How to Design Duct Work for a 3 Ton Air Conditioning system

Ductwork design for a 3 Ton air conditioning unit.
Have you ever wondered if your ducts are sized right? Follow along to get a crash course in duct design.

[@teachmeeverything3344]

Thanks for all the love guys. I'll be straight with you, I uploaded these videos then life got busy and to the backburner it went. I was pleasantly surprised to see so many views and subscribers when I remembered about this channel. I'll put some time in to answer your questions and start making some more videos.

[@noneofurbus3339]

1:10

[@noneofurbus3339]

13:20 13:21

[@richardlong5482]

Dude thanks a million!! I have stressed about this project, never done it before, but you really helped me understand how to build a quality balanced system.....you're a life saver!

[@pastorlee7908]

I couldn't agree more! @@richardlong5482

[@skytiger6671]

My 24x48 house using 3 ton, but starting in the middle 28-8 ying out to on each side reducing to 20-8 will this be enough reducing? 3 bed 1 bath large kitchen and LivingR on one end.

[@joshbibler]

I just wanted to comment and say thank you for making this video. I've watched dozens of videos the last few days about this subject as I'm redoing some old ductwork in my home and adding a few new ducts as well. The calculations for reducing the trunk to increase pressure was very helpful.

[@ClarkeLSmith]

This is an amazing explanation of how to design and HVAC ductwork system and how the reduction of the trunk size allows you to push more air to other rooms farther away from your system. Excellent!!!

Thanks so much. You clarify lots of questions. I will design my 3.5 TON ductworks for an AirBNB place with this helpful info. Thanks again.

[@robertsmith2956]

Don't want working AC, so they will leave.....

[@TheAbdallah89]

excellent job explaining everything in details. I'm a Mechanical engineer by the way and currently doing HVAC Service residential. I really enjoyed the Video. thanks a lot.

[@jeffdeluca1153]

I worked in an HVAC dept years ago doing this stuff. You have done an excellent job of explaining and re-educating me on this topic. Well done!

[@kathrynpe1]

Thank you, thank you, thank you. My all male coworkers are cool and all but only so helpful. This info isn’t a fly by tutorial or something that in passing one can quickly grasp. But your video and my duct calculator will definitely be a gem moving ahead. God bless you!

[@WarrenHenry]

Thank you for posting this. For laypeople like myself this is valuable information

[@amolbhagwat6816]

This video is amazing and is answering lot of my basic questions about duct work planning. 👏👏

[@Parabellum-X]

This is my favorite KZbin video. Seriously. I've probably watched it about 20 times so far, just trying to get this information embedded into my brain. I've watched COUNTLESS videos here on KZbin about Duct Design, Sizing Ductwork, Calculating CFMs, etc. You name it. But this video in particular, in regards to the specific information that I am looking for, has been the most informative. Hands down. (I graduated college 2yrs ago (for HVACR), and, although I have performed a few change-outs and NUMEROUS service calls, I feel like my next step to advancing as an HVAC Technician is to learn how to calculate heat loads and unit sizes, and sizing ductwork.) Thank you so much, @teachmeeverything3344, for the time and consideration that you have taken out of your busy schedule to produce such wonderful content. And thank you SO MUCH for not playing annoying rap or elevator music throughout the entire video. 😏

[@FooDaki]

Thank you for the video , excellent explanation

[@IADHDD]

One of the best videos I have watched. Very clear and easy to understand. I really enjoyed your video and I will watch it at least 10 times more lol

[@trueforum378]

Limit of watching it is 5 times per person. Please!

[@crackerjack2010]

very good explanation on calculations

[@Michael-fp7so]

This is one of the best explanations on this topic I’ve found!👍 Good shit bro , preciate you!

[@BandofBrothersSpike]

Keeep the vids coming boss they’re great especially hvac related

[@teachmeeverything3344]

Thank you. I am jumping back into it, life kind of swallowed me up this year.

[@sergioramirez9025]

Sir this by far is the best video ever explained 👏 thank you so much 🙏

[@hubercats]

Excellent video! Thank you.

[@twochickshomestead3439]

Best video I have found!

