R-value in duct

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Old 02-27-05, 03:57 PM
Tman
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R-value in duct

Would it make sense to increase the R value of my ductwork? My HVAC unit is in the attic and the ducts are flex type with R-6 insulation. I live in OK to give an idea of the temps. The longest duct runs are 80'.

On a cold day I measured the air temp at the closest and farthest register and were 105 deg F(15' run) and 95 deg F(80' run) respectively. Where possible I bury the ducts in blown-in insulation to help, but 75% of the longest runs don't run on top of the blown-in insulation.

I have sealed all ducts so air leaks are not a problem.

If the duct R value should be increased then what product/method would be best?
 
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Old 02-27-05, 05:46 PM
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80' of flex? WOW

The easiest way to increase the insulation on the ducts would be to replace the R-6 flex with that having a higher R-value. I know R-8 is available. Don't know if anything better is out there or not.
It just blows my mind (what little I have left) to hear of an 80' flex run. Around here, we are allowed a max of 14'.
 
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Old 02-28-05, 06:46 AM
Tman
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I'm not sure adding R-2 difference would be worth the cost and effort. I would sooner install a larger dia R-6 over the existing, but even then would it be much of a difference? Is a 10 deg difference between the closest register and farthest within "tolerance"?
 
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Old 02-28-05, 10:51 AM
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Wink

Boy 80' is a long way for it . Any L's are turns like in it thats 5 more ft of drag for each. How about a bigger pipe to it. Or why isnt the furnace more in the center of the home??
I have sealed all ducts so air leaks are not a problem.
The heat guy should have did that . With 2 bands on the flex pipe and mastic all around it.

ED
 
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Old 02-28-05, 12:18 PM
Tman
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The furnace is towards one side of the home and is really as close as it can be to the center. It might be closer to 70'. The attic is partially finished so has some obsticles. Originally the "heat guy" had 1 12" duct feeding the entire side of the house(longest runs), then branching to 5 ea 8" to feed each register. That side of the house had reduced airflow so I ran a 10" in parallel which fed 2(farthest from the unit) ea 8" ducts and left the 12" with 3 - 8" ducts.

The "heat guy" not only didn't seal the ducts I found 2 instances with the splice smaller than the duct. The duct had split becuase of the severe neck and was drawing or blowing air into the attic.

Airflow is better now but not great. I'm curious about the heat loss. Any suggestions?
 
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Old 02-28-05, 02:29 PM
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well first with flex duct code here is pull the liner down and over the metal pipe take off band it then pull the insulation pipe down over this and band it then mastic.

If I have this right. A 12" pipe cant feed 5 8" pipes no way. just a rough way to look at pipe is

A 6 "=100 cfm , 2- 6 "=one 8 " pipe at 200cfm
now just 2 - 8" pipe =a 10" pipe at 400 cfm

So that 12 " pipe he had should only have 500 cfm from it are in it for heat.
left the 12" with 3 - 8" ducts.
12" pipe 500 cfm 3 -8" pipe need 600 cfm still short 100 cfm

kick it around ED
 
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