In HVAC, how many lines can you run off of one main line?

Question

I ran 3 6" lines of duct from a 14" line to 3 different rooms and the rooms are not getting cooled

Answer 1

I am going to give you a mini-crash course, given that your question is more complex that you might think.

You have about 400 cubic feet per minute of air movement from each ton of air. A three-ton unit, for example, has about 1200 c.f.m's of air. Your 14" line can handle about 1000 c.f.m's depending on the length of the run. Three 6' lines, drawing less than 200 c.f.m's apiece, certainly haven't drawn down all of the capacity of the 14" duct, so we have to start looking for other answers.

1. How are the lines run? Is there a triangle, or a tapered junction box that transfers the air neatly to the 6" runs?

2. How is the 14" duct tied into the air handler? Is this trunk line serving other lines as well? Is this the only line off the plenum (ductwork at the top of the air handler)

3. What size is your unit? Is it large enough to service the living space? (you need abbout 1 ton of air per 500 sq. ft.)

4. What is the condition of your equipment? Is there sufficiant refrigerant? Is the evaporator coil clean and unobstructed? Is the compressor functioning? Is the condenser coil (outside) deteriorated? Is the filter clean? (washable filters are a big problem)

5. What is the condition of your insulation? Do you have r-30 in your attic?

6. 6" ducts will typically only do a small room. What sizes are you trying to cool?

7. Are there any tears, seperations, or major leaks in your supply ducts? Is your return air chamber tightly sealed?

8. What is the orientation of the three rooms in question? Do they have a lot of glass? Do they get a lot of sun?

If you can get me some answers to these questions, I'll help you get your rooms cool..

Answer 2

If you do not taper the size of the main line as it runs you will loose velocity. The air will actually slow down.

Go to your local Wal-Mart Super Center tonight and look at the exposed duct work in the ceiling. It tapers down every so many feet to keep the air velocity constant.

mike

Answer 3

lots of things to consider, u dont want all 3 ducts on the same side of the main......u dont want them within 2 ft of each other.......u dont want to be any closer than 2 ft from end of main......ur unit can only push out so many cfm's, u might have exceeded this with 2 many openings( ducts to vents) tell me how many ton ur unit is and i can ell u how many ducts u can run off of it......just had a 2 1/2 ton put in we could get 13 6" runs out of it!


lic. gen. contractor

Answer 4

I used to design hvac systems for residential housing. Pay attention to AIR BOY HOSS he is right on.

Answer 5

There are just some things the home owner shouldn't try themselves, this is one of them. This is a comfort product.

You would not go buy a Yugo and try to fix it up yourself to make a Lincoln out of it , would you?

Most designers don't think "hey this guy might add 3 rooms to his home next year, I better oversize the system to be there for him."

Answer 6

you should run two of your takeoffs then you reduce the trunk line by 2 inches and then run your 3rd takeoff .so at the end the trunk line will be 12 inch.you can buy reducers you would need a 14 to 12 transition and a 12inch end cap and of course a piece of 12inch duct

Answer 7

you need to get rid of 400cfm per ton of cooling. I believe a 6" line will carry 150 cfm. if it is an additional room that you have added then your unit is probably undersixed. use 600sqft per ton to figure.

Answer 8

(1) 6" duct is only big enough for a small bathroom. What size rooms and what size unit are you working with.

Answer 9

This is a design problem. But I think the required air (cfm) and cooling required (heat load) is not attained. It is best to go back to design.

Answer 10

you don't say how big the unit is sounds like you have maxed out this unit