HVAC Return Air Ducting

63 responses

  1. Tim Carter
    March 30, 2012

    Barb, you get my Furnace Checklist.

  2. Chris
    March 31, 2012

    Good information, thanks. I am considering remodeling my basement. My basement has a cold air return that services the first floor. It runs across the basement ceiling, perpendicular to the ceiling joists, sitting right below them. This return spans my entire basement. during the remodel I could just box it in, but the size of the return after boxing and then covering with sheetrock or paneling would really create a low-point in the room. The return is rigid foam about 12 in x 12 in, so its bottom point is below the main beam that supports the first floor joists. Are there any options for changing the shape of that return to a lower profile, so it doesn't come down a full 12 inches?

    • Rick
      April 16, 2012

      I have the exact same question. The return ducts in my basement are exactly 1/2" below the top of my head (ask me how I know). I'm hoping that a shallower, wider duct would give me an equivalent volume of airflow with less frontal lobe damage. Could I swap my 8" x 14" duct with 5" x 22-1/2", or are there fluid dynamics issues that would make this not an equivalent amount of airflow? Thanks!

  3. christopher
    May 9, 2012

    I am remolding a 97bottom year old farm house and i put central heat and air in it so i was wondering if you can put the cold air return duct in the ceiling in one corner and the actual duct in the ceiling in the other corner.

  4. jim
    September 24, 2012

    i live in an 2 bedroom 1200 sq ft hi rise condo in atlanta ga. the hvac unit is in the center of the apt and supply lines are install up high ( since AC is predominately needed due to climate and the heat is easily retained from the concrete mass. the closet is only slightly larger than thewidth/ depth of the hvac unit. suplly is facing upward. the return is located on the bottom of the unit directly facing the floor. directly underneath the unit is a 50 gallon electric water heater. the space between hvac and water heater below is 3 inches. even if i switched out water heater to something like point of se on demand i am at a loss of how to run return lines from the two bedrooms. the living room is fine as this closet is in the living room. is there a way to use the skinny tube return?

    • Tim Carter
      January 6, 2013

      Jim, your question requires lots of typing, plus I have some questions for you so I can give you the correct answer(s). I only do pithy answers here in the comment section. If you want to protect the investment you have in your house and not waste time or money *hoping* you make the right decision, you should talk to me on the phone for just 15 minutes. It'll be the best investment you've ever made in your home!

  5. Herman
    October 6, 2012

    I have a two story home. The duct work for the return upstairs goes nowhere. The main floor has no duct work and only runs through the joists in the wall is this normal?

    • Tim Carter
      January 6, 2013


  6. Amer
    November 19, 2012

    I have bought my house and the bed room was added but there is no return vent.. The room is cold almost 10 degrees less than the rest of the house ,,
    Should I open ahole in the bottom of the closet that lead to the living room and but admell fan to blow the cold air from the bottom to act like return

    As I know cold air stay down and hot up what I think ?

    • Tim Carter
      January 6, 2013

      your question require lots of typing and I have some questions for you. You should consider investing in a 15-Minute Consult with me. Look at my cart for that. Lot's to discuss. I just do short pithy answers here.

  7. Damon rauso
    January 22, 2013

    We have a 7year old house, very little or no cold air return upstairs. The registers are there, but they never cut through the floor to the basement. They, heartland homes, placed plumbing pipes through them and braced them at the floor with 2x4's, blocking everything. I ran a snake with a camera down them. Is this legal or correct or do I have a solid complaint against them? I'd rather not cut through all my newly painted bedrooms on my dime. Really frustrated with the mechanics of this builder.

    • Tim Carter
      February 24, 2013

      This is a code issue. You know who to call.

  8. Gregg
    March 10, 2013

    I just did an inspection on a contemporary house built in 1985. It is a 2 story house. The heat is hot water / base board. There is central AC. There is only one Return (which is on the second floor directly under the air handler). What are the implications of this? Also, when we had the fan running the air coming through the vent was VERY loud. It was a wooshing sound. Is this normal? how can we preven this?

    • Tim Carter
      April 1, 2013

      Gregg, Walter, this is what my 500-Second Consult is all about. Click the Shop icon at the top of the page!

  9. Mick
    April 20, 2013

    I live in a single floor ranch house in the northeast (Western NY State). I'm building a fully insulated, airtight, multi-use soundproof room in my basement. It will mainly be a home theater/lounge type area. The space is not a pure rectangle but almost. The last 10 feet of the room shifts over about 6 feet. In total it's about 3450 cubic feet of area. I have a capped off 6" supply run stubbed out about 2 feet from the furnace plenum that was intended for heating/cooling the basement.

