Guidelines on when to repair or replace duct work?


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Old 08-12-09, 12:37 PM
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Guidelines on when to repair or replace duct work?

Does fiberboard and flexible duct work have a limited lifespan?
How do you decide whether to replace it or repair it?

When my parents had a new AC system installed last year, they kept the old duct work which is 30-35 years old.

This may not be related, but the system seems to be cooling very slowly (three hours to drop from 80 degrees to 77 degrees). The air temperature at a ceiling vent was about 70 degrees in a room that was about 77 degrees.

The duct work in the attic has a few places where the rectangular fiberboard duct with aluminum foil on the exterior had lost its foil. I think the foil may have deteriorated in those places. I can feel air passing through the fiberboard where the foil is missing.

In other places, the foil appears to have separated from the fiberboard so that the foil puffs out, but it doesn't appear to leak.

There are a few joints that have come partially untaped and wouldn't be hard to retape.

At least one slip in joint (where the rectangular duct meets the air handler and on the duct side of an anti-vibration flex joint) is leaking notably (possibly because the fiberboard duct expands outward when the system is pressurized).

The smaller round ducts which have a foil covering and feed the ceiling vents seem to be mostly intact, but I haven't inspected them closely or looked to see what's going on inside the foil. I think these are flexible with fiberglas insulation under the foil, not steel-walled, but I'll have to take another look to be sure.

What do you think? Repair or replace?
 
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Old 08-12-09, 01:29 PM
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Not really a pro on this, but the problem with the duct board breaking down doesn't sound good, since you are only seeing the outside. As for the leaks, they are costing u double, what leaks out has to come in and what comes in has to equal what goes out. Sounds like time to get an estimate.

Plus, a 35 yr old system most likely wasn't balanced as it should be today, so besides fixing the problems, you can improve the system .

Bud
 
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Old 08-12-09, 03:11 PM
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Metal is always better, but you can fix the duct board by re-taping the seams and coating the inside. Coating will put a rubber like paint on the inside. The benefit to doing this is it is a lot cheaper plus the coating has a mold inhibitor in it. Flex 35 years old replace!
 
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Old 08-14-09, 06:41 AM
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Thanks for the feedback! That'll help when I call a contractor for help.
 
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Old 08-15-09, 09:09 AM
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Are you equating the length of time it takes to drop the temperature to bad ductwork?
It could be that there are some ductwork problems but the length of time it takes to drop temperature is not necessarily a good gauge of problems.
It would be expected that if your a/c was properly sized the pull down time could be normal.
For maximum efficiency, especially in an area that has high humidity longer run time is a good thing.

When I install residential a/c I am careful to explain that when the a/c is used that it is not meant to have the unit turned on when the owners get home from work to a scorching hot house.
Could this be the case with your parent's a/c?

If you expect hot weather turn on the cooling in the morning and set it for whatever their comfort level is.
It will come on when the cooling load is lower and not be struggling with a large amount of heat.

Could be there is a problem but might be worth checking.
 
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Old 08-15-09, 12:09 PM
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That's an interesting analysis, Greg. My mom has been opening the windows to get some fresh air in the house in the mornings, so she's clearly not allowing the system to maintain a constant temperature/humidity.

I has been very humid, so maybe the system cooling slowly is a good thing.

We're having a tech come look at the duct work Monday. It'll be interesting to see what he thinks.
 
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Old 08-15-09, 01:55 PM
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No, it is cooling slowly because it has to work twice as hard!

The problem in humid climates is that it could take as much as 50% of the a/c's capacity to remove moisture.
It is always nice to have a window open but it is contrary to having an air conditioned space.

Not sure if they are concerned about power bills but there is a high price to be paid the way they are doing it.
 
 

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