Flex versus Rigid Ductwork ??????


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Old 12-28-03, 01:31 AM
chazmarcus
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Flex versus Rigid Ductwork ??????

hello everyone,

I'm doing a total gut rehab on a 9000 sf brownstone in Harlem. 7 units over 6.5 floors. Right now we're looking at Ruud rooftop condensers for their ability to do 65' vertical runs without oil traps? and Goodman gas furnaces in each unit. My question here is about duct work - it appears that using FLEX duct is cheaper, easier to install, and with a DIY video something worth trying. BUT is it shooting myself in the foot? what's the poop about flex duct being harder to clean, higher friction levels thus reduced air flow?? etc etc I'm sure all the pros and cons are true, but what really matters here? My floor plans are flat, long and thin - -each floor is 20 x 75 - - brownstones are built long and narrow - - but for one tiny bedroom in the back of the building, most of my runs will be 6 to 20 feet long. If flex duct is so easy and cheap to install is there anything to worry about here in my project????


thanks for your thoughts -I love the forum.
charlie marcus -harlem NYC
 
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Old 12-28-03, 05:40 AM
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The 65' run w/out oil trap is for the suction line for the A/C itself, I hope you are not installing this yourself??

As for the duct, stay with the ridge metal pipe. Yes, the flex are cheap and easy.. But you get what you pay for. I've pulled one too many flex jobs out and replace it with metal to get the proper air flow.

If you want the good air flow, you will need to use larger than normal size flex vs sheet metal duct. so I am sure you don't want to take anymore space than you need to for your apt units.

A short run with flex is OK, but not for long. They got those "bumps" inside from the coiled wire to supprt the flex. They act like bumpers.. when air gets bumped around alot, it don't flow very well.

Kinda like driving your car down the bumpy road vs a new smooth road.. What road do you go down on the fastest?

Is the Roof top unit cooling all of the apts in the building? or One condensor unit per apt?

Make sure your apt had it's heat load/lost cal done, and duct sized for total comfort. :-)
 
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Old 12-28-03, 08:10 AM
chazmarcus
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flex versus rigid duct work

Jay,

thanks for you suggestions - all good thoughts - we're having one rooftop condenser for each unit - and believe (hope/trust) the ruud machines will handle the longest vertical run of 65'(as promised bytheir tech staff) -we're planning to install whatever we can safely and cheaply and have the pros come in for the important stuff to make sure the system is absolutely correct -

I understand what you said about the bumpy roads, but you also mentioned if we do go with flex, we should size it larger than the rigid to allow for the bump factor - -if we upsize appropriately, do you think flex is okay, if not ideal, or still a very poor choice? you said flex is okay for short runs, but not long - -how many feet is a short run in your book? the longest run we have is 35 feet and it is to a small back bedroom with 100 cfm - -because we don't want to drop a very large living room ceiling just to accommodate a small bedroom we are planning to run 4 or 6 inch flex through the joists and not drop the ceiling at all . . . thus a long run with flex . . . is this ridiculous and we should jam the rigid in, or not such a big deal because it's only 100 cfm????

and just to have it hammered into my head . . . I understand good versus bad air flow, but what's the downside to bad air flow? does the machine burn out immediately? after 10 years versus 20? if you upsize the flex duct don't you immediately compensate for this? sorry for being annoying. and lastly, can you clean flex duct - if not, is it such a big deal???

thanks
charlie

ps - - any other opinions or thoughts are greatly appreciated -even if you agree with jay, but especially if you don't !!!!! I post on other forums and sometimes once one reply is given, others don't chime in. as I"m spending near a million clams on this total rehab I want to get all the annoying questions out of the way during the design phase so I can spend my construction phase crying and pulling out my hair!!!
 
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Old 12-28-03, 10:56 AM
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Lightbulb Duct work

A 65' lift for an AC condenser on the suction line and no trap. Boy Id have it put down in black and white for sure . That it will last 5 years. Id add oil traps to them at each floor, but maybe the top unit there a trap right at the coil. On units that high up the head will run lower all the time.

Dont know the layout you have but with the duct work all inside Id for sure use metal duct and 6" metal pipe up in the joists for all runs. Forget this flex pipe and do the job right. It will pay you back in the long run in fuel cost you will save.

My .02 cents ED
 
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Old 12-28-03, 01:16 PM
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I'm guessing these will be rental units? If they are and your not mearly trying to "flip"the brownstone then go with hard pipe for better airflow otherwise you'll be getting "cold calls" from your tenents at 2:00 am
 
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Old 12-28-03, 06:54 PM
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IF you were to go flex, you may have to run 8" flex, and overall that will make it about 10"..

Just stay with the good old fashinon metal pipe to get it done right.

With a poor set up, the flex will shorten the life of the HVAC system.. Over heating the heat exchagner too much, fan motor burns out.. poor heating, and cooling results..
 
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Old 12-28-03, 08:35 PM
chazmarcus
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flex versus rigid duct work?

okay okay, I get the message - -hard pipe it is - !!!!! but about the 65' run - the condenser will be roof top - above the handler - - and the Ruud specs and the Ruud technicians say with a thermal expansion valve a 65' run without an oil trap is a piece of cake. THOUGHTS??? rheem/ruud (raka or uaka series - ) THAT SAID . . . is an oil trap on every floor (every 12 feet) such a big deal??? it's just a little "p" trap thing right? could it hurt to put it in, EVEN THOUGH the manufacturer assures I don't need it? Does it make sense that a TXV and manuf. specs should be reliable????

when I done framing y'all can come see my rigid duct work and pat yourselves on the back. I do plan to flip a couple units right away, but the others will stay in my portfolio for a while - no need to be cheap right off the bat.


thanks so much - - now solve my 65' vertical run questions !!!!!


charlie
 
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Old 12-29-03, 11:14 AM
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Lightbulb A/C

Like I said will they put it down on paper that the oil will get back up to the compressor in the 65'??????????????? I dont see how it can hurt to put them in. All the trap should do is help get the oil back up to the units on the roof.

You said 12 ceilings. We would set the unit on a box 2' or 3' high. line it with duct board and cut the return grills into this [if you can]. sure cuts down on the blower noise this way.

ED
 
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Old 12-29-03, 05:36 PM
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condensers 10 feet or more above the evap coil must have traps. even if the contractor disagrees, what does it hurt to install it? 2 more sweat joints and a premade trap. increases oil return, and will prolong compressor life. i would put just one in, i have never put in more than that on one unit, there may be benefits to every 12 ft, i do not personally know. ruud/rheem are the same, i like them.
 
 

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