    I plan to "y" or tee that stub and run two separate supply runs. Each will use a damper door for seasonal flow control adjustment. Each run will supply one end of the room. Given that, I expect I'll need one return line for each supply line introduced to the room.

    I've been told I don't need return air vents in the room but I'm not buying that. I've read that the returns should be 1.5 times the supply volume to assure proper air flow but I've also read they should be equal. I assume that refers more to the trunk line volumes so I was simply expecting to run two 6" return lines to the room from the existing cold air trunk to match the two 6" supply lines. I thought I'd place the return registers opposite each other (about 15 feet apart) midway between the supply registers so they each draw from the center of the room. Is this a plausible location?

    I may be over simplifying things but will using two 6" heat/AC supply runs and two 6" return lines be sufficient for my room volume? I'm not looking to rework my HVAC system I just want to make sure that I can heat and cool the room to a reasonable comfort level. I don't have zone heating which means the thermostat is upstairs so this new room is going to be at the mercy of the upstairs living area but hopefully, I can use the dampers to achieve a decent comfort level for watching movies or just hanging out with friends.

    Any advice as to whether I'm on the right track or not?

  10. jeff
    June 18, 2013

    Its a rule in hvac if you have any unit change out say from three ton to now a three and a half ton ac that the return air duct is at least 18 inch to 20 inch return remember its only going to give what it gets so if you have no velocity out of the registers threw out of the home look to insure that its not running on too small of return air flow duct and filter grill air can

  11. jeff tech
    June 18, 2013

    When ever you change out a split system unit were the condenser or out door coil is changed in order to get the seer rating seasonal energy efecancy rateing value of this the indoor coil must be matched to the new condenser and or change the metering device in some cases this may be posible but its allways best replace indoor air handler and evap coil to get top performance out of the new seer ratio wich is nothing more than a larger out door coil.

  12. Peter
    July 17, 2013

    I had an addition put on to back of house and the return air vent was along the baseboards on the wall underneath a window, this ultimately became the opening into the addition, thus closing off the return vent. I would like to place a floor register for return air just next to the doorway using the floor joists as the return in the basement ( which is how they currently are). Problem is the space I want to put new return falls right next to current return bay. How can I transition the new return vent into the old bay ( joist are 2 x 10 ).

  13. John Aikenhead
    November 14, 2013

    I have a 4yr old two storey home in Ontario Canada, I am finishing the basement with 1 Family room - 1 Office/Bedroom - 3pc washroom plus workshop and utility room areas. Ive installed the heating ducts already but I need to know if its essential to install cold air returns in all or some of these basement spaces, if so does it need 5" ducting or will 4" be sufficient. Also in the basement should the side wall duct grills be at the top or bottom of the walls, in the rest of the house on the ground floor the return air registers are at floor level while on the top floor bedrooms the return air registers are at the top of the walls.
    Any assistance appreciated.

    Rgds; John/

  14. Tim
    January 29, 2014

    I have 6 inch supply line for HVAC in two new bedrooms but can only put in 5 inch lines for return without some major remodeling how much problem will this create?
    Thank you.

  15. nancy hummel
    May 18, 2014

    We have a 2 story house with a crawl. Recently we installed
    heat pump duct work. the installer installed duc vent into floor not wall.
    Is this ok? Should the duc be installed above the floor for better
    operation? thanks nancy

    • Tim Carter
      May 22, 2014

      You need one of my 15-Minute Phone consults. Too much to type.

  16. Gary
    June 17, 2014

    I have a two-story house with a walk out basement. The basement is 10° colder in the summer then the rest of the house can I put a cold air return in the basement? I would like to circulate the cold air from the basement to the rest of the house it's a finished basement.

    • Tim Carter
      June 18, 2014

      Yes, you can do this to capture the cooler air. Just be aware that in a few days the basement will be hot and humid. What's more, you could set up a condensation issue in the basement as you introduce lots of humidity on the cooler wall and floor surfaces. Mold follows condensation.

