Basic Return Duct Sizing Made Simple

In this video, I talk about return duct, you need 200 square inches per ton in order to keep the noise down across you return grill. you air conditioning and heat pump system has to breath. the math is 200sq in X 3= 600sq in
always size return duct work at .5 static pressure
use a duct calculator the more grill space, the quieter the system will operate more efficient
Thanks for watching!
- David
[e-mail]
Follow me on Facebook: / david-jones-ac-1010702...

[@briankolek3641]

Thank you guy.. it’s about time someone post the right engineered design from a HVAC contractor, i am one also, sick in tired of going on jobs and seeing miss installations by piping size, thank you again… i liked to give thanks to being taught by my brother in law , who literally the best for sizing all aspects , residential, commercial, industrial.. we did it all

[@gregorysampson8759]

I like this guy because he thinks it through. so many guys out there just do the easiest quickest way and go home.

[@soup3r]

This is an interesting approach from a contractor's POV. Over-sizing the return duct is good practice because you are on the suction side of the fan where the static pressure in the duct and through the return register is more difficult to overcome. You also get the benefit of sound reduction. You aren't actually returning double the air, as the fan can only suck in what it is supplying. You are simply returning at a lower velocity.

[@GunHolsters]

Thanks for clearing that up. He lost me when he said he was returning twice as much air. Now it makes sense.

[@joestube8887]

Great job David, you are amazing at what you do thanks for giving back to the newer techs trying to learn.

[@flamefurnace]

Thank you for sharing your knowledge, David.

[@fighkb]

Hi David I can not thank you enough for those videos, they really help me a lot to be a better technician.

[@Azcooler1]

David, thanks for these videos. I am using them to train some of our inside staff. You have made this stuff simple to understand. Thanks again, John Newell @ Johnny Cools!

[@jonesacnaples]

Thanks I glad it helped, airflow is the most important part of learning about Hvac. Keep in touch.

[@robertpsargent]

Thanks for this video...it makes figuring out the return system a lot easier.

[@chokumYnai]

Thanks for making the video David. I wondered how these calculations were made. I'm attempting to finish my basement on my own. Wish me luck.

[@ryszardjacekrusniak7993]

Thank you for sharing your knowledge.... invaluable.

[@juancarlosdesoto2652]

I like your teaching David! It has helped me a lot

[@surginerc]

Really enjoyed both of your videos. I am Universal certified but have never done actual duct design until now on my own home workshop project. I have studied ManualsJ,S, and D. Just trying to get the proper TEL and sizing down so I can verify pressure/noise/friction/ and velocity. Thank you for your help in making videos

[@ericcarrizales8144]

Very helpful straight forward video for techs

[@lamansavin]

I been looking for this for some time, thanks

[@richt8297]

Helped me a lot. Thank you so much. I have a 3 and a half ton hot air heat + ac unit and 2 16x16 returns. Looks like I’m going to have to add some returns. I was thinking 3 12x12 one for each bedroom with a 9 inch flex for each.

[@Uglylizard66]

Great Video. I'm a self employed A/C tech love it great work.

[@charleshinesley225]

This helped me out! Thanks!

[@robertpray1064]

Great Vid. Undersized intake can really impede a system. A property I had in TX had problems from opening day with the third floor ac. Filter was mounted on the ceiling of a hall closet with a door vent. System would ice up. Immediately noticed the closet smelled like attic air. I added a 30"x 30" from the adjacent ice machine space, sealed the duck sides on the ceiling, and sure enough, system was quieter and efficient. Similar problems at a historic hotel restaurant in Austin where we found intakes almost non existent. Immediate relief! RM Pray, FMP

[@justinsizemore6200]

Taught me more than college did. Thanks!

[@jonesacnaples]

You are very welcome my friend. I hope it helps.

[@deplorablepat8332]

Nice job George

[@danielshvacphiladelphiallc11]

nice job love your work

[@calal2804]

This man knows what he is talking about... period

[@vickerley4271]

I like this it explains it simple to help people

[@coolramone]

Thanks David. I had a 5 ton with a 24x24" return filter with 0.3" w.c. static pressure that eould not keep up on hot days. I added another return filter & grille and it now has .17" w.c. and keeps up (temp) on hot days. 👍

[@johanfry1]

even thought this was made seven years ago, its so imformative.

[@jonesacnaples]

This kind of information is good forever I’m glad it was helpful thanks for watching

[@bklynwam]

Hi David. Your videos not only provide knowledge to the new hvac tech but educate home owners. You have been doing a great service. I have a question for you regarding the correct sizing of the return air duct work. Is it possible to have one return located in the hallway on the second floor in a two story house? Or you do recommend having individual returns in each rooms and one in the main location for the rest of the house..