[@will-smith-nh]

Excellent video. Thank you.

[@pineychristian]

Good video.. at least it gives an idea to people to see how much time is needed for people like me and millions of others need to design a duct system or entire HVAC system only to hear complaints about the pricing. There is a lot time need to design and build a material list to properly quote customers. This is the stuff that goes on behind the scenes at night when we don't get to spend time with our loved ones to make money .....

[@chuckpugsley759]

Whiney prepper, count your blessings that you have a job.

[@teachmeeverything3344]

Truer words have never been spoken. When I owned my company the divorce rate for owners of HVAC companies was about 30% higher than the national average. There is a lot of blood, sweat, tears and mental exhaustion that goes into the trade. I have a deep respect for anyone tackling it and family life simultaneously.

[@elijahgoodman3140]

Excellent job filling in the blanks I've had designing a duct system. I've always known about keeping velocity high, but what I didn't know was any velocity under 600 fpm was the point you needed to downsize ductcto keep static up. Thanks alot, truly.

[@pastorlee7908]

Very, very informative! I now can feel totally comfortable designing my own. Thanks so MUCH!!!

[@ianwalbert8727]

great video

[@will_hunt_music]

Awesome tutorial and great detail! Thanks! Only thing that would have been nice to add is notes on where/how to put the return ducts

[@oindigenous9990]

Great video, New subscriber!!!

[@maxrovira7315]

Thanks you for your time and for great teaching to be able to carry my out my project

[@AngelofOntario]

Wow, that was an AWESOME explanation & AWESOME drawings!!! I’ve been wondering how they size ducts as I’ve been thinking about how to change how our 2 bedrooms are heated, to make it more efficient (don’t ask, previous DIY owner). I LOVED THIS video! THANK YOU!!! Your explanations as you go along are very easy to follow & understand, while being thorough! Not everyone has that gift. Makes me also better understand how potable water plumbing is sized too. And possibly how to get more velocity at my furthest faucet! Do they have the same slide charts for water pipe sizing??? Also, in residential heating with a natural gas, forced-air furnace, I use grate filters at the points of exit into the rooms to cut down on winter house dust. And I change them monthly. Is that stressing my heating system?

[@r.t.7925]

Good info .Thanks

[@juanrafaelsoto9342]

Excellent thanks

[@trentongibson4417]

Would love to see a heat load calculation from scratch or even a mock one . I know people's software that they will use will be different but the process should be very similar thanks

[@billmiller5075]

Good job

[@ironmike7160]

Yup. Have to transition smaller each section to keep equal amount of velocity of airflow. I worked HVAC for 5 years.

[@maxa9246]

Dude, make the next one the cold air return! :-) Great Video.

[@thedahlins]

Thank you! Your calm, clear way of describing this is very informative! And along the way, you give us a tutorial in how to use a duct calculator. Much appreciated! If you have time to do another video, would you describe how to determine CFM's for various rooms, optimum register placements and the pros and cons of hard duct vs flex duct. I'm about to do a job for a client where the exterior wall of the living room faces south. The room has a vaulted ceiling, where the ridge beam prevents running ducts above the windows facing south. An option is to place registers high on the north wall in attempt to get air movement up and over to the windows and the south wall. The house is on a slab. Any thoughts would be appreciated! The layout of the room is similar to the 300 cfm room in your drawing, with the ridge beam running the length of the room.

[@cookingblue22]

VERY informative and helpful lesson for the DIY person out here. I am curious, however, how the length of each section of duct affects the calcs and performance? Thinking shorter runs the better, but what do I know??? Also would love to see your process for designing the return side as well! TERRIFIC video...thanks so much!!!

[@michaelmorassi4953]

Nice video. Noticed you used a .08 FR for the main trunk line, but no the branches to the individual rooms.

[@isaiahwalker4065]

Thank you

[@MadMotorDoc]

Duct calculator a must ! Just started doing my homework on this stuff , explained & understood everything but the lenghts of each trunk runs before having to reduce ? 8"x 24" X ?. Im converting an abandoned hydronic on slab to forced air through attic.

