Would a bigger takeoff increase airflow?

[fat bob]

Originally posted by dash
Ducted return large enough,or a good retuen path??

I'm okay there. Rarely close the door.

[core]

FatBob

That sounds like your problem.

1. To much flex.
2. If designed at (.1"WC friction) then 9" run = +(-) 320 CFM. 6" run = +(-) 110 CFM.

110 + 110= 220 CFM
320 - 220= 100 CFM (that may be availible in that duct)

Increasing the 9" to 10" would be a huge waste of time and money.

What does the 2 6" runs feed??

[fat bob]

Originally posted by core
FatBob

That sounds like your problem.

1. To much flex.
2. If designed at (.1"WC friction) then 9" run = +(-) 320 CFM. 6" run = +(-) 110 CFM.

110 + 110= 220 CFM
320 - 220= 100 CFM (that may be availible in that duct)

Increasing the 9" to 10" would be a huge waste of time and money.

What does the 2 6" runs feed??

The 6" go to two 6x8 one-way registers. The registers were the 3-way stamped kind but I replaced them which helped a little. I don't think the registers are the problem. I also think the installer was pretty careful to seal the ducts and avoid unnecessary bends and kinks. I just need to get more air into the 9" trunk.

[core]

Originally posted by fat bob
The 6" go to two 6x8 one-way registers. The registers were the 3-way stamped kind but I replaced them which helped a little. I don't think the registers are the problem. I also think the installer was pretty careful to seal the ducts and avoid unnecessary bends and kinks. I just need to get more air into the 9" trunk.

Why would you think the installer was careful???? To me 20 to 30 feet of flex duct is careless! To much friction loss in flex duct.
To maximize these 6" ducts you may have to remove the flex and run insulated hard pipe.

How big are the rooms that are feed by the 6" ducts?

[dash]

Originally posted by dash
1.Move it to the top of the pleunum,add a damper if it's too much air flow.



2. http://www.fantech.net has booster fans that work and with static readings.current and desired cfms ,will size the fan for you.

[Edited by dash on 08-02-2005 at 03:29 PM]

If you can repalace the tri-box with a wye,that would help,if is all unaccessable ,number 1 or 2 above will solve the problem.


The TEL of a Tri-box is high enough that ,just chaging the takeoff,will not make enough difference,I had previously assumed this was one 8/9" run to a grille.

[fat bob]

Thanks Dash et al. for your suggestions.

[tinknocker service tech]

since there is no pratical way to replace the intire duct it would seem to make sence to move it to the top of the pleun. make it a 12 in take off anr run the flex as clos to where it goes under the floor. it will start at about 550 to 600 cfm and restrict down to hopefuly 400 at the arrow head. this should increase the stadic pressure of the flex. then install another outlet in your bedroom if you can. this will give you more air flo the trying to get though two six in runs. you will now have three six in run and about 375 cfm give or take a little. the question here that is important is how many cfms will the system deliver
what size is the airhandler. model # of unit will help because if it is not big enough what ever you do will make no differance at all

[fat bob]

Originally posted by tinknocker service tech
since there is no pratical way to replace the intire duct it would seem to make sence to move it to the top of the pleun. make it a 12 in take off anr run the flex as clos to where it goes under the floor. it will start at about 550 to 600 cfm and restrict down to hopefuly 400 at the arrow head. this should increase the stadic pressure of the flex. then install another outlet in your bedroom if you can. this will give you more air flo the trying to get though two six in runs. you will now have three six in run and about 375 cfm give or take a little. the question here that is important is how many cfms will the system deliver
what size is the airhandler. model # of unit will help because if it is not big enough what ever you do will make no differance at all

I've got a Carrier 4 ton variable speed air handler that can get me slightly over 1600 cfm according to the manual. Set at 350 cfm/ton it gives excellent cooling except to the master bedroom.

[udarrell]

Originally posted by dash

Originally posted by udarrell
Use the 10" takoff and Run the 10" duct out as far as you can then go the reducer to 8 inches. That "might" (too many unknowns) get it closer to 250-cfm but will increase the velocity through the boot and diffuser.

http://www.udarrell.com/proper_cfm_b...g_systems.html

The friction rate quoted in your site is not applicable to all duct systems,many need to be designed at .06 or .08,as opposed to .1 or .09.A manual D design will give you the correct FR for every job.

You are right, I should have emphasized that they use the manual D, and will make a note of that on that page. Actually, I should have made a note that the chart sizes actually vary from .06 to .1, not very professional, huh.

It was meant to be a rough guide for equipment owner's to do a ball park check on the sizing of their duct systems.

The pros ought to know to use the manual D for ductwork.

When larger residential A/C equipment is installed many times the duct system ends up being undersized. Thank you "dash" for jacking me up, I should have explained the variation parameters involved in the rough chart.

[markwolf]

There is a little more than just size to consider.Is it an adjustable takeoff or straight?Is the take off leaving the plenum in the same direction as air flow?Is there a scoop take off directly below the take off?Is it placed to close to a blank end of an a-coil?You can put in a twenty inch take off & see little improvement if it is not installed properly.