[@HansHVACR]

Great videos. Thanks

[@jonesacnaples]

Thanks for watching. My friend

[@flyonbyya]

Very helpful!

[@billsomrak]

Well, at least you put my mind to ease with your explanation for a 1600 sq. ft ranch. I live in a small 2 bdr. 600 sq. ft. ranch (on a slab) which has a single cold air return to the central AC/Furnace of 16" x 20" (320 sq. inch.). Extrapolating from your dimensions for a house almost 3 times the size of my house, I can see that my single 320 sq. in. return is adequate.

[@jonesacnaples]

Hope it helped. Thanks for watching .

[@tonycalabrese9315]

Great info.

[@nofixedaddressbyknowhope260]

I used to work with this guy... What's up David...i used to work with you way back in the day.... 2002-2003 time about....with Marcel and Dan...small world!

[@bwjtsa]

It's nice to see an HVAC professional. I'm only retrofit and service as new construction in my town is dead. Almost every system I see every day is operating at well over .5 total external static. Typically .8 and above return static. Every system replacement I sell and install includes some type of duct modification. Load calculations and duct design are a must. Today I saw a 8 year old LP furnace (100K BTU) with a failed limit. The bottom of the furnace was cut to connect 10' of 12" flex to connect to the return trunk. One option I gave, besides a new furnace with duct modifications was to solve the return air issue and replace the limit. Another contractor convinced them the limit was all they needed. Why is our industry so inundated with uneducated professionals? It's not rocket science but it is an exacting one.

[@jaimecorrea6854]

Awesome 👏

[@bksingle]

Thanks for posting! I love how you're passionate about doing this. You making a mark in the industry.

[@jonesacnaples]

.06 may be to low, you may not have enough supply pressure. Between .1-.2 max is what usually works best.

[@stevehenry9053]

hell yes you rock

[@jonesacnaples]

I do not use the air Ductulator to the exact cfm I always round down for example 8" is 200 cfm instead 230 as it shows on Ductulator. The same with 6-7 9 ECT I always add one more than I need. I have done this for years with no problems. Thanks for asking.

[@lulupokes]

short and sweet

[@jonesacnaples]

That's correct return is designed at .05 static pressure and supply at .1 static. You to return more air for static pressure loss from pressure loss through duct work, A coil, blower motor, and then all the supply duct sometime will be a couple hundred feet. Hope this helps . Thanks for asking.

[@jonesacnaples]

Put the thermostat on the top floor. Make sure you a return on each level. Out more air on top floor so it will stay cool in the summer.

[@NestorHVAC]

Hi David, I like your videos and I know I'm late commenting on this one but I noticed that the duct size that u show for return it was for hard duct or round metal I notice when I was doing calculations for a job and I thought I was missing something. Thanks anyways. Nestor

[@koreymayo8884]

18 inch flex duct for a 3 ton is correct for 1200 cfm, but 14×14 is too small. It would be 16x16. and 9 inch duct is almost 300cfm. I would size the grill and air filter 20×36 for 720 square inches. 14x14 square duct or 16 inch round duct is 1000 cfm for a 2.5 ton unit.

[@JohnnyJr396]

Davis I was taught in school to use friction rate .1 for supply and .05 for return. Some people say to use .1 for both. Technically aren’t we supposed to use the longest run and figure the pressure drops of the register on that run , the return grill on that run and any other drops on that run to find the friction rate to use? I believe I know why people use the standard .1 , . 1 or .1 , .05 for residential but would like to know what you think . Thanks

[@sarvmit]

Hello, We have a unico ML4860-BL1-ST model with 4-5 ton capacity. Per unico, they mentioned that they go by 250 CFM per ton, so suggest we use 60 sq inches per ton for return area . If i use that I get 5 (ton) * 60 = 300 sq in However, if I go by 5 (ton) * 200 = 1000 sq in. This is a huge difference and am trying to figure out what should be correct return air.

[@rbeldua]

thank again for sharing... can you tell pls which is the best location for a return diffuser. ie. wall side with windows? wall side without windows? :)

[@orlandezpierre2461]

Hey mr. David id like to know what i am missing on my trouble shooting method..ima go into detail..when your ready

[@jamesmartin5970]

David this is a great help on sizing the static pressure to me. But I am twinning two 5 ton unit together and the static pressure @ 0.1 supply pressure @ 4000. @ 400 CFM supply rectangular ductulator 30x16 duct and @ 0.5 return rectangular 40x16 duct @ 400 CFM but I cannot get the grille space right for the return @ 200 CFM pre-ton I need help .will you do a video on twinning 5 ton unit.

