How to Add a Room Air Duct with Speedi-Boot – Part 3

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How to Add a Room Air Duct with Speedi-Boot – mount the vent boot and connect the flexible air duct. This project is continued from Part 2.

The Speedi-Boot™ air vent is fastened by nailing the feet of the telescoping arms to the wood joists with the pre-installed nails (you can also use screws). This is where the Speedi-Boot really shines compared to a raw vent boot where I’d have to measure, saw and mount 2″x4″ blocks to build a box frame.

Speedi-Boot Ceiling Joist Mount

Remember, the Speedi-Boot can be mounted perpendicular (as shown) or parallel (turned 90 degrees) to the joists by removing the telescoping arms and reinstalling on the other side as illustrated in Part 1 of this project.

Mounting the Speedi-Boot Air Vent

Installation Tip: Lightly press and hold the Speedi-Boot against the drywall for a snug fit while nailing the feet.

Flexible Air Duct Installation

Push back the fiberglass outer jacket to expose the coiled-wire inner core of the air duct. Slide the inner core over the Speedi-Boot as shown.

Slide the Air Duct Inner Core over the Speedi-Boot Air Vent

Secure the duct to the Speedi-Boot with metal foil HVAC tape. Smooth out all wrinkles for an air tight seal.

Seal the Flexible Duct to the Speedi-Boot with Metal Foil Tape

Slide the insulated outer jacket over the neck of the Speedi-Boot and secure with a zip-tie. Trim off the excess tail of the zip tie.

Secure the Insulated Jacket to the Speedi-Boot with a Zip Tie

Lay the flexible air duct as straight as possible and cover the Speedi-Boot with insulation.

Speedi-Boot Air Vent and Flexible Air Duct

Take care to provide a generous radius on all bends in the flexible air duct to avoid kinks that would restrict the air flow.

Rigid Air Duct Trunk Line Take-Off

The new air vent branch line will be connected to the rigid air duct trunk. I strongly recommend against splicing into an existing supply branch line if at all possible, i.e. making a wye or tee connection, because this can result in insufficient air flow for comfort. However, if you’re installing a return air vent, then splicing into an existing branch line is OK.

Place the starting collar in the center of the rigid duct and trace a circle to cut the mounting hole.

Trace for Starting Collar Take-Off on Trunk Air Duct

Cut the starting collar hole with a utility knife.

Cut for the Starting Collar in the Rigid Air Duct

The starting collar is installed with the mounting fingers inside the air duct.

Insert the Starting Collar – Fingers Down

Reach inside the starting collar and bend the fingers outward 90 degrees to secure it the rigid air duct.

Spread the Mounting Fingers to Secure the Starting Collar

The starting collar is sealed to the rigid air duct with metal foil tape.

Seal the Starting Collar with Metal Foil Tape

Use only minimum length plus an extra 2 feet of flexible air duct needed to span the distance from the air vent to the starting collar. Cut the outer- an inner jackets with a utility knife or scissors, then cut the spring wire on the inner jacket with wire snips. My pocket Leatherman has wire snips at the base of the pliers tool. Fasten and seal the inner duct with metal foil tape.

Fasten the Air Duct Inner Core to the Starting Collar

The air duct installation is completed with a zip tie.

Zip Tie the Flexible Air Duct to the Starting Collar

Here’s the new flexible air duct branch line:

HVAC Flexible Duct Branch Line

HVAC Flexible Duct Branch Line

The attic work is now complete.

Ceiling Air Vent Installation

I’m ready to install the air register on the new Speedi-Boot vent. Speedi-Boots have dust covers to keep out spray paint, dirt and debris. The dust cover is especially helpful for floor installations to keep out dirt during while remodeling or adding a new room.

Speedi-Boot Air Vent Ceiling Installation

Remove the dust cover:

Remove the Speedi-Boot Dust Cover

Speedi-Grille™ Ceiling/Wall Air Register

The Speedi-Grille register is a high quality product that snaps into the vent boot, holding itself in place until permanently fastened with the included screws.

Speedi-Grille™ Ceiling/Wall Air Register

The secret to Speedi-Grille are the two sheet metal spring tabs that run the length of the register. Just press the Speedi-Grille into the Speedi-Boot and it snaps into place until the mounting screws are installed. I really liked this feature because I’m always dropping screws while trying to hold a standard grille in place.

Speedi-Grille™ Spring Tabs for Snap-in Installation

Speedi-Grille installation is a snap (pun intended). Now I’ve got two free hands to install the mounting screws.

