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Douglas, HVAC Technician
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4 or 6 Ducts. Which is better I just had Central A/C

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4" or 6" Ducts. Which is better?

I just had Central A/C and Heat installed and most of the rooms connected to the end of the Trunk line barely push out any air. There are currently 5 Registers/supply's (Cat Room, Bathroom 1 & 2, Computer Room, Baby Room) connected to the end of the trunk line, all seem to be 6" ducts.

The lines in the beginning of the Trunk (my bedroom) pumps out an enormous amount of airflow, so much that I leave the vent closed.

my question is, When do i use 4" or 6" ducts? I believe the beginning of the Trunks have 6" and 7" and towards the end are all 6".

I need to increase the airflow at the end of the Trunk Line (the 5 registers). I got two different answers when i went to Home Depot and Lowes. One said I need 6" ducts at the end while the other says to use 4" ducts because the airflow is the weakest and I am just moving cold air.

Do I leave the 6" at the front and change all at the end to 4"? Will type more in a sec
Fortunately....or rather unfortunately (depending on how you look at it) the guys at home depot and lowes have no expertise in HVAC or they would be working for an HVAC contractor making twice the money. No worries, half the jack legs calling themselves HVAC contractors dont know airflow either. Around here all you need is a ballcap and a pickup truck to call yourself a contractor and get a liscence too.

The problem is that the duct was not properly graduated to maintain velocities. Whoever said 4 needs a kick in the shins. The other guy is partially right, but should have elaborated.

Duct systems need to be designed to maintain static pressure in the duct and keep the velocity in check. The air, as you can see will take the path of least resistance. By keeping the static up in the duct system (Static is the outward pressure that keeps a basketball pushed out all the way around). The static will force the air to where you want it to go.

Personally I would drag the installer back and make him do it right. He is supposed to sell solutions, not units. The unit cant do anything by itself.

I know this is not your home but look how the duct is reduced after a run is taken off (Blue Ducts). This is how it's supposed to be. Air is dumb.... you need to tell it where to go. Taking a closer look, the engineer specified how much air he wanted in each room and designed the duct to deliever just that amount. The number on the bottom of his notes show the CFM (amount of air), the number on top tells the contractor what diffuser or register to use and the size of the pipes are spelled out. (Again some schmuck with a ball cap thinks he knows this and in the end has no clue). Your system may be 2 stage which will only exagerate a bad duct system.


Customer: replied 6 years ago.
doh I finished revising my question and it didnt take lol Give me one sec, I am uploading a design and questions. Uno Momento por favor
ok shoot
Customer: replied 6 years ago.

4" or 6" Ducts. Which is better? I just had Central A/C and Heat installed and most of the rooms connected to the end of the Trunk line barely push out any air. There are currently 5 Registers/supply's (Cat Room, Bathroom 1 & 2, Computer Room, Baby Room) connected to the end of the trunk line, all seem to be 6" ducts. The lines in the beginning of the Trunk (my bedroom) pump out an enormous amount of airflow, so much that I leave the vent closed.


my question is, When do i use 4" or 6" ducts? I believe the beginning of the Trunks have 6" and 7" and towards the end are all 6". I need to increase the airflow at the end of the Trunk Line (the 5 registers).


I got two different answers when i went to Home Depot and Lowes. One said I need 6" ducts at the end while the other says to use 4" ducts because the airflow is the weakest and I am just moving cold air.


Do I leave the 6" at the front and change all at the end to 4"?


Is using a 6" to 4" reducer the same or do i have to replace the actual piece at the trunk.


Here is a PPT drawing of my home. Please note that the Trunk/Unit are actually the same but since I have a crawlspace running alongside my home, It was just best to draw it twice. Also, it may look reversed but the 1st page is drawn horizontally and the 2nd vertically. Just picture the crawlspace in your head to line up the direction in your mind.

I am not concerned about the bathrooms, primarily the Computer Room (1st floor), Living Room (1st floor), Main Bedroom (2nd floor) and Baby Room (1st floor).

My beams run in the same direction as the arrows for ductwork. If you need size dimensions, I would my house is roughly 35 feet by 60 feet. 35 feet is the direction of the arrows.


I would prefer to reduce the Main Bedroom and increase flow to the Living Room (directly below Main Bedroom (2nd Floor) and Computer/Baby room (Below Cat Room on 2nd Floor)

Customer: replied 6 years ago.

Ohh this is my home lol my oil heater went so it was going to cost just as much to replace and move my oil furnance as it was to install Central Air and Heat.


At first, I had them come back because i was barely getting anything out of the end of trunk registers. They used so much Flex, We literally cut off about 6 Feet of Flex at each register. I thought that would solve the problem, it did make it better, but not efficient.