[@daniellefantigrossi8910]

Great videos! . How would I figure out how many CFM's a 30"X6" floor return grille is good for? I understand that the square inches are 180.

[@lallymcghie9312]

I have a little under 2000 sq foot home. Return air is in hall next to master bedroom. When ac kicks on it sounds like a jet plane taking off and wakes me up at night. After looking at your videos I know I need another return air grill but will this cut down on the noise? It doesn't whistle but is loud...also until is in attic and very close to return air. Master bedroom is also much cooler than rest of house.

[@robertprice7877]

David, I just had a 3-ton Trane unit installed they put in a 20X25 return grille which I know doesn't work with your 200 rule. Before I talked to them about this are there any other factors involved here I need to take into account?

[@MrGarcon98]

our class did not go over this very good at all back in the 90s ? just make it cold lol ,,,,good important video though !

[@jaybenson5574]

What is the best way to heat and cool a two level home with a partial basement an d crawl space, 900 sqft 1st level and 550 on 2nd

[@hvac917]

Hi David Quick question, you are sizing this with the metal ductulator but using flex for your returns, in another video you use the flex ductulator to calculate the size. I am asking because I am in the process of redoing my return duct system upstairs and going to be using 10"metal that I had calculated out to 300 cfm's @ .05 Static in a few areas. I am going to insulate the metal as its in an attic so obviously the flex would be easier to run but im guessing Id have to go to 12" flex to get the 300

[@jaymorrison556]

Great Video. i have a question though. i have a single story ranch with the hvac in the basement. i have 1 return duct going to the hall on the main floor. There are 2 partially finished rooms in the basement that have heater vents, but no returns. The rooms are approx. 250 sf. i want to put a return duct in just to help with circulation. any suggestion as to the min size duct i should use? i have a low ceiling, so i am trying to make it work.

[@glenc2353]

If you want to add more returns, is it okay convert the jump ducts into returns instead of making new holes in the ceiling?

[@smille5291]

Had my house duct work all redone this last winter I have a 4 ton unit. Live in Florida. The guys put in another return so I have a 2- 20x20 flex returns but they go into a 22x14 smooth duct (plenum) trunk from the ceiling to the air handler should they have increased the trunk size also?

[@camilo11315]

Greta video, Thank you for sharing your knowledge. I have a Question for you if that's ok. A contractor in our Area is putting a 3.0 unit in our first level but we originally had a 2.5 unit. I told the contractor that we didn't need a 3.0 Ton unit because the all ducts were build for a 2.5 tons. He told me that we should no have any problems. Can you put a 3 Ton unit without affecting the duct work? Would the machine be able to breath? If he alters the intake duct then he has to increase/decrease the outtake ducts right?

[@SombraLocs]

Someone told me to use .08 for both Return and Supply, he said using .01 for the supply was the old way of doing it. I can't really remember the exact reason, but he mentioned that it had to something with the newer units coming out. What is your say on this?

[@mememmine997]

OK! My unit is a three ton it came with a 14" tap for the return. 3tons×400cfm=1200 total cfm needed. But a 14" tap came with the unit but it will only allow 700cfm into the unit. If I need to run two returns say 2x14" =1400 cfm how far away from the unit on the 14 main do I need to put a "y" to connect two 14's to the "y" to lower the pressure.

[@SteveDuce]

How did you terminate the added return ducts? My house is very similar and same size and I only have an 18 by 18 return on a 14" duct in the hall. I plan to add 3 returns, one in each bedroom using using the same size return ducts here. Did you run them to one box and then another larger return to the unit (sizes of box and duct?) or did you run them to the existing return and expand the register?

[@lynns.4819]

We just had a 4ton-2stage Trane installed in our Houston home. Return grill is 20x30. We do not hear whistling, but will this be adequate for the unit over time, or will it make the unit work harder?

[@fredcarlos9425]

David, here in Houston, I design my supply @.06 FR and my Return @.05 FR, what is your say on these design numbers?.

[@NinjaTruther]

I agree with oversizing the ductwork... but, what cfm per ton are you using as well? Sizing the unit to the house, and why? Old school rule of thumb was 400cfm/ton. But, I see some people using other numbers on pages like 600cfm. Does this have direct correlation to the R-value of the home?

[@heidis8713]

Do people complain that they have to get several different filter sizes? How would you remedy this? Change filter area? Modify the return duct size? install a damper? Thanks!

