I have been reading these forums for a while now. If you guys dont mine I now I have a few questions. First let me tell you what I have and what I have had done.

10 year old house, So Cal. 10 seer unit 3 ton unit. Aprox. 1450 sq. ft.
Old flex duct was gray with R 4.2 rating in attic. Return grill 20"x20" with a 14" flex return Supply.

Just had all ducting replaced with R8 silver jacket flex. New bar style supply registers. I like these, more air-flow. I also had a bar style return grill installed. Less restriction grill. Went from a 14" flex to 16" flex return.

I have (4) 8" supply and (4) 6" supply to the house.
According to information I'm reading on this site I shoud of gone to an 18" return. If i'm doing this right I have supply air at 1200 cfm? My 16" return is ony 900 cfm? is this correct.

Thanks for advice in advance!

Any help would be appreciated.

 

You have it right for sure A 3 ton can get by with a 16" metal BUT a 18" flexpipe. Also you did good going to the silver also. Out of the box here. A good AC duct work will have about 70"sq per ton going in and out of the unit.

ED

 

Thanks Ed for the response.

Another question: If I increase the 16" to an 18" return, will I have to change my 20"x20" return grill?

Also is 900 cfm correct for 16" flex?

Thanks!!

 

20 X 20 grill good for a 18" flex return

900 CFm good for a 16" metal pipe

 

I'm glad to hear that the 20"x20" grill will work for the 18" flex.


The old 14" flex return was about 10' long run. The new 16" flex return is about 40' long run with (3) 90 degree bends. I was told this would make it quieter, which it did.

I also have been told that increasing the return size from 16" to 18" flex will make the blower motor work harder which in return uses more power (current). Is this true?

I had about 7 companies come out and give me estimates, and they all have different stories!

Again thanks for the help.

 

I'd say the blower will work less hard, because you're decreasing the flow resistance of the return duct.
(my 2 cents)

 

16" to 18" flex will make the blower motor work harder which in return uses more power (current). Is this true?
Just think about it. With the bigger pipe there now the blower dont have to work as hard. So it draws less amps so it dont cost as much to run.

Could you work the 3 90o say more like 45o in the turns .Or big wide 90o that helps also.

ED

 

Yes, I think I can do 3 45 degrees. Does longer runs like the 40' feet for the return hurt?

 

Bends, lenght and diameter all work together to determine the amount of drag. More bends and longer runs add drag.
How did your return line go from 10' to 40'?

 

Installers added the long run to make it quiet. It was never noisy to start with. It is super quiet now. Will the 40 foot hurt?

If I change the 16" to 18" flex return should I make it shorter run? I will try to make 45 bends instead of 90 bends.

What improvements will I get if I change the return to 18" flex from 16" flex?

 

Like said if you can cut in down in length do it . yes the 45o helps on blower noise.
A 3 ton at 1200cfm calls for a 18" duct in flex 16 " in metal. If and when you put in a new blower unit for sure get a V/S blower . You cant hear or know when they come on.

ED

 

I don't think flex duct is intended to be used for such long runs. Perhaps I'm thinking of something different, but isn't flex duct flexible? So why have elbows at all, if you can gradually turn the corner. Flex duct also has a greater resistance to airflow over ridgid duct, as it's not as smooth on the inside. But you compensated for that by going to a larger diameter.
I can't come up with any actual numbers on any of this, but I'm just trying to use my common cents here.

 

You're right it is flexible so they are bends not elbows. It sounds like 40' is to long.

What are the benefits of 18" over 16" flex?

 

What are the benefits of 18" over 16" flex?
Cause thats what you need for 3 ton with flex pipe for the right cfm. Thats what the books call for.

ED

 

Thanks for the help guys!


Any thing else I should do or things I need to know before I complete this job?

 

actually, 18 flex is borderline depending on bend and lenghths. and a 20x20 is small especially if its a filter grille. and your total output cfm is only 960.

 

Iam thinking the same thing. 20x20 is to small for a 3 ton system. 20x25 would be a better choice.

 

schulze31,

Will a 18" flex work if I keep th 20"x20" filter grill?

Is this enough CFM for the output registers of your calc of 960 cfm?

16" flex = cfm?

18" flex = cfm?

Thanks

 

Can I calculate 16" and 18" CFM?

Do you mine me asking how did you get 960 cfm's?

Thanks again.

 

I went into the attic and bad news. I can't increase my return grill size, because the fire sprinklers are in the way! If I increase the return flex from 16" to 18" flex I will have to use 18" flex R6 not 18" flex R8 because of area around return in the attic. R8 insulation is very thick.
It would be very difficult to add anothe return because of all the fire sprinkler pipes.

So will it hurt to 18" flex with a 20"x20" return grill?

How to calculate 18" flex CFM and 16" CFM?

Thanks Again!

 

Have had and use all the time for one return on a 3 ton a 20X20 filter grill that 400sq" - 15% for the grill gives us 340sq". Even at 80sq per ton that Still over by a 100sq " for the filter.

ED

 

960 is your supply cfm. 8" runs are 160, and 6" are 80. and since you are not even up to 2.5 tons on supplly you can probably get by with 18" i think its max cfm is 1200. i dont have my filter calculater on me. but i am stil not convinced a 20x20 is satisfactory. i know that if there was a choice i would put in a 20x25. but again you aren't running 2.5 tons so you should be fine. but you said earlier that you think they run 40 feet of 16. most flex i have seen comes in 25' boxes. if possible i would try and stay with one box! your noise should still be fine, if its still to loud with as little air as your pushin you can probably get away with lowering your fan speed.

 

Thanks Again guys for all the help.

Ok! I was off on another project for a while. It's called work. I hate that.

I went back to the attic and looked at the connection of the 16" flex to the blower box. The 16" takes alot of the space on the blower box. I might get a 18" flex collar if I'm lucky, not sure it will fit.

Question: I was looking for a rectangular to a 18" round transition. I have looked everywhere with no luck. Does anyone have a clue where I can buy a rectangular to a 18" round transition?

 

Any sheet metal shop can make you one. Phone around there. If the box on the unit return is ductboard just make a bigger one so a 18" round fits it.

ED

 

It's just a sheet metal box with a blower and a door to get to blower. There is just a 16" hole for the flex collar. It's a vertical furnace in the attic.