Return air

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  #1  
Old 07-03-05, 06:20 AM
chris4me
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Return air

I am working on a old two storie house that I have.Upstairs ac units and duct work is in the attic and everything ok.Downstairs unit is in the basement and duct work and return air run under the house.Something has torn up almost all of the return air duct work,and the unit is pulling air from out in the open instead from the house.I cannot get under the house to rebuild as big of a box duct that was there for the return air,and am thinking of using round flex duct,but I do not know how big the return air duct has to be.is there a size requirement for return air ducts?Can someone help?
 
  #2  
Old 07-03-05, 07:26 AM
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Return Duct

The size of the flex is going to depend on the size of the unit & the length of the run. If you can post a make & model of the outdoor unit & the length of the duct you need, we can tell you what size you would need.
 
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Old 07-03-05, 02:28 PM
chris4me
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make and modle

The make of the unit is a Lennox forced air furnance.Model:G1205-137-3.
The length of the run is 14 feet.
 
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Old 07-03-05, 07:33 PM
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Model #

Sorry, I guess I was not clear on which model # I wanted. I need the make & model of the outdoor A/C unit.
 
  #5  
Old 07-04-05, 11:48 AM
chris4me
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outside unit

The outside unit is a Lennox condension unit: Modle: HS16-651U-3P,Serial no: 5185607137.
 
  #6  
Old 07-04-05, 01:09 PM
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Return Air

The outdoor unit is 5 tons, at least as far as I can determine from Lennox's web site. For 5 tons, you need to move 2000 cfm of air. For that much air, you would need 20" flex & I don't think they make it that large. You could use 18" metal, 2-16" flex (if you can find it), or 2-20x8" rectangular metal ducts.
Of the choices, the 20x8 is going to be the easiest to find.
 
  #7  
Old 07-04-05, 03:12 PM
chris4me
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thanks

Thank you for the info,I now know what I need to do to repair the return air.Thanks again,this is a good forum.
 
  #8  
Old 07-04-05, 03:37 PM
chris4me
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replacing duct

Grady,I just checked I have room to put in one 10" flex or metal duct,can I put a return air vent in the basement next to the unit,or does it have to be inside to pull out hot air?
 
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Old 07-04-05, 08:30 PM
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Return Space

You indeed have a problem. It really needs to pull air from the conditioned area. Even a 10x10 will only give you 500-600 cfm.
I just re-read your original post & realized there are two systems. One for upstairs & this 5 ton for downstairs. To have 5 tons of cooling for one floor is a lot. This place must be really big, poorly insulated, &/or in a very hot climate. Something I've had to do a couple of times is dig a trench to get my ductwork in. If this is an option, you want to line the trench with something waterproof. Keep me updated on your progress.
 
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Old 07-05-05, 03:53 PM
chris4me
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duct size

if I can get more inside I'll put in what I can.The old duct was about a 20"square box but it probably had beern torn up for several years from the way it looked,and the unit was pulling return air from the basement. What will happen if I cannot get the right amount of return air,say if I had to stay with the one or two 10" flex ducts,will it hurt the unit,or will it cut down of the flow of conditioned air?
 
  #11  
Old 07-05-05, 05:27 PM
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Lack of return air

It will indeed hurt the system. Lack of air flow is a major cause of indoor coils freezing. If gone unchecked this can lead to the compressor getting slugged with liquid refrigerant & cause severe compressor damage.
 
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Old 07-06-05, 03:24 AM
chris4me
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duct

If I can I'll Put in two 10" flex ducts,but unless I go anouther route that is about all I can put in.That and if I put anouther return air location inside the house.Can you have two return air locations?
 
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Old 07-06-05, 08:43 PM
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Multiple returns

Yes you can have more than one return, in fact it is preferable. 2-10" flex ducts is going to give you about 600 cfm total. You still need another 1400. A single 16" round metal duct would do the trick for the second return.
 
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Old 07-07-05, 03:49 AM
chris4me
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new duct

I'm going to put in anouther return air duct and I'll make it as large as I can.
 
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Old 07-07-05, 07:00 PM
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Originally Posted by chris4me
I'm going to put in anouther return air duct and I'll make it as large as I can.

"as large as i can" isn't going to be good enough. this is not really debateable. you need 2000 cfm or you will need a new a/c compressor. period. if you can't make this happen yourself you'd better call a pro.
 
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Old 07-08-05, 03:40 AM
chris4me
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duct size

I will put in more than one return air duct to get what I need.
 
  #17  
Old 07-08-05, 04:22 PM
chris4me
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thank you

Grady,thanks for all the help and the time that you took to guide me thru this thing.I have put in three 10" flex ducts and might even put in one more.Thanks again for your help.
 
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Old 07-08-05, 05:01 PM
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Originally Posted by chris4me
Grady,thanks for all the help and the time that you took to guide me thru this thing.I have put in three 10" flex ducts and might even put in one more.Thanks again for your help.
is this all of your return?



3 @ 10"o= about 900 cfm.

i hope you have more.

flex duct is bad enough, but in a crawl space it gets attacked by critters. could this be what happened to the original duct work? you might be replacing this again. put in some metal duct.
 
  #19  
Old 07-08-05, 05:40 PM
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Tinner is right about the amout of return, you are still WAY short. If 10" flex is what you are going to use, even if I do prefer metal, you are going to need six of them.
 
  #20  
Old 07-11-05, 03:52 AM
chris4me
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ok

Ok thanks for the help,I'll look in to putting in metal duct.Thanks to you guys for all the help.
 
 

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