run from the HVAC closet (near the center of the home) to the outside where an old Swamp Cooler type system used to sit. That has long since been abandoned for a more traditional system but the duct work seems to be original. The metal ducting that runs through the center of the house above the main hallway (in the attic). Looking inside the ducts, you can see they are well made. I don't know exactly what you call it, but inside the ducts there are curves and chutes built into them to seemingly route the air with as little obstruction as possible. Of course they are very dirty inside, but since they are metal, it seems one could do a decent job of cleaning them. The big problem is that in at least one area, there is water damage in the sheetrock ceiling. Initially, I believed this to be due to a leak in the bottom of the duct blowing cool air into the attic and causing condensation. However, I've now pulled down the sheetrock and couldn't find any obvious leaks (they may still exist but they are at least smaller than I expected and will require more detailed inspection). I did observe some rusting on the bottom of the ducts, so now my theory is that its really due to poor insulation causing a cool duct to come in contact with a hot attic resulting in 50 years of condensation.

Assuming I don't decide to just scrap the whole ducting system and replace with flexible duct (I believe this all I can afford), I think my approach is going to be to seal the duct joints with proper shiny backed tape and then attempt to reinsulate the duct. I was thinking of spraying the rusted areas with some leak spray stuff I've seen at my local HD. The ducts were/are insulated with fiberglass batt type insulation, so I don't know if this will solve the problem. Another potentially big issue is that the existing ducts sit basically right on the 2x4 fur down in the hall way. Obviously there is the old insulation between the duct and the 2x4s, but clearly over the years it had been compressed and likely lost some of its effectiveness. But, this presents a problem of how to squeeze some sort of insulation between the existing ducts and the 2x4s. I am considering sliding 1" fiberglass duct board under the ducts, and then running batt insulation back over the top and then making sure the fill the space between 2x4s with fiberglass. I think it will be a challenge to find an inch between the ducts and 2x4s, but maybe it can be done. I also considered substituting 2x3s for the 2x4s to give me more space but am hesitant because this area is used for attic crawl access.

So, like I mentioned above, should I try and rehab the existing ducts or is it more trouble than its worth?
I have a bid for about $2000 to rip and replace the existing system with r8 flexible ducts. I'll get at least another bid, but that doesn't seem outrageous. It seems to me that a flexible ducting system might not do as good a job of distrubiting the air as my existing trunk and branch style system. However, given that the ducts are in the attic, I wonder if the flexible ducting's likely improved insulation would outweigh any "flow" reduction I would see from such a system.

Finally, can I just replace ducts without replacing the cooling/heating system itself, or is there a reason why replacing ducts might create problems with replacing the rest of the system later?

Sorry for the long-windedness. I can provide diagrams/pictures if it would help.

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Old 04-01-2013, 09:57 AM   #2
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1950s remodel-New Ducts or repair existing?


flex duct bad, metal duct good, but if the duct is 50 years old, it past its life span. Additionally, there is a boat load of ductwork science about size and shape etc that was not around in the 1950's, thus, I would suggest new metal ducts designed with manual D and manual J. Of course, you like all of us, are on a budget. I think that for the time being, you should insulate (its cheap), seal as best you can (also cheap) and use the existing ducts till the old system dies, which might be another 10 years, if your handy.

in the mean time, read up on ductwork static pressures, design (manual d and manual J) and how to run flex. I believe that once you read up on flex, you will not want to use it.

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Old 04-01-2013, 10:29 AM   #3
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1950s remodel-New Ducts or repair existing?


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should I tear out the existing ...and replace it with flexible NO
should I try and rehab the existing ducts YES
Get a pail of duct mastic and a pack of sheet metal screws.
Tighten up everything and then seal all the joints.
If any of the sheet metal is damaged anywhere... replace that bit.
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Old 04-01-2013, 10:59 AM   #4
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1950s remodel-New Ducts or repair existing?


Thanks for the replies. I am reassured to hear support for my initial thoughts about flex duct. I think if i keep the metal ducting the biggest trick is going to be reinsulating it because of the small space between the duct and the 2x4s. Unfortunately, lowering the ceiling anymore than it already is isn't an option. I also can't raise the ducts without having to raise all of the registers in the rooms. Is 1" fiberglass duct board the best option to go under the existing metal duct, or is there some more space efficient approach I should consider?
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Old 04-01-2013, 05:53 PM   #5
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1950s remodel-New Ducts or repair existing?


i would redue all the duct that you can its time you can also size it better for more effiency to the furnace and the house.... ben sr
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Old 04-02-2013, 12:57 AM   #6
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1950s remodel-New Ducts or repair existing?


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Originally Posted by Dtrain View Post
Thanks for the replies. I am reassured to hear support for my initial thoughts about flex duct. I think if i keep the metal ducting the biggest trick...
The suggestions to replace the duct are ultimately the correct path...
the question is when it's most prudent to go down that path.
(and what to do until then)

Quote:
condensor/compressor/gas furnace system itself is nearing 20 years old and will eventually be replaced...
You're term "eventually" needs to be firmed up.

When the equipment gets replaced will be the best time to do the duct work because you can then size that new duct job based on the specs of the new equipment.

hth
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Old 04-02-2013, 08:29 AM   #7
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1950s remodel-New Ducts or repair existing?


Most ductwork from 20 years ago is not sized for today's furnaces and how much air they move. You certainly can get some added efficiency by sealing up the ducts and reducing the duct leakage, but going larger will work better with today's AC systems.

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