[@danieldougherty9791]

So on the ductulator, you use .10 static for the supply then .05 for return?

[@chezisaac8833]

thanks i have a 5 ton unit i had replaced but they did not replace the return. i have a 5 ton goodman that i love after my unit from the 70s just recently died lol. Im pretty sure the duct sizing on the return is undersized. please any information would be appreciated thank you

[@hlyautey]

I guess you missed the lecture on control volume. You return the same amount of air as you supply. Air in equals air out. You oversize return duct so there is less pressure loss per 100 linear feet. The reason being that the atmosphere is the moving force for the return air vs the blower being the moving force for the supply air, which is higher.

[@johnvanlom7112]

david I have a 2 ton package unit Goodman 14seer where's the txv or metering device located? and. unit has no charging chart book says 11degree sub cool at95 ambient 11supheat

[@jonesacnaples]

I have two, Basic Duct Work Design, Basic Duct Work Layout, look at the theses let me know what you think?

[@NinjaTruther]

Why exactly a 14×14(or 200) on the return grille instead of the rule of thumb 12x12(144)... is the difference the duct being flex or hardpipe?? or maybe the difference in filter from the old blue media filters to the newer pleated stuff?

[@sheilabarker433]

Do you recommend using jumper ducts or transfer grilles between rooms that don't have a dedicated return or do you just undercut the doors. We are building a house with 6 bedrooms and I can't see how I can make this work without having transfer grilles or jumper grilles all over the place. Can you use a wye on a return to pull from two different rooms or does that not work?

[@fl0wmastr]

200 square inches per ton x 2.5tons = 500 square inches. my current return is 21.5x21.5" = 462.25 do i need a bigger return to have my ac return humidity? my ac cools to 74 but humidity is still 60% in florida. ac runs plenty and should take humidity out but doesn't. what do you recommend?

[@FecesandGiggles]

If the ac was over sized and per ton of the unit the main (non addition half of the house) has ample grill space and ample size flex and you are simply utilizing a return air to combat positive pressure-not wanting all the cold air inside to blow out when opening a door outside on addition half what size flex/ grill for a 200 square foot room with 8.5 ft ceilings?

[@nocomplainadana]

Mr. Jones: I noticed in one of your other videos that you are (or have been using) Quietflex duct. I agree with your .05 return and .1 supply static. I watched another video of you designing a 3 ton systems' return. Using your duct-a-lator, you have chosen 1-14" @ 700 scfm, 1-9" @ 200 scfm, and 1-10" @ 300 scfm. Those figures are wrong. The duct-a-lator you are is for hard pipe. please go to Quietflex's website and look at their calculator. correct: @.05" 14"=528 cfm 9"=173 10"=227

[@kidadd]

If you're returning twice as much air as you're supplying, where's the air coming from? You'll be creating negative pressure inside the house, and thereby drawing in air from outside the house envelope. It'll come from outside, or the crawl space, or the attic. So if you're cooling the house, you'll be drawing in lots of moist, hot air from outside, making your AC unit work harder. Or am I missing something?

[@sogriz4003]

If floor opening is smaller than grill size does that affect calculation? (ie 3"x14" floor opening with 9" x15" grill size)

[@mackendw]

I have a mobile that has a 2.5 ton AC unit. the main air supply to the ductwork uses 10" flex duct. the return air uses a single air return duct grill size of 12 X 20" with 12" flex duct back to the AC unit. The size of the mobile is approx 900 sq ft. Can I use 10" flex duct on the return without any problems?

[@user-zr7tl7fw8g]

Is it ok for me to add air return for each bedroom if so what size should the duct be?

[@Arieldny]

return should be calculated at .05 per ton or .5? thanks.

[@lizicadumitru9683]

Is it a good thing that you're returning twice as much as you're supplying?

[@eddiesellers8522]

great stuff David. as a rule of thumb do you use 1 cfm/sq ft in round metal? and do you use that rule on new construction?

[@jonesacnaples]

depends on ceiling height, and windows .75 is safer on new construction try to get help with your local contractor on advice see what he thanks

[@jessetrujillo3069]

So after all that it comes down to...If the grills don't whistle ..it must be ok

[@GAROmx]

One of my rooms whistles what do I do?

[@julianwang921]

Hi David, I reviewed your another video and you mentioned 1200CFM for supply air and 2400CFM for return air, so it doubles the air. Why in this case, you got 1200CFM return air but still stated that double air flows. Could you explain that?