[@armintahoori6524]

Very good 👍

[@yoelocuba5900]

It is a good explanation. How do you size return air duct? Can show me please.

[@austindearman2296]

I partially closed my registers in certain rooms to redirect flow to the larger rooms where the air is needed more. Then proceeded to drive screws into the registers so they can't be adjusted when people walk across them or the kids try to mess with them.

[@Je1gonzalez]

You don't have volume dampers on your collars?

[@trueforum378]

Great video! Question: how do you calculate the. LENGTH OF DUCT? example your 8x28? How far does this rectangular duct go?

[@AdamMarsh-vz2iw]

Great video. Question is if you were to add zoned heating to this or any system would it affect or change this setup. I'm particularly wondering how it would affect a 2 zone system where you had upstairs and downstairs on two different zones? Do you need to change a bunch to account for flow and CFM, or is that why a bypass is added somewhere in the system to allow for limited flows? thanks for any help or explanation on that.

[@centrumsolutions4744]

Thanks for the this video. Are those room CFMs total equivalent length and friction rate? Also, are they based on ASP or?

[@rickcantu4501]

Hey brother great video very good explanation , I live in Texas and we usually only use 1 sheet a duct board to make plenum and branch every run with flex. That’s 95% of Texas. Does this also work in that kind of designs? I’m a service technician don’t really do duct designing or manual J or D

[@hvacando5609]

Is there a chart or pdf that shows the El for every single fitting? If so can you please let me know? Also can I find out the El for a specific fitting by calculating myself?

[@donaldpape1435]

Where are you getting your friction loss calculation from.... Rule of thumb? TEL,ASP,ESP? Plus loss per 100' of duct. I do love that slide rule though.

[@IADHDD]

I do have a question for you, I am installing a Mini-Split Concealed Duct system (1800 BTUs) which if I use the "400 CFM per 1 ton", it will be 720 CFM system. However, the indoor unit has 3 speeds (H/M/L) and the CFMs are 529/450/370 CFM. I am not sure what CFM I should use as a starting point to design the duct? Do I design it for 720 CFM which is the 18K BTU, or should I use the supplied CFM for the indoor unit? Even if I was to use the CFM for the indoor unit, am I supposed to use the H or M or L?

[@adleysworld7227]

Sorry for all the comments. The reason I asked is I jus had a unit installed in my attic, They T'd off the supply trunk and ran the Trunk perpendicular to my long side of my ranch style house int he middle. They ran long flex lines to all of the supply lines. Each trunk left and right is about 8 feet long with one said having 6 take offs and the other with 4. This is a 2.5 ton AC

[@darkflux]

this was very edicational! the only thing i didn't understand was, on each of your trunk runs, how do you know how long to make each section of trunk? for instance, your initial trunk size was 8 x 28, but how did you know how many feet long to make it before dropping down to 8 x 20? i'm sure there is a logic there, i just missed it.

[@vqgang3575]

Would it be better to put a 4 ton or 3.5 to reduce the amount of reducing plenums u add ? or would u have to use a 3 ton due to the sq ft of the home ?

[@felixmilburn4594]

Great video, can you do a video for the return using this project/example as the guide?

[@Martin-py4kx]

I would also love to see this aswell! This was a very educational video, and I may look into learning this professionally to expand my skills in the trade

[@Je1gonzalez]

Did he ever make a video for the return? I would like to see it

[@jesse4553]

Me too!

[@RichardT123456789]

Hello, How would you calculate a mobile home split system with an inside down flow air handler 3 ton . No return . The Mobile home is 16X80 single wide with 8 vents total. The original duct is the rectangular sheet metal, and it is degrading.The mobile home is 1999 model . The local supply shops do not have the sheet metal available & I am considering using Flex duct & I am not sure what size or may I even use flex duct in replacing the sheet metal . Thank you

[@hubercats]

I’m curious as to how this design process changes if one is trying to design a two floor duct system, under the assumption that the two floors are connected (in the sense that air can flow freely between the two floors via a staircase, for example).

[@FoXHounDXD]

Friction rate is not static pressure..... and you do not assign it based on duct type. You calculate it based on the total equivalent length of duct and available static pressure. Screwing this up, makes your "design" nothing but guess work.