Speedi-Grille™ Air Register Snap-In Installation

The Speedi-Grille air register is attached with two screws that fit into the register pilot holes on the Speedi-Boot. The screws clamp the register and Speedi-Boot together, compressing the foam rubber gasket of the mud ring to the drywall for an air-tight seal. Compare this to a raw vent boot that leaks air into the attic.

Speedi-Grille™ Air Register Installation

Standard Air Register Option

Standard air registers also work fine with Speedi-Boot, just install the normal way with a screw- or socket driver.

Attach the Air Register to the Speedi-Boot Vent

The Speedi-Boot is huge improvement in air vent installation speed, fit, finish, and quality.

Thanks for reading,

Bob Jackson

Copyright © 2019   Reproduction strictly prohibited.


  1. James September 5, 2010 at 1:40 pm - Reply


    I’m currently going to try and do this project myself to my own room because it becomes like a sauna in there sometimes. I have a question though. Since you know so much about this I was wondering if there is any how to on Return Air Vents that you could make or recommend. While I do feel that this WOULD improve the air cooling in my room I still thing that one of the biggest problems I will have will be that there isn’t enough circulation in my room either way. The Main (and ONLY) return air vent is downstairs in the middle of the living area. Nothing upstairs. Meaning a return air vent in each room upstairs could really improve the airflow even. So what recommendations can you make on that?

    • Bob Jackson September 5, 2010 at 2:39 pm - Reply

      Return air vents are installed more or less the same way as supply air vents. You could do one of several things:
      1) Shorten the door so there’s a 1″ clearance between the carpet/floor and door bottom for the return air to escape.

      2) If your building codes allow, use the wall cavity between the 2×4 studs and sheet rock as a return airway. You’ll have cut a 1 foot high access panel in the drywall at floor level to cut through the 2×4 sole plate and subfloor to complete the air return to the 1st floor and so on to the basement, or where ever you can tie into the main air return duct work. Not easy and not the best method due to leakage between adjoining studs, but you won’t waste any floor space when the job is complete. Be sure to choose a vertical cavity where there’s no electrical outlets, light switches, etc.

      3) Look around the attic for an existing utility cavity between the walls down to the basement. If so, you could install a ceiling return vent and flexible duct.

      4) Build an 8″ square column in an inside corner of the room to conceal a 6″ flexible air duct. You only need to frame in two sides and cover it with drywall to hold the new flexible air duct. The finished product will look like a square post in the corner as often seen in commercial office buildings where a support column is located. You’ll need to plan carefully and evaluate your floor plan above & below the room to confirm you can reach the main air return duct. If you’re lucky, the 1st floor room will also have a corner at the same position as upstairs to continue the framing, drywall and duct work. You’ll have to cut holes in the floor and ceiling of course, so plan very carefully.

      5) Consider a window air conditioner unit. Not pretty but it may be your best option.

      Just about anything can be accomplished with creativity, planning, time and skill.

  2. Shawn March 23, 2011 at 5:20 pm - Reply

    Thanks for this I’m going to try and install another vent in the master bedroom. I’m wondering why you choose 4″ materials instead of 5 or 6. Like the 4″ duct.

    • Bob Jackson March 23, 2011 at 8:09 pm - Reply

      I installed a 4″ duct because it doubled the A/C vent capacity of this average size room over the existing 4″ duct, and there was less risk of unbalancing (i.e. starving) another room by using a smaller duct. The air flow can be balanced (or regulated) by the partially closing the louvers with a larger diameter duct, but there’s an increased tendency for noise from whooshing or whistling air.

  3. Shawn March 24, 2011 at 2:54 pm - Reply

    The duct working currently being used in my attic seems big at least 6 or 7″. I’m not sure if I should try and match that size or go smaller since it’s just an additional vent to a room. What do you think?

  4. Rob April 1, 2011 at 8:17 am - Reply

    How much play room is there between the register and the boot? I want to install this in a drop ceiling where the tiles will hang about 2 inches below the joist. Do you think this would work?

  5. Marcelo Gomes April 4, 2011 at 7:51 pm - Reply

    Thank you,Bob. I am finally encouraged to add a vent in my son’s bedroom. It is on top of the garage and is hot like hell. I already insulated the garage door and doubled the insulation in the attic. It helped, but didn’t solve the problem, as we live in South Florida and the summer easily reaches the high nineties, sometimes goes over 100F. I am also planning to move an existing vent in another bedroom, which is located over the bed, to another spot on the same room. I want to use the same duct with a new speediboot. I looked at it but it looks like it will be very difficult to uninstall the old “raw” boot. It was installed when the house was built 5 years ago, using some kind of metal frame between the joist and the drywall. How can I properly seal this boot and keep it quiet there? I will leave the grill there as well.