I didnt draw Returns but there is a huge return in the hallway on the 2nd floor, One in the Cat Room, One in the Main Bedroom, and One in teh Living Room.



As an example of what I was saying... (I cant upload any files, but I took a snapshot of what I was saying after a quick modification.)

The industry should use ACCA manual D as a reference... but here... look at this.

Customer: replied 6 years ago.

If you need to know the model its a 3 Ton Goodman 16 Seer w/ Heat Pump and I believe they did a 4 Ton blower? Not quiet sure.


i believe all vents are 12"? and 2 x 10" vents in the Living Room.


I already have the ceilings opened cause the electrician cut one of the ducts when he ran wiring for my new electric water heater. I also already removed the flex from the Baby Room and have both 4 and 6" hard ducts ready to install. Plus since Im doing it, I am going to properly insultate each piece as i run them.


As for getting the installer to come back... well, thats what i get for going with the cheaper company. Its just one of those things where you say to yourself, i dont want to deal with these people anymore and if you want something done right, gotta do it yourself heh


Yah i see what your saying about reducing, Forgot to put that in the drawing but there is one point, about halfway where it looks like they reduced it a 6"? Unsure of amount but it does get a bit smaller and stays that width until the end of the trunk

Customer: replied 6 years ago.

So I guess your saying leave the ones at the end as 6"? I dont mind shrinking the ones in my bedroom to a 4" if that will increase airflow. I was also debating on removing one Register from my bedroom since that room gets VERY hot


I bought all the necessary pieces of 4" and 6" I just want to try to make the airflow better at the end heh.


Another option is installing one of those in-duct Fan things? Or should i just leave it all as is and just insulate the duct pieces I can get to that run between the joists. the ceiling is already open

in a nutshell a 3 ton machine needs to move 1200 cfm of air. It needs to move this at about 700 feet per minute in the main trunk. (there's more to it).

is you main duct rectangular or round?

I cant answer your last question just yet.... if we are going to do this, we are going to do it right.
Customer: replied 6 years ago.

ok, its a 3 Ton Heat Pump and I believe the Blower is set to 4 ton. As for the Main duct, if you mean the Trunk it is Rectangle. and wrapped in some insulation cover.


All Lines to registers were mostly Flex Duct but they came and replaced a lot of it with the hard metal duct and removed excess flex at each register. The whole house is only 1700 Sq feet i believe.


All are 12" Registers I believe except the two in the living room which are 10"

Customer: replied 6 years ago.

I am going to run upstairs and measure the size of the rectangle trunk and where it shrinks down a size. brb 2 min.


GOod thing my walls are still down. Had wood walls upstairs and i tore it down because i was redoing all the drywall & electrical.

Customer: replied 6 years ago.

ok so the Trunk is 9 Inches High and 16.5" wide. Roughly right where the stairs are on the 2nd floor is where it shrinks to 9" high and 12" wide

Are you saying that the first drawing you provided is correct or an example of an incorrectly one? Do you think that the solution really is to just shrink the main trunk line down a bit more? And if so, how easy is that lol

ok rectangluar....

1600 CFM for 4 ton, but I wouldnt know why they would do that unless you have like no humidity in the summer to contend with.... Lets spit the difference for arguments sake to 1400 CFM

1400 CFM to move at 700 FPM would require a main trunk starting at like 10 x 35" or 8 x 50 or 12 x 28.... as a couple runs are taken off at say 110 cfm per run, that leaves us with 1070 cfm.... this would need to be reduced to 10x24 or 8x34 or 12x20...take 3 more off at 110 cfm and we are down to 740 which will be about 10x17 or 8x22 or 12x14...take 3 more off with 110 cfm each for instance and we have to supply the last 3 with the remaining 310 with a 10x7, 8x8 or 12x6

A 6" flex duct will only move about 70 cfm if stretched tight. Hard pipe 6" would get you about 100 cfm. Hard 7" will get you about 140 give or take.

Your 9x16.5 is about half what it should be.
Customer: replied 6 years ago.

Just measured the main trunk start and it is 20.5" wide, 9" high. Ill have to go scout to see where it drops to 16" wide and 9" high

So basically your saying the problem isnt necessarily the actual duct work, its the trunk. So leaving them all at 6" should be sufficient?

Right at the start of the trunk there are 3 lines coming off it (2 in Family room + Kitchen)

and it looks like 4 lines coming off the trunk when it is 16.5"

and 5 when it is 12 " wide

Uhh, yes and a little no, but more yes.

Like I said, the air is dumb but you really cant wish it to go where you want it, you have to force it there.
Customer: replied 6 years ago.