[@romanh5066]

GREAT VIDEO. QUESTION: IF THE VELOCITY FOR METAL IS AROUND 800-1000, WHAT IS THE RANGE FOR BRANCH FLEX DUCTS?

[@teachmeeverything3344]

Typically around 600 FPM anything more will result in whistling and wind noise from the registers.

[@helipadny1]

Probably a stupid question.. My air handler ( 2ton system) will be in the middle of the house. most of the load will be on one half ( 500 cfm's) and the rest for the other half.. Do I size the trunks coming off the plenum (tee) accordingly?? 500 for one side and 300 for the other. or is one a one size trunk ( 800cfm) straight down the middle and just branch off from there?? thx!!!

[@centrumsolutions4744]

For every 300-4000 CFM drop the main duct need to be reduced.

[@HVACKnowItAll]

Did you calculate your TEL and ASP to get your 0.8 fricton rate or are you using a rule of thumb?

[@edwardjanowiak]

I’d like to see that part explained also 🤷🏾‍♂️

[@alexzapata692]

Ditto lol

[@joshuaseaton4526]

He is using rules of thumb with the ductulator. Could be correct, but also could be very wrong.

[@teachmeeverything3344]

@@joshuaseaton4526 True, .08 is pretty typical, but not always exact. This house is also completely fictitious and the air volume requirements are made up.

[@vqgang3575]

can u show a video explaining how that plenum would be set up in a attic

[@riazmalik2956]

Excellent explanation. How much the ductulator cost. Very handy and quick way

[@teachmeeverything3344]

You can get the Super Cool Slide rule from their website or on Amazon. They also have a free app.

[@MJubeily]

The duct that is drawn is the 8'' or (28'',20''.. etc) ?

[@retsbew8954]

You should follow “Manual D” and fill out your friction rate loss sheet, so you can determine what friction rate to use. Otherwise, it’s just a guess. Maybe you did that, but didn’t show that part?

[@elro2590]

Is it acceptable for the last branch duct to have a slightly higher velocity than the one before it? Wouldn't this cause backflow?

[@jamescusack2136]

Is friction loss the same as static pressure? My air handler has a “rated static pressure of .50inwg” and a “mad static pressure of 1.00inwg. How do the two correlate?

[@jessicawang7095]

Thank you so much , What will be the recommended cold air return size in this design, if you use only one (including length, how far )

[@teachmeeverything3344]

I'm working on the return air video now. Keep an eye out for it soon. In the video I am doing both a room return and central return.

[@chrisw4562]

Thank you for the great video. I am wondering, though, does the calculation of the pressure drop not need to take into account the length of the ducts going to each room? The calculator says pressure drop at 100 feet length, but most of the ducts appear to be 15-20 feet long. In my house, the duct length varies from under 10 feet to 40 feet, and the result is artic winds blowing in one room, while the other only gets a light breeze. It seems to be designed wrong and I want to correct that, but wonder how to calculate the proper duct sizing vs. length

[@mattandrew6531]

That's a great question.

[@travisnelson8175]

Generally you don’t run a flex duct more than 25’

[@HobbyOrganist]

My problem is my house never had central heat or A/C and I put a furnace in the basement and did the ducts as best I could with 6" round, but the ceiling is only about 6'4 in the basement and the layout is a cross-T with 2 valleys on the roof, that means the floor joists run one way in one part of the house and 90 degrees the other way in the rest, plus a main support beam that sticks down about 4" where the two joist systems meet. My floors are also porcelain tile and I'm not cutting more holes in the floors! Going to the attic is out of the question with ceiling vents- I have R100 insulation up there and the access is a TINY hatch barely big enough for me to squeeze thru and it takes a 10' ladder to reach it. I want to put a new furnace and rect ducts in

[@teachmeeverything3344]

The ceiling height in the basement can be hard to work with. Sometimes you have to decide one or the other (correct and adequate duct sizing or headroom). As I see it you have a couple of options. A ductless system without duct would be at the top choice, this eliminates the need for vents when you use the highwall units (or other non-ducted options) second would be a short depth duct either custom made or custom ordered, probably 6". Sometimes you have to get creative with the runs to make them work, and it can more often than not be a huge pain. Concerning the floors you can use wall packs, going up through the bottom plate on the wall and putting the supply air vent on the wall above your baseboard. Feel free to drop me an email if you have more questions.

