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Old 06-16-2009, 09:16 AM   #1
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Trying to troubleshoot low airflow to kitchen


Hey guys, great forum

I've been battling low airflow to the kitchen for the past 18 months since we moved in (3050 sq ft 2 story). I'll have to get the blower details later, but it's a Carrier installed about 8 years ago. It's a 2 story, and the kitchen run is the longest of the bunch (I'll have to get pictures later) with 8-inch flexible from the blower. I'd estimate that the horizontal run in the attic is about 45 feet, which then wyes into 2 6-in ducts that drop down 10 feet into some kind of sub floor register box, then on down to the kitchen. I had a guy out yesterday who recommended replacing a large section of the 8-inch with 12-inch from the blower, then using a 12-8 reducer about 25 feet downstream. Basically saying it's way undersized for feeding such a long run. Is there a general rule of thumb stating where your step-downs should be in relation to X feet away from the blower, depending on the CFM? I did add an inline booster fan to the 8-inch before the wye and that helped a tad bit. He also recommended cutting off some excess of the 6-in ducts to remove the "scrunching" and rounding out any curves. Also to enlarge the access hole to the subfloor registers and check for any leaks. Luckily these are accessible because part of the attic is a walk-in room with these ducts in plain view.

The outside unit is a 20 year old 4-ton (upstairs 3-ton) and the rest of the house still stays cool. Last year in the Houston summer our bill was in the low 400s, however we were paying about 15c / kwh (10.8 c this year ), so really that's not bad considering how blasted hot it gets here. Sorry for the long post, thanks for any advice.


Last edited by texsn95; 06-16-2009 at 09:18 AM.
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Old 06-16-2009, 10:52 AM   #2
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Trying to troubleshoot low airflow to kitchen


Probably should run a 10 or 12" the whole way to the 6" flex lines.
Increasing the exposed/visable part of the 6" to 7" would also help a lot.

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Old 06-16-2009, 10:53 AM   #3
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Trying to troubleshoot low airflow to kitchen


I would say the main problem is the amount of flex duct. Flex duct really shouldn't be longer than 15 ft. due to the reduction to airflow. I would recommend replacing the flex with metal insulated with sleeves. Also recommend up sizing the duct to the wye if the trunk line will support it.
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Old 06-16-2009, 11:10 AM   #4
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Trying to troubleshoot low airflow to kitchen


In reality.
Nothing wrong with a 100' flex duct run. It has to be sized for its TEL though.

There are many areas that run flex trunk lines. Air flow is good. As long as they sized it right.

From what the OP describes. The sizes ran, are too small even if its ran in hard pipe.

He has a TEL on that supply run of over 200', not including the length of his.
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Old 06-16-2009, 01:40 PM   #5
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Trying to troubleshoot low airflow to kitchen


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Originally Posted by beenthere View Post
Probably should run a 10 or 12" the whole way to the 6" flex lines.
Increasing the exposed/visable part of the 6" to 7" would also help a lot.
Do they make wye reducers that go from 10-12 down to 6? And what exactly do you mean by your second sentence?

So it sounds like he was correct in what he said yesterday, sometimes you just never know if what you hear is BS or not.

In regards to adding the larger duct itself to the blower, is that best left to a professional? I've never pulled a duct off from that end so I'm unsure what's all involved. The rest of the house cools pretty well, so I just want to make sure that by adding a long run of larger duct, I don't majorly compromise airflow to any of the other rooms.

Are there any good sites out there where you can input data such as blower cfm, duct lengths, diameters, # of trunks, etc to try and correctly size a sytem? Like an interactive calculator or something. I'm an engineer myself but don't have much experience in flow assurance (air or fluid) so this is interesting to learn about.

Thanks guys
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Old 06-16-2009, 02:35 PM   #6
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Trying to troubleshoot low airflow to kitchen


Wyes are made in most common sizes. May not be normally stocked in your area.

The 6" ducts, if you can reach/access them, shold be increased to further reduce restriction.

No such site that I know of.

All other supplies will be effected. How much depends on how restrictive they are or aren't.

May or may not also have to add a return. To make up for the increased air flow to the kitchen.
The blower can't deliver anymore air then it can draw in.
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Old 06-16-2009, 02:59 PM   #7
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Trying to troubleshoot low airflow to kitchen


Thanks beenthere. What do you need to actually add it to the blower? A certain collar? Do you just widen the existing 8-in cut out and fit it into the blower?

Also yes the 10ft of 6-inch is accessible then it disappears into the subfloor box, so I could run a 12 to an 8 wye and replace the 6s with 8s. Then I'd need an 8 to 6 reducer at the box or else an 8-inch box i guess. Or maybe the 8-inch can make a good fit to a 6-inch box with enough tape and / or zip ties.
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Old 06-16-2009, 03:23 PM   #8
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You need the take off or start collar for the size duct yor running.

And then have to cut teh opening to fit that take off/start collar.

Can't crimp 8" flex to a 6" tab, and have a good seal.
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Old 06-16-2009, 03:44 PM   #9
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Trying to troubleshoot low airflow to kitchen


Quote:
Originally Posted by beenthere View Post
The blower can't deliver anymore air then it can draw in.
Do you suggest again to remove the air handler cover, defeat the interlock and then check airflow?
At least this isolates the problem between replacement air and supply air but it may not be called for here (?).
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Old 06-16-2009, 04:16 PM   #10
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Is this stuff any good?

http://bestbuyheatingandaircondition...lver_flex_duct
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Old 06-16-2009, 06:17 PM   #11
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Yes thats good stuff.
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Old 06-16-2009, 06:19 PM   #12
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Quote:
Originally Posted by Yoyizit View Post
Do you suggest again to remove the air handler cover, defeat the interlock and then check airflow?
At least this isolates the problem between replacement air and supply air but it may not be called for here (?).
From what he says. Its the logest runs not getting air.
So its a high static problem on the suppply side that needs attention first.
After that is corrected. His return will be able to draw more air.
If its not enough. Then he needs more return.


If he was having trouble wth shorter runs. Then removing the blower door would be a good test.

But he said they all have good air flow.
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Old 06-16-2009, 07:51 PM   #13
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Trying to troubleshoot low airflow to kitchen


Quote:
Originally Posted by beenthere View Post
From what he says. Its the logest runs not getting air.
So its a high static problem on the suppply side that needs attention first.
After that is corrected. His return will be able to draw more air.
If its not enough. Then he needs more return.


If he was having trouble wth shorter runs. Then removing the blower door would be a good test.

But he said they all have good air flow.
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Old 06-17-2009, 09:04 AM   #14
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Trying to troubleshoot low airflow to kitchen


What's the cheapest place to buy this online? Or is it just as cheap to go through a local supply shop? Also I can't seem to find a 50 foot section of 12-inch, just 25 feet, is that the longest I can get?
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Old 06-17-2009, 09:32 AM   #15
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Trying to troubleshoot low airflow to kitchen


25' is the standard. You won't find it in longer lengths.
A hard pipe of the same size is used as a coupling for longer lengths when needed.

Usually, a supply or 2 are within 25' of the unit. Then wyes serve as the coupling.

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