Give me one sec, I kind of understand where you are going with this. I just want to get the total length of my trunk and the distance between the trunk and register.


brb 1 min

I have to eat... I'll be back shortly... wife says so.
Customer: replied 6 years ago.


since I know im taking up a lot of your time. If i was to give you a $40 tip and provide you with the exact length of my trunk (or max space available for the trunk lengthwise) and provide you with the exact distance from the trunk to the registers. Could you provide me with the trunk dimensions and where I need to decrease it? Not sure if you have that book Manual D - Residential Duct System. it doesnt have to be perfect, just your guestimation would probably be better than what i have now lol

Frankly, since my walls are open, me re-doing the trunk would be a lot easier than me trying to redo all the ductwork to each room. I have full access to 85% of the trunk except at the end there is about 4-5 feet I would have to crawl to but is still accessible.

I can just completely remove the trunk and make adjustments to it and just reconnect the registers/supplies since those pieces will be there already waiting to be connected

I can always run a trunk within the beams horizontally if you think that is better . My house isnt that big and with the way the crawlspace is, Ideally it would be nicer to just keep 1 trunk and leave the register/supply ductwork that is already in place.

As you can see by the drawing, that one trunk locations basically directly hits 8 (Main Bedroom x 2, Living Room x 2, Family room x 2, Computer room, Cat Room) of the 12 registers without running a lot of ductwork.

The only ones with more than roughly 4-5 feet of duct are the Kitchen, Both Bathrooms and Baby Room.


Ill go measure while you are eating.

so how do the ducts get to the second floor?

How does the duct connect to the unit? Is this a packaged unit or a split system?

Edited by Douglas on 1/18/2010 at 12:12 AM EST
heres what I need....

Do another powerpoint single page

Do the downstairs in one color and overlay the second floor in another color Lets say brown for first and green for second. Put the registers in the rooms using the same colors as the walls.

I dont get the crawl space on the side does that work, is this a space between row houses or somthing or what am I missing?
Customer: replied 6 years ago.

Whoops Sorry, was doing the PPT.


Basically the crawlspace runs alongside my house and is roughly 3 feet wide by 3 feet high. Some people use it for extra storage space but its pretty much unfinished and kind of like an attic crawlspace but running alongside my house on the 2nd floor. Why its there, is beyond me lol I actually have the crawlspace running along both sides but I removed the irrelevant one because it is hard to get to if we were going to run ductwork.


The Indoor part of A/C Heater Unit is in my attic which is directly above the Family room. The Trunk is connected to that unit and runs into the crawlspace. Both pieces are located on the 2nd floor. So that one trunk line does both 1st and 2nd Floor.


On the 2nd floor, all the registers and supplies come out of the top of the trunk to feed the upstairs registers. All of the ones downstairs come out of the bottom/underneath the trunk and in my Joists to get the the rooms like the Baby room, 1st floor bathroom, and Kitchen. All other registers - Computer Room, Cat Room, Main Bedroom, Living Room are literally either 2 feet above or below the Trunk Line. The 2nd Flr bathroom Basically shoots across from the trunk to the bathroom by utilizing the attic. And the Family room (1st flr) is literally below th attic so they just ran 6" ducts in the attic and just cut holes to install the register.


I think the confusion is coming in because the 2nd floor was drawn Horizontally and the 1st floor the other way but if u use the Trunk as your point of reference just turn one page so that they are literally on top of each other.



ok let me digest this for a while....
Customer: replied 6 years ago.

hopefully this one makes a little more sense. I rotated the second page so you can basically see how they line up or AC_and_heat_design_Revised[1].ppt

Ok, I overlaid it.... Dont worry about where the rooms are, you can visualize the rooms

On this picture the upstairs registers are in purple and the downstairs are in yellow... Does this look right?

Customer: replied 6 years ago.
Yup looks about right
ok I'll work on the sizing... it may be a little while.
Customer: replied 6 years ago.

Sure, thanks again. XXXXX mistaken, i thought they said my blower can go up to a 5.5 ton blower but im not 100% on that. You would think this house should be easy to heat since 80% of the vents are right above and below the trunk. There are really only 4 vents that have to travel any distance (Bathrooms, Kitchen, Baby Room)


the Family room ones I was probably going to move to the other side of the wall to eliminate the extra 10 Feet of duct they put down so those will only be about 5 feet directly to the left of the start of the trunk.


I have to run to walgreens to get baby detergent. If you answer while im gone or need any additional info, I can text that from my phone since it sends me a message.