    • Bob Jackson April 5, 2011 at 12:17 pm - Reply

      So you’re going to abandon an existing vent & vent boot so you can reroute the ductwork to a new vent, and want to seal off the abandoned vent.

      You can seal off the abandoned vent boot after disconnecting the ductwork by cutting a piece of fiberglass insulation about 2″ larger than the vent boot. Insert the insulation into the vent boot with the paper side facing the room, just like a picture inside a frame. Tape/seal the edges of insulation paper face to the inside of the vent boot with metal foil HVAC tape. Reinstall the air register to cover up the hole. This will insulate and seal the abandoned vent boot.

  6. Marcelo Gomes April 6, 2011 at 10:54 pm - Reply

    Yes, Bob. You understood my layman language into what I wanted and gave me the best answer, very fast. I’ll do it this weekend. You are the one. Thank you again.

  7. TiaZhan June 23, 2011 at 1:23 pm - Reply

    Wow! I was reviewing your instructions and had to stop and say thank you for the 110% clarity. This seems so feasible for even a simple housewife such as myself. (My husband specializes in sports stats and kindness but nothing handymanish.)
    I will try this rather than purchase a portable ac unit!
    Thanks again, Bob!
    Hey…what do you know about damaged wood floors? Hee hee, if your ever in Orlando, please stop by.

  8. James Bailey July 10, 2011 at 5:45 pm - Reply

    Thanks for the excellent tutorial! I was able to follow it and install a vent pretty easily. I ran in to two problems I thought I’d share. First, the nails that come with the Speedi-Boot bent up too easily when I was hammering them into the ceiling joists and I was unable to get them all the way in. Next time I’ll use different nails. Second – Home Depot doesn’t sell the Speedi-Grille for some reason so i bought a standard one. The screws that came with the standard grille were too big for the pilot holes in the Speedi-Boot. Fortunately I had a couple of screws that fit. I went with the 6 x 10 vent.

    Thanks again for this tutorial.

  9. Jesse July 18, 2011 at 2:36 pm - Reply

    Hi Bob

    Great site, I’m really enjoying reading all of your “real world” tutorials. Question about this one though… how do you know if your AC unit has enough capacity to handle another new vent? That is, could adding another vent somehow compromise the system? Or would the only risk be slightly lower AC pressure in all the other rooms serviced by that unit?

    • Bob Jackson July 18, 2011 at 4:57 pm - Reply

      I’m pleased you found my site helpful.

      > how do you know if your AC unit has enough capacity to handle another new vent?
      The Air Conditioning Sizing Chart is a good reference to check if your AC system is properly sized for your home. A properly sized AC system should have sufficient capacity (i.e. tonnage or BTUs) to support a couple of new vents, as there’s so many variations in floor plans between homes of comparable total square footage, window area and quality of insulation. At the opposite end of the spectrum, adding more vents won’t make up for a undersized system that struggles to keep house cool.

      For example, a co-worker says his AC system is undersized and won’t cool the house below 80 degrees F when the weather is in the mid 90’s; the home inspector noted the system was “barely” sufficient when he bought the house – but it was mild weather and he didn’t realize the impact until the summer heat was on. By comparison, the two 3 ton units on my home can drive the temperature down to the high 60’s in the same summer conditions – if I’m willing to pay the huge electric bill!

      > … could adding another vent somehow compromise the system?
      It could if the new branch line were taken off another branch line. This is why I ran the new branch line(s) from the main trunk ductwork. One or two new branch lines from the trunk shouldn’t be a problem, but a half-dozen new branch lines could be. For example, suppose your finishing a full basement and need to install new ductwork; supporting that much new living space may require either a larger or a new dedicated compressor and air handler. In my projects, the total living space hasn’t changed.

      > Or would the only risk be slightly lower AC pressure in all the other rooms serviced by that unit?
      The air flow to the other rooms could be reduced and needs to be re-balanced by adjusting the louvered vents to equalize the air flow so all rooms are comfortable.

      Thanks for asking!

  10. George P August 20, 2011 at 12:21 pm - Reply

    Planning on adding a vent to a newly converted screen room to sunroom. Although there is a vent neaby in an adjacent area you seem to shy away from splicing into it to add the line. You recommend cutting into the main line…..BUT….my main line is flex duct. there is no rigid box or plenum to cut into. So how do I add a line cutting into the main flex using speedi products?