  17. Frank
    June 19, 2014

    Multiple return ducts? So, bought house. It's 30 years old. Looks like the added the central air later on in it's lifetime. Blower unit is in the attic . It's a 2 story raised ranch. In the attic, it looks like a quick , lazy job. One end of the attic, is the unit with a 2x 2 feet intake duct in the ceiling of the hallway just under the unit. The outgoing air duct goes from one end to the other of the attic with several off shoot ducts to the outer walls of the upper rooms. But is also just holes in the ceiling of the upper floor. Just along the outside wall. Is this correct installation. I myself, think not. We're should the intake duct or ducts be located. And the out going air ducts. Should they be in ceiling or walls? Upstairs, downstairs, etc... Cheers!

    • Tim Carter
      June 19, 2014

      I suggest you read all my ductwork and HVAC columns for your answers..... All the info you need is right here at my website.

  18. Jane
    June 29, 2014

    I had a new 3-ton unit installed in 2010. It replaced a 2.5 ton. My house is 1200 sq ft ranch with ducts in the crawl space. The original duct work is 14 inches and I have been told I need to increase to 16 inches. I have broken straps and sagging and it looks like something has been chewing on the cover. Do I need to repair the 14 inch or tear it all out and install 16 inch? My return in floor is 14X20. Is this correct size for a filter?

    • Tim Carter
      July 5, 2014

      You simply get the correct filter that fits into the rack with no gaps.

  19. Steve
    July 1, 2014

    Another possible return air duct is an abandoned chimney. I have a 90 year old house with a chimney running up the center of the house. It was knocked below the roofline when the roof was shingled last. I knocked it down even more, built a cap for it, and ran insulated flex duct to each bedroom, then made a connection between the chimney and return duct in the basement.

  20. Tarek
    July 3, 2014

    Hi, I have a two story house one year old and the bedrooms upstairs are colder in the winter and hotter in the summer by close to 10 degrees depends which room. Of course the builder sends his HVAC company to tell you everything is working within spec, the house temp not comfortable at all. I noticed the the return is using the cavity which is drywall and joist but doesn't have sealant anywhere just an opening. How do I know if the problem is with the return? Another thing that I have noticed is I have few ducts for the heat/cold are running on the outside walls. Please let me know what you recommend.

    • Tim Carter
      July 5, 2014

      I recommend you purchase one of my 15-Minute Phone consults.

  21. Karen
    December 11, 2014

    Tim, we just bought a 47 yr old ranch in St. Louis, MO. It's 1000 sf, 3 small bedrooms and 2 baths (one very tiny). There is only one air return vent in the hallway; it's 14" x 30". Should I replace it with vents in all the other rooms? 3 Bedrooms and living room? We have a kitchen but no dining room. Those are all the rooms. The vent in the hallway is huge and almost touches the two bedroom doorways on either side of it. I want to put new 1x4 trim around the doors but it won't fit because of the big vent. Thanks.

    • Tim Carter
      December 11, 2014

      If you want the best heating and cooling experience, you'll have your supply registers on exterior walls and a return air in each room on the opposite wall. The air is PULLED through the room. You NEVER put a return air in a kitchen or bathroom.

  22. Joe
    December 26, 2014

    Hi Mr Carter , I know this can't be right, this was done before I purchased the house . The previous owers had some work done from a well known co in the area and they added I think two or three return ducts one is at the highest point in the the two story house in the hallway. but It runs down from the attic to the basement in the square box that the chimmeny runs .

  23. James C
    December 30, 2014

    Hi Tim,

    Thanks for the info. I am confused by one thing, you say to use wall cavities as a return but a lot of other thing I've read including a code for homeowner's book says this is a no-no because there's no way to make an adequate seal. I have a small ranch (900 sqft footprint + half finished basement.) Since the walls are all being redone anyway, should I just put in the proper vent boxes?

    • Tim Carter
      December 30, 2014

      Believe me when drywall is screwed to the wall studs that creates the return air duct, it' pretty sealed!

  24. Karen
    January 2, 2015

    My house is about 60 plus years old there was horse hair round the windows for insulation . Humidity can b very high in the winter we constantly have to run a dehumidifier in the house and nothing can b against the outside walls in the rooms or mould can appear . Can u run wall returns if house only has a crawl space

    • Tim Carter
      January 8, 2015

      Yes you can if you have a great installer.

  25. Kevin
    March 14, 2015

    Good information.

    Your section on reversed air flow in older homes was almost funny. When I read 80 year old house, ducts in internal walls, and particularly the one giant return air vent, I felt like you were sitting in my living room as you wrote it. Having never seen such a large return air vent (15" x 30"), it was researching the proper size of a return vent that brought me to your site. I say almost funny, because at the end of that you say all wrong.