[@michaeldean8409]

there are many different ( Air Duct Calculator's ) TRANE,LIMA, when you line the air volume numbers up the sizes of ducts are different WHICH ONES CORRECT

[@happyj313]

What was the square footage of the house of this design

[@judahexodus7958]

Great video but I would’ve love to see the sq-ft of the room on there.

[@number1pappy]

Maybe I missed it, but what is the square footage of this house?

[@elijahgoodman3140]

One question though, will you have a issue moving air out of the furnace into that elbow? In commercial we used turning vanes, what say you?

[@teachmeeverything3344]

Turning vanes are always suggested, this video is more about duct sizing than assembly practices.

[@frankgall6]

What was the blower door number?

[@darwincabrera7668]

How did you find your cfm per room

[@danmazzeo7662]

Any reason why you didn't show return duct work? do you have another video on the return? Thanks Dan

[@teachmeeverything3344]

I had planned on doing one for the return air yet somehow this channel slipped through the cracks, I will work on finsihing it up since there seems to be some interest. Thanks for watching!

[@adleysworld7227]

Just so I better understand. Basically the first section of trunk 1200cfm at 940 FPM could support about 12 7" Takes offs? At what point do you say it's time to reduce is it after a ton of air has been removed from the trunk? Why not run that trunk 24 feet and be done with it. Is it you just loose velocity?

[@syedsaadali2945]

What's about the return air?

[@michaelfisher7835]

Why did you put the furnace, where the air has to make a turn, before the long straight main run? Wouldnt it have been better to have a straight shot without the turn?

[@Ifeguy]

Of course it would, but in real life you don't always have the option of placing the equipment in the most ideal location. Presumably you couldn't control that

[@michaelfisher7835]

@@Ifeguy Understood. However, if the architect did his job, he would design the home so as to make sure the HVAC was in the best area to be the most efficient. Been there, done that.

[@Je1gonzalez]

@@Ifeguy If is in the attic or basement he should have enough room, specifically if he has room to run all that duct

[@teachmeeverything3344]

Over the years I have had to put units in some terrible locations, they never leave the HVAC enough room and the designer usually draws a 3' square closet to do our work in. With that said this is a fictional home and layout, I use it for an exercise in the classes I teach. We draw the equipment in various locations.

[@kmrwoodworks]

How did you perform the load calculation for this example?

[@ktmkilljoy]

by multiplying the standard of 400 cfm by the tonnage of the unit, then dividing that by the square footage of each room. this example shown would look like this 3 ton x 400=1200, assuming this is a 1500 square foot house 1200 cfm / 1500 is .8 then take .8 and divide by the room square footage to give you your individual room values.

[@idanny76]

I have high static pressure on a new 3.5 ton heat pump. I know for sure the flex duct is the issue. I have two 12" coming out of the supply in opposite directions. I know I need an 18" for a 3.5 ton heat pump. I did a diagram layout of the duct in the attic. What would happen if I just replace the two 12" to 18"? Ignoring the downstairs, because everything is between the walls. The problem was even worse with an undersized return. The original and only return was a 24x18. The total static pressure on the unit was .9 W.C. I was able to drop the TESP to .66 by making the return 30x24. Since there is so much static pressure and the unit TESP is a .2 and I need an 18" for a 3.5 ton for 1400 CFM. How big does the supply plenum should be and what shape? Should the supply plenum split to 3 18" going out to each of the 3 distribution boxes? Do the other three distribution boxes also need to be larger? Looking at the triangle shaped supply plenum I have very little room up for a rectangle plenum. I think a 3 feet ductboard supply plenum should work. There is a foot from the Air handler to the ceiling and I think 2 feet from the attic floor to the roof. Now I am looking at two types of designs. Design #1 keeps the 18" ducts straight by having a single 18" coming out of the supply and then a distribution box splitting it to three 18". The sides would be coming out straight to the to side boxes and one going to the third box with a slight curve. Design #2 splits to three right at the supply plenum and goes to each box individually, but all three will have a slight curve. I will not be using any metal plenums..... it will be all flex duct and ductboard boxes, because of budget. I am not trying to make a perfect system, but trying to remove the restriction from the 12" undersized ducts installed and replacing the flex duct that was patched with mastic with new ones. Another question about the installation..... I have the flex duct running up and strapped to the roof for support. Since the supply plenum is low to the floor due to the roof..... Can the flex duct be run on the attic floor? Can you tell me which layout makes more sense and less restrictive? Thank you. Layouts: Current i.ibb.co/Ld9kmPM/Flex-Duct-Layout-and-Size.jpg Option #1 i.ibb.co/GHyS00g/2022-07-05-0002.jpg Option #2 i.ibb.co/PNN8mSB/2022-07-05-0001.jpg Air Handler Specs ibb.co/17VvF4B Video of attic kzbin.info/www/bejne/n3rReoFubtZrfbc

