In My Case: Flex Duct Outperforms Steel Duct - HVAC - DIY Chatroom Home Improvement Forum

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Old 06-07-2013, 09:01 AM   #1
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In my case: flex duct outperforms steel duct

I recently reworked part of the radial design sheet metal ductwork in my unfinished basement to improve headroom and appearance.

I used metal fittings to connect flex duct to the plenum for runs to room registers. Being careful to support the flex and manage the bends in runs of about 20-25 feet.

Result: The air temp from the registers supplied by the new flex duct is about 52 degrees whereas the air temp at registers supplied by the remaining sheet metal ducts is 59 degrees. (the sheet metal had a lot of mastic and was wrapped in insulation but the insulation had deteriorated in some place due to age)

I was really shocked at this result based on all the information on the net saying sheet metal is superior to flex.


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Old 06-07-2013, 09:28 AM   #2
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Glad to hear that you were able to make it work!!!
bottom line is that the product is not too bad, if installed correctly. sounds like you installed it very well. One of the drawbacks is that I do think cleaning, later down the road if necessary will be hard. The temp check you did is really dependant on a lot of other variables, but if you are happy and the system is running correctly, good job!!!!

I would really be interested if you had before and after static pressure numbers, as the increase in temps could be caused by a higher static pressures, slowing the air down over the evap, giving the air more time to release its heat and pickup cooling. but thats just a guess.

did you increase the size of the flex duct by one size as you replaced the steel ducts??? I believe that most mfgrs suggest that you go up one size when using flex. Example. 6 inch steel = 7-8 inch flex.


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Old 06-07-2013, 11:01 AM   #3
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I agree with your comment on cleaning. There was a good amount of dust in the ducts (especially returns) after 40 years. I think it would be easier to replace the flex at some future point than try to clean them. I think having a media filter is also a good idea to reduce dust in the flex.

I based the size of the flex on what I thought the room needed instead of just upsizing from the existing sheet metal duct size.

I think static pressure has increased some because there is a bit more air noise coming out of the returns.

There were also some significant gaps in the plenum sheet metal from when a new AC/heater unit was installed. I patched those holes, but still amazed the flex duct is delivers such colder air than the sheet metal ducts that are still in place. I may get around to replacing those in the fall. Again, my primary driver was redesigning the layout so I can finish the basement.
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Old 06-07-2013, 11:16 AM   #4
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sheet metal is superior to flex is a generic statement and is related to the resistance to air flow. flex will have more resistance and slow the air down resulting in a colder temp. you got lucky as 50-52 is a great supply temp and 55 is more the normal and 59 means you have way too much flow. what we want to check is the temp after it leaves the A coil. should be between 50-55. after that it all does it's own thing depending on type of duct/length of run etc.

actually you got REAL lucky as where you are you must have high humidity?? and if you can run 52 deg it probably is 50 off the coil which means you are getting maximum dehumidification.
"Cut it twice and it is still too short".

Last edited by yuri; 06-07-2013 at 11:20 AM.
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