How many 5 inch ducts for a 16 inch trunk line?

[johneic]

I put a packaged HVAC system in my barn. It has 16 inch supply and return trunks/tubes from the manufacturer. I have routed them into the barn and it heats and cools the main area fine.

I have bought round metal ducts in sizes 16, 14, 12, 10 and 9 inches. This will look like what you see for HVAC in warehouse/big box stores, run suspended above, for their HVAC. This will run 40 feet inside of my 50 foot long barn. Simply to distribute the air front to back better.

I have decided to use 5 inch takeoffs all along the length of this decreasing size tube. It will just be the 5 inch takeoffs so there is no duct length loss (friction rate loss) to consider. I may put a grill cover on the takeoff later.

I have read that if I am putting lengths of duct on the main branch these CFMs apply, 50 CFM for a 5 inch, 85 CFM for a 6", 125 CFM for 7" and 190 CFM for 8" but these would not apply to me as I am not putting in duct and will not have the associated friction loss. But if they do apply, I would need 24 five inch take offs to handle 1,200 CFM?

I find the online duct calculators too complicated for my simple set up.

So, how many 5 inch take offs are needed to handle the air provided by a 16 inch, 1,200 CFM main trunk?

I assume the 16 inch main truck is sized correctly (CFM) as the manufacturer supplied 16 inch round holes on the side of the gas pack.

 

[johneic]

These are what I am looking at, the 5 inch or the 7 inch, both with dampers

Speedi-Products SM-SDL9D 5 5-Inch Diameter Round Duct 9-Degree Saddle Take Off with Gasket and Volume Damper - Vents - Amazon.com

Speedi-Products SM-SDL9D 7 7-Inch Diameter Round Duct 9-Degree Saddle Take Off with Gasket and Volume Damper - Vents - Amazon.com

 

[Birdhunter1]

If it were me I would go with 7 or 8" take offs with a damper in them and choke them down as needed to get the airflow where you want it. You can always choke airflow out of a bigger takeoff while you cannot increase air through a smaller takeoff.

 

[johneic]

I agree, I can buy fewer 7 inchers than 5 inchers too.

 

[dadster4]

It depends on how far of a "throw" you want...what is the distance from the main trunkline to about five feet off the floor and ten feet from the outside walls?
12- 5" saddles will handle 1200 cfm and throw the air some 15' or so, and be a little noisy...12- 6" saddles will throw the air a shorter distance and be much quieter.
A 5" saddle will put out about 100 cfm when just screwed to the main trunk (with 12 installed), a 6" will provide about 150 cfm with 8 installed. I would stick with 6" if more duct or a register may be added in the future, it's a common size, where 5" is a little odd. The higher the main trunkline is...the more they should be angled down, 45 degrees or so if it's 12' high.

 

[johneic]

Thanks Dadster!, I think I am going to go with five 8 inch, that is one takeout for every reduction in size, each 2" reduction in main trunk size restricts airflow by 200 cfm and and 8" is 200 CFM (If I am correct in what I read on the internet, anyone please double check me). My ceiling is 8.5 feet high so these will come out about 45 degrees (I guess?) at about seven feet off the ground. The building is 22 feet wide. The main truck is offset to one side, about 24 inches from the wall. The line will start seven feet from one end, I can make it any length to the other end, thinking about making it end seven feet from the other end. Insulated overhead garage doors at each end (R3?). I spend most of my time in the middle of the building so that is where I am pointing most of the output to (Or should I point the end ones towards the overhead doors? to refill those areas with HVAC air if the doors are opened? and compensate for the lack of R value in the doors?)

I would like to beef up the R value of the doors but I have not yet found a good way to add insulation to an insulated door. A blanket would catch/hold all of the dripping rainwater that comes from the hinge space between the panels in the door when opened. (This is the subject of another thread though.)

I am also making a 4 inch tap for a very small office (6x9) and may make another tap to point towards the back door.

 

[dadster4]

I think your plan should work fine, don't bother blowing at the doors, you will have plenty of air movement to mix the air once you close the door. Keep your 8" saddles about a foot from the reducers as you go down the line, and the last saddle 18" from the endcap at the far end. With the sizing of your duct you planned your system should work out great...better than some homes I've seen! And I've seen plenty in 30 years HVAC. :thumbsup:

 

[johneic]

Big thanks Dadster4 on the location of the saddles, I had no idea where to put them. Here is the start of my job, I only ordered the saddles after we finished talking about them here so I will not have them until next week. I have time to do anything different, so anybody add any ideas please. Here are four pics, first is before, 2nd and 3rd are the first three foot 16" section hung by chain using sheet metal screws, and the fourth is my shop from the view of the duct, a lot of work went into this place, I am almost done with this job, so happy!

 

[johneic]

I plan to tape the plastic duct to the first section but do not plan to tape the rest as a little air leak would not hurt, I want to keep it neat and clean looking. I could use silver tape if needed.

 

[kenmac]

Just an FYI. I have no idea how much flex you are using, but just know it will cut your CFM's down a bit . you shouldn't have any problems though. I have a 18'' round metal with about 6' of flex.I have many 5'' take offs. I get plenty of air flow.