Hopefully its just the trunk that is causing the problem cause it would be nice to return the $400 worth of duct stuff i bought and just get a newer, bigger trunk plus materials to reconnect each supply back to the trunk




Because of some of the real short runs, you will have to put balancing dampers in the take offs to the runs so you can balance it. You dont want to depend on the registers as the air has already started down the pipe...
you'll laugh at this, but it works when you dont have the fancy tools those of us spend thousands on....

When you have it all piped in, you can use the balancing dampers to get the airflow set.

Read and bookmark this article.... You can set the airflow on each run. It takes a little math but it works....
Douglas, HVAC Technician
Category: HVAC
Satisfied Customers: 3089
Experience: Manufacture Rep for Major Brand, Technical Trainer, NATE
Douglas and other HVAC Specialists are ready to help you
Customer: replied 6 years ago.
Okey dokey. I'll be home in 15-20 min won't let me open any attachments or pics on my phone. At the cash register now, be home soon
lol you sound like me... driving around with GPS, Laptop, blackberry.
Customer: replied 6 years ago.

Gotcha. Thanks a bunch but I just want to make sure i understand this diagram.


So where you indicate 6" or 7", I know that is pretty straight forward for the actual duct work but when it comes to the actual connection between the end of hte duct and the register, do i need to specifically look for a 7" register or at that point, can i use a 7" to 6" converter etc.


For the trunk, is the 1200, 790 and 410 the CFM that is remaining at that point? or does it mean something else. Does how long each section is matter? Or can i pretty much just wing it from the point indicated on teh design or after it passes the last register for that group.


Im assuming the 10x24, 10x15, and 10x9 are the sizes in Inches. Dumb question but, is there a specific name for Trunk so when i go look for it in Lowes or Homedepot do i need to call it something else?


And the 120, 110, 100 etc I assume that is the umm Feet per Minute or CFM each register should be pumping out?


When you mention dampers, I briefly read through that link you posted, so basically im putting in Dampers not so i can change them around freely whenever i want but to balance out the airflow throughout the entire system? And basically I can close the wall up cause i wont need to mess around with the dampers once it is set?


And the last dumb question, does it really make a difference if I have all the supplies coming out of hte side or top or should i really have it come out of the bottom for all the 1st floor registers?

Customer: replied 6 years ago.
not sure if i need to post so it allows you to respond or not. I dont know if justanswer closes the topic once i accept heh
you can use an increaser at the end if you need to for the register boot.

The ducts are in square inches. Just make sure the volume is the same give or take a very small amount. So if you were converting 10x24 thats a cross section of 240 square inches. If they only have 8" duct then you would need 8x30.

The 100, 110 are cfm measurements.

Yes the dampers will allow you to fine tune it. Because some runs are longer than others, there will be variables. Start with them all open and adjust accordingly.

It doesnt matter where they come from, just try to keep the nubmer of elbows to a minimum.
Customer: replied 6 years ago.

Thanks again for all your help and advice, now I just need to go return two truck fulls of duct and go get the correct stuff. I hope When i disconnect the trunk from the A/C unit that it isnt that hard to go from the 20"x9" to a a bigger 24"x10" or whatever combo makes it 240 square inches. So it doesnt matter how tall or long it is, as long as it comes out to the same Square inches per section?


What are the common sizes available to people at local hardware stores?

Dont know, I never bought any duct at a hardware store... Always made my own. (better made and less expensive).

You can make your transitions out of spare metal. Just make sure it's sealed. You will want to apply mastic to the seams to prevent leakage.

Customer: replied 6 years ago.

hmmm do you need any special tools to make your own? I recall Home depot selling like sheets of the metal, wasnt quiet sure how to turn that into a trunk.


Is the tool similar to this s/stores/servlet/ProductDisplay?storeId=10051&productId=100648037&langId=-1&catalogId=10053&ci_sku=100648037&ci_src=14110944&cm_mmc=shopping-_-googlebase-_-D27X-_-100648037&locStoreNum=4163&marketID=34


Along with like umm S Cleats or something like that?

Haha that's one tool the others looks like this....


Customer: replied 6 years ago.

LOL well I dont think ill be picking one of those up anytime soon LOL


Ok, last question, When you say 10" high x 24" wide, how long does that particular trunk need to be or does the length not matter cause a 10"x24" by 5 foot piece will have a higher square inch than a 3foot etc

same thing.. just more joints.... As long as you get past the take offs that are supposed to be on that run.
Customer: replied 6 years ago.

ok, that makes sense. Ill just pretty much follow the design. Its pretty much like how it is now except I will be making the trunk a bit bigger. Thanks again, ill leave you be now heh.. plus i have to clean the mess before my wife comes home :P


Ill let you know how it turns out.

Customer: replied 6 years ago.
unless you live near PA then come drop those machines off :P
uhhh no....

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