[@YoBitchChose]

I only read the first paragraph but 12" round flex is ALOT of SUPPLY to one grille, 18" would be ridiculous, unless you're talking about your return size?

[@caden01691]

People spend a lot of hard years learning trades, nobody is going to come along and borderline design your system for free on KZbin and You clearly don't know enough to do this on your own. Call a local HVAC contractor like everyone else.

[@teachmeeverything3344]

@@caden01691 I 100% agree, especially being a former contractor.

[@pineychristian]

Hopefully you use hard pipe duct off your branches than used flex for the last 10ft to 5ft. Flex duct was never designed to be used for the branch runs as a total run duct. It was designed for acoustics purposes to quiet down the air flow but , we see it all the time now. So people tend to think it is a normal thing ....

[@philllsxga.7737]

Correct. You should use as little flex as possible. But it's everywhere and we'll over 10 feet..

[@LC-jy8mj]

SMACNA ( sheet metal air conditioning contractors national association) recommends no more then 8 feet of flexible duct.

[@pineychristian]

@@LC-jy8mj very good point. People really don't have a clue. Flex was invented in 1970 by a man in NJ it was really designed for acoustic purposes , not for ease of installation.

[@teachmeeverything3344]

I typically kept it to 10' in residential, and less than 5' in commercial. It does do a decent job quieting down ductwork as well as making some of the connection points easier. With that said just because something wasn't originally intended to be used for a purpose does not make it unacceptable for that purpose. You can adequately size flex duct runs with this in mind, even more so with the wide use of ECM motors these days.

[@pineychristian]

@@teachmeeverything3344 you really can't "use flex more so with ECM motors these days". I've been in the hvac field for over 15 years now and seen many ecm motors fail prematurely because of being installed with too much flex duct on a system cause of the increased static pressures. Older ecm motors would " hunt " trying to find the speed they needed to operate meaning speeding up & down . Now they just fail. If you have constant torque ECM motors they will most definitely fail early on a system with too much flex duct. Flex duct flows less by design than hard pipe duct system. 6" flex duct and 6" hard pipe do not flow equally per 100ft . This is why they make Flex duct calculators now.

[@SolarDrew]

How do you know how many CFMs are needed per room? You said you'd need to know BTUs per room first, then how many CFM per BTU?

[@teachmeeverything3344]

The CFMs for each room are determined by the heat gain and loss for that room. This is figured up using a load calculation tool that examines things like building materials and environmental conditions to determine the heat load for the home.

[@JB-yq9bn]

Low velocity is only an issue if you don't insulate the duck work.

[@MrAbYounes]

what about return?

[@giuseppepandolfo4815]

How many square feet is this house?

[@maxrovira7315]

Can you design a 2 ton system, 4 ton system, and 5 ton system please

[@deathrowak6702]

If you have that same 3 ton system on a house 2500sq ft house with 21 heats would you match the main duct sizing to the air handler or size the duct up to 2100cfm

[@goldenretriever6261]

What do you mean by "21 heats"?