    I just recently bought the house, and while I wouldn't say it in disrepair, it does certainly need serious updating and has been neglected by previous owners. After reading your article, I now realize that I am going to have to get a HVAC guy to overhaul my heating/ac system, and that because of the serious ineffeciency of my current system, it is much more of a necessity than just an improvement. I also now have more of an idea of what I will need to ask the contractor and the kinds of things to look for and ask about to maximize efficiency. Thanks for the information, at least until I have to pay the HVAC contractor. All I really wanted to know was if my return was too big. LOL.

  26. karen
    April 5, 2015

    I have a question for you guys. I am renting a little cottage that previously had electric baseboard heat. They have now installed a furnace which they seem to have used some of the old duct work for the installation. I'm noticing a slight moldy smell in the house now which is very similar to what the basement smells like, mind you it is a dirt floor basement. Anyway, I investigated the return duct and indeed they used old ductwork. when I take the grate off of the duct, I can see there is a very dirty black fiberglass like lining inside of the duct that seems to be the source of the smell. My question is shouldn't the company that installed the new furnace have also installed a new return duct or at least ripped out that dirty filthy insulation and replaced it? Do you think I can just rip that stuff out? could it be asbestos? any suggestions comments/feedback?

  27. Deb
    May 12, 2015

    Our return on the second floor is part of the same run as the return on the first floor (both feet into the handler in the basement). Will this EVER draw enough air from the second floor of a cape cod?

    Secondly, the return is just part of the void in the walls, and on the second floor it is open to the bathroom plumbing, as well as all floor voids and one bathroom (!). Could I DIY the metal sheathing for the return, do you think? I'm small enough that I could crawl down in there and screw and seal joints....

    Thank you,
    An uncomfortable, but enterprising, gal from Tennessee

  28. Dale
    May 22, 2015

    I have a 1,100 square foot raised ranch with forced hot air. The duct work runs between the two floors - lower level has ducts in ceiling while the upper level has the ducts in the floor. I am looking at adding central air - will it suffice to just use the existing duct system or should i have the system installed in the attic with ceiling ductwork on the upper floor?

    • Tim Carter
      May 23, 2015

      This is a little complex. I don't have the time here to answer you in the detail required. If you make a mistake here, you'll be uncomfortable and the fix is THOUSANDS of dollars. You may want to consider my 15-Minute Phone Consult.

  29. ariel
    May 25, 2015

    I currently have a Trane 4 ton unit. My return air is a 20x25x1 with a duct diameter of 16". The tech told me the size was good enough and did not need a second one. Is this correct and if I need another one what size do you recommend?

    Thank you!

    • Tim Carter
      May 25, 2015

      Read my past columns about AC Sizing. Type that into my search engine here. You have to do calculations to determine your Heat Gain. Once you know that, then you can get the correct tonnage. Do NOT GUESS or HOPE it's right.

  30. Ken
    May 28, 2015

    I have a 60+ year old colonial in Connecticut with central heat/air. Supplies and returns at bottom of walls in most rooms downstairs and upstairs. I also have a laundry chute we don't use in the upstairs landing, between all 3 bedrooms. Upstairs is very warm in the summer. If I were to convert the laundry chute, are you suggesting I convert it to a return-air instead of a supply? Furnace/air handler is in basement.

  31. Roger
    June 29, 2015

    If each bedroom of a 3 bedroom house have A/C supply and the only return is in the main hallway, what should I be checking for in terms of overall system functionality?

  32. Aleks
    August 19, 2015

    Hi, I'm currently finishing our new build and noticed that our HVAC contractor put a vent right next to the front door!!! I was told by some friends that it is absolutely a bad idea to have the vent by the front door since every time we open the door we would loose hot/cold air?. When I asked my builder and contractor about it I was told they always put a vent by the front door for the same reason my friends who are also contractors warned me about, that when we do loose he warm air we need the vent to be close by the door so that it could hear up the room quicker. can you please advise of having a vent by a front double door is acceptable and if their is any exception to have one there?
    Thank you

  33. Fred Bacher
    September 7, 2015

    I live in a nightmare house constructed in 1987, San Diego, 2- story, 2100 sq ft. cathedral ceiling in living and dining room. 4 ton AC and 80% carrier (1987 model). They really botched the cold air return and eventually located it in the family room about 5 feet away from the furnace and water heater sitting on a 2 foot tall "box" in the furnace room. The return was way too small and I've managed to open it to only 300 sq in. I think I can double this by moving the water heater about 6" and adding another bar grille filter assy above the first. But they are both downstairs, which is typical of SoCal construction. Should I make every effort to put on on the 2nd story? It might cost a fortune if it's even possible. Up stairs is always too warm in the summer. Thanks