[@ttali211]

Julian Wang yes I want to know this as well, in another one of your videos you mentioned that you double the amount of return air compared to your supply, which confused me. Are you saying that because the return is at .05 static pressure that it's basically double due to the supply is at .01 static pressure? Could you please clear this up David? Thank you, Thomas PS.. I really appreciate these videos, I just passed my master license test here in VA and haven't done much duct design but am trying to do so because I'm about to do my moms house and moving the air handler in the attic as well as re-doing all the duct work and want to make sure it's done right. Also could you maybe make a video on when it's appropriate to reduce the plenum size and the appropriate size to reduce to.

[@josephzoumadakis9743]

cause return is half the static of supply therfore double so they say

[@ToothyGus]

By using 0.05"/100 ft you allow the duct to be larger which causes less velocity in the ducts for the return. Remember the volume of air flowing through the system is always constant. The only things we are allowed to modify are velocity and friction losses.

[@tommiefletcher9870]

Not static friction rate and you do a lower friction rate for the return because the F.A.U.has a blower not a sucker. You are not returning 2x more air if you have a 3 ton blower you are going to have somewhere around 1200 CFM if you have enough supply duct.

[@theotsavoussis6282]

David When i was in college i was taught three types of duct sizing VELOCITY REDUCTION CONSTANT PRESSURE STATIC REGAIN Why in your video when you use constant pressure sizing do you have supply ducts at 0.1 static pressure and Return ducts at 0.05 static pressure? Shouldn't they both be 0.1 static? If you draw in twice as much return air than supply air won't you get drafts? Thanks

[@srokalapy]

+Theo Tsavoussis I wish to know what is Davids answer. I was told by some expirienced HVAC tech that I should use 0.1 for supply and return.

[@theotsavoussis6282]

Correct that is the equal friction method Friction is the same i.e. EQUAL on both supply and Return ducting

[@ashiqueashique941]

Yes I want to know the same thing..#Theo

[@buielectric]

3 tons x 200 sq.in =600 sq. in . How you say 600 cfm , ...? thanks

[@DanO181919]

I have a 25 x 30 return grill on my 5 ton carrier unit. I should have at least 1000 sq/in on my return and I only have 750 sq/in. My return grill does whistle. That doesn’t really concern me as much as my unit in the attic will sweat pretty bad. I’m thinking my small return is acting like a dirty filter or a dirty evap coil, which will put less of a load on my evap causing it to get to cold thus causing my unit to sweat. Should I add an additional return? What are your thoughts? Thank you.

[@DanO181919]

I should have at least 1000 sq in on my carrier 5 ton split system, yes?

[@newfut8713]

Are you available for questions

[@fehrhardt9368]

At 0:52, I believe the author had already stated, 200 sq in of return vent area per ton. But then 3 * 200, is stated as 600 CFM. Does this not make sense?

[@Fraustyair]

It makes sense, 200 square inches of return per ton, 3*200 is 600 square inches of return air, you only need half of the cfm's per ton. Each ton gets you 400 cfm's (rule of thumb) so he's installing a 3 ton system, which get you 1200 cfms total divide that in half for you return air and thats what you need in square inches of return vent area. Hope this helps!

[@shadovs1]

22*16

[@shadovs1]

i have return flex duct 16 inc for 2 ton unit. is it enough? cos i hear whistling

[@ttali211]

shadovs h what is the size of your return grill?

[@cutabove9046]

I believe the duct calculator your using is for metal ducts. If your using flex duct in the system you should us a calculator specifically for flex. OR, am I wrong?

[@luisrico146]

its for both

[@apetty1207]

Valid question though

[@kyhnsacheating169]

It's for duct with a smooth interior. It is not for vinyl or metal flex. Ribbed duct has more friction than smooth duct.

[@shabeelaa]

Hi where i get trane air duct calculator

[@snail5341]

www.amazon.com/Trane-TD1-Ductulator-Sleeve-Calculator/dp/B01BUGXB7G/ref=asc_df_B01BUGXB7G/?tag=hyprod-20&linkCode=df0&hvadid=198090983914&hvpos=1o1&hvnetw=g&hvrand=14338980978886305148&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9013880&hvtargid=pla-359474957905&psc=1

[@carlsaberhagen628]

I have noticed there is no *rule* in sizing ducts. Every *pro* has their own method of sizing.

[@garylargent8557]

There is definitely a rule of thumb for the good techs. They may not take the exact same steps but they typically have very similar outcomes

[@wcroot]

Does the length of the return ducts matter?

[@jonesacnaples]

try to keep them no more than 25 feet

[@jonesacnaples]

no not most of the time

[@ajayababupodila3604]

William Root j