[@tonyedwards9972]

2500 Sq ft house would need a bigger unit

[@seeqr9]

@@tonyedwards9972 almost need a second unit at that point

[@Je1gonzalez]

You'll need a 4 and half or 5 ton unit or two units to match the sizes of the house... (Period)

[@teachmeeverything3344]

A 3 ton would probably be a little light depending on what region you lived in. In Minnesota it would probably be fine. One thing about refrigeration is that it is very picky about the airflow it has. If you want it to work like it is intended it needs 350-400 CFM per ton. So a 3 ton unit would run 1050-1200 CFM. Anything more or less than that will cause the refrigeration cycle to operate incorrectly.

[@mikeerin5031]

So many of these videos are done with ducts coming through the floor. That’s not typical of homes in my part of the US.

[@teachmeeverything3344]

The only real difference would be the location of the ceiling registers. Typically they would be located closer to the center of the room, but still more toward the outer edges. The same types of reductions would apply with duct board or Sheetmetal. If you are using a flexible duct system it would be significantly different.

[@chasadmin-oo3jq]

GOOD STUFF

[@weld3z]

In my country we use 3 ton in one room

[@adleysworld7227]

At what point do you know you need to step down. It is every 4 takes offs? Why not run the 8x28 the entire length then? Is it you just lose velocity ?

[@Anthony-rp3qh]

When you dump cfm at a take off, you downsize to keep fpm and static pressure correct…basic manual D

[@andyreyes6464]

how do you find the length of every supply duck and plenum

[@kathrynpe1]

It’s in the video. You can get a calculator from your local supply store. Depending on cfm and velocity he explains it pretty accurately though if you watched it. Just trying to help

[@annettelopez6028]

How do you determine when to reduce the main duct size?

[@Yiffboy]

The ductulator will tell you, after you enter the ammount of CFM you need and also when you reduce,t his way you know if you go smaller on your run( more than likely and 99%?) from what i've learned.

[@edwardjanowiak]

Your supposed to reduce when your velocity drops by half, that is the guidance of ACCA Manual D

[@pineychristian]

☝ this is the correct answer .. I know a lot of people use the old rule of thumb of half your branches off the main but , that's not the right way either

[@teachmeeverything3344]

Once the velocity dips down to around 500-600 FPM in the main branch I size the duct work down. This is done to bring that velocity back up. Without the velocity (speed of the air) you wont ever deliver the conditioned air where it belongs.

[@thomasromero8149]

why .08 instead of .05 or .10? thanks.

[@teachmeeverything3344]

It's a rule of thumb that we use when we do not know the Total Equivalent Length or Available Static Pressure.

[@danpmatz]

That a 2 ton system 28x8 is huge.. Your unit is only going to be 14 or 17.5 Also you should always start at the end and work your self back to the unit. This duct is tooo big you will have very poor air flow. Also if you dont put a Radius, fitting, at that first elbow.You are going to have significant pressure loss resulting in more lack of air flow.

[@austinfoster6718]

I don’t understand how you assumed how long you could make your duct work, I get sizing down but could I run a closed line 2000 feet without sizing down lol

[@Seedavis397]

I love cold air blowing on me when I’m dropping a bomb your wrong

[@garylarson1279]

I have a masters in HVAC technical education and this method does not reflect proper duct sizing according to Manuel D. However, if you’re going to use this method, you need to put balancing dampers on every run or you’re going to have improper air flow. Sorry but don’t believe everything you see on KZbin.

[@user-kc3qf9sj2x]

Yeah but if he calculated the Total effective length first it would be too confusing. Why worry about fittings and pressure loss when we can just set the slide somewhere between .08 and .1.

[@CrazyPeiceOfPie]

You spelled "Manuel" like the name.

[@baron1c]

He had balancing dampers……..

[@rottenheavenly6245]

Starting with i have a masters in HVAC is basically saying i have totally no practical experience but I’m so smart and know better than everyone else.

[@christophercurcio4535]

Could you possibly do a duct calculation for my house .

[@mikeerin5031]

Pretty sure 850 CFM minus 250 CFM is 600 CFM and not 550 CFM….