  34. William Varner
    October 4, 2015

    I am currently installing a air circulation system in my 2300 square foot home. I heat with a wood heater in the center of the home and it is to warm in the center, but the ends of the house are chilly. My plan is to place 2 registers on each end of the house and use them as return air. each one would connect to 8 inch flexible insulated duct which would run about 35 foot each to a Y. The inline fan is a 10 inch fan supposed to deliver 780 CFM. I would connect the motor to the 10 inch on the Y and exhaust the air over the stove with 10 inch flexible ducting. This would circulate warm air through the home. My question is will flex duct be sufficient on the negative side of the fan or would two 8 inch duct not be able to handle the vacuum? Is my sizing in the ducts adequate? Any help would be greatly appreciated!!! Thank you very much.

  35. George
    March 10, 2016

    Thank you! This was very interesting to read as I have an exam tomorrow on HVAC 🙂 Cheers from Toronto, Canada.

  36. Kent Irwin
    March 21, 2016

    I'm buying a condo, 1750 SF, 2 br 2 bathr, totally open floor plan 15x29 great room with vaulted ceiling to 2nd floor loft bonus room nothing else on 2nd. No cold air returns anywhere, in main first floor hall big wall vent down low I assume is a return. Central gas heat and AC. What and where do you rec returns?

  37. Maya
    May 12, 2016

    Hi. I'm having a home built and the return vent for the second floor is actually in my master bedroom on the ceiling. I have never heard of it being placed in a bedroom before. Since its located there, the builder made cutouts for vents over the two bedroom doors. Is this common? I'm use to it being in the hallway area. Should I be concerned?

  38. Andy keyz
    May 14, 2016

    I have a 2200sq ft home with 4 levels, it was built in 2012. I have 6 return vents in the home with the one being closet to the unit being the only one with a real suction return vent connected. Is there a law or ordinance when a builder builds that they are required to connect these vents? They put grill. overs over cut outs in 5 out of 6 rooms which are located upstairs.

  39. Sara Croft
    June 26, 2016

    Silly question. I have a cabinet I want to put near my cold air return. How much space should I leave?

  40. Earl
    August 13, 2016

    I have a two bedroom 1200 sq.ft bungalow with floor registers in all rooms with two large 6"x24" floor grills in the central hallway.

    The one grill draws return air from the open concept living-dining room as well as from two registers in the sunroom. This grill is farthest from the furnace fan.
    The second grill within 24" of the return to the furnace fan and draws air from two bedrooms and two bathrooms.

    Q. Would it be better to close off part of this one closest to the furnace or the first one or neither?

  41. mike
    August 13, 2016

    Hi I was wondering if it OK to run my apt ac system without one of the ceiling vents installed? I took it off to clean the large amount of buildup dust, and when it kicked on it seems like there is a lot more air coming out of the duct than when this vent is in place, which is rusting anyway. thank you

  42. Dennis DeBoer
    May 7, 2017

    My supply's are on the floor and my returns run all the way up the wall with a vent opening low and high. Each one has a grille and a register but not all rooms have them installed the same. I'm trying to figure out if the register with the close-able louvers should be installed on the top or the bottom. I would think in the winter I would want to draw the heat down through the bottom vent so would want the louvered vent installed on top so I could close it in the winter months. I guess that would mean in the summer I would open the top and I would be returning through both openings. Am I correct in my assumption?

    • Tim Carter
      May 8, 2017

      I just revised the column above and cover your situation with new content. Look for what I say about the Location of the return air grills / ducts.

  43. Frank Nicoletti
    May 9, 2017

    Hi Tim, I love your Ask The Builder site. Recently I had my A/C and heater replaced and noticed that in the return duct just above the unit in the basement of my ranch house there is a return at the top just under the floor a return duct that is 15in wide 10in high is that a good idea? I just noticed because of the location of the unit. Thank you for any info you can give me, Frank

  44. Ed Pohlman
    May 22, 2017

    Extra bit of helpful info: returns (vents) are to be at least ten feet from a gas or other combustible furnace so as to avoid back or negative pressure and draw carbon monoxide into the supply registers. Keep this in mind when renovating a basement.

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