    • Bob Jackson August 20, 2011 at 1:20 pm - Reply

      Cut the main flex duct in two where you want to take off the branch line for the sun room and install a Tee Wye. Pull the insulated main duct and branch duct lines up tight on the Tee Wye to cover and insulate the metal Tee Wye. You will need cut a circular pattern in the side of the main flex line so it will fit properly and cover the exposed metal. can custom make a fitting to your dimensions if a standard part is not in the catalog. (Disclaimer: I have no experience with A local HVAC supply house may stock the same Tee Wye.

      Thanks for reading!

  11. Gary December 13, 2011 at 9:00 pm - Reply


    Thanks for all the great advice.

    Question: I am finishing my basement into a large “L” shaped room (approx 12ft W x 35ft L), a bathroom (5ft W x 13ft L) and a back room (12ft x 12ft). I was planning on adding a 6″ register in the 12×12 room, a 4″ register in the bathroom, and 2 – 6″ registers opposite each other in the “L” shaped room approx 20ft apart. Does this sound about right? Also what size return would you recommend, 8″ or 12″? Thanks

    • Bob Jackson December 14, 2011 at 11:00 am - Reply

      The total square footage of new air conditioned space per your room dimensions is 629 square feet. The first step is to verify your AC system can handle the extra square footage with this sizing chart: Air Conditioning Square Footage Range by Climate Zone. AC 4 Life also offers professional Ductwork Design Service to take the guesswork out of a complex issue.

      To figure the proper duct sizes yourself, use the Residential Air Duct Sizer and Velocity Calculator. You’ll want to size the supply and return air ducts to stay under the maximum air flow velocities in the table shown on the top right part of the Ductulator screen.

      The big question for using the Duct Sizer calculator is: “How many Cubic Feet per Minute (CFM) do you need for a room of a certain square footage?” This depends greatly on where you live (climate zone), how well the home is insulated, what’s in the room that generates heat such as refrigerators, computers, lights, etc., and how many people will be in the room. See the Simplified HVAC System Design presentation by the University of Florida Environmental Technology Dept. For residential systems, start with 1 CFM per square foot of floor space.

      1 CFM per square foot of floor space seems to be the rule of thumb used by contractors:
      Of the 79 contractors providing a CFM per square foot estimate, 42 of them (53.2%) use 1.0 CFM/square foot — although with a great deal of variability within the group. A value of 0.8 CFM/square foot was the second most popular response (10.1%), and a value of 1.5 CFM/square foot was the next most popular response (8.9%). The mean was 1.04 CFM/square foot.
      Reference: How Contractors Really Size Air Conditioning Systems by the University of Central Florida.

      So, back to the large L-shaped 12ft W x 35ft L room is 420 square feet of floor space. At 1 CFM/Sq.Ft, this suggests you need 420 CFM of ventilation.

      Assuming two supply ducts in the large long L shaped room to distribute the air flow, 210 CFM per duct is needed for a total of 420 CFM. Enter 210 CFM into the Ductalator Airflow Supply field and 420 for Return value and press the Calculate button. The result is 8.68 inch diameter for the two (2) flexible ducts and 11.24 inch diameter a single return duct. Rounding up, the calculator calls it in rounded figures 9″ supply and 12″ return flex duct.

      As a sanity check, I examined the HVAC supply ducts in the larger rooms of my home. A room similar in size yours has two 4×14 inch floor registers supplied by two 8 inch flex ducts with a single 16×8 inch return grill.

  12. Gary December 14, 2011 at 10:20 pm - Reply


    Wow, thanks for the awesome info and resources! In your room similar in size to mine, with the two 4×14 inch floor registers supplied by two 8 inch flex ducts, what size duct does your single 16×8 inch return grill have? 12 or 10 inch?

    • Bob Jackson December 15, 2011 at 11:12 am - Reply

      The 16×8 inch return grill uses the 2×4 stud wall cavity for a return to a vent boot in the subfloor that connects to an 8 inch flex return to the rigid trunk line.

  13. Dave January 6, 2012 at 3:21 pm - Reply


    Thanks for advice. I notice in your article you say to avoid branching off an existing line, rather use the main. One my third floor, I have a 10 inch elbow off the unit going to 10 inch flex to a tee with reducers to two 8 inch flex to registers. I would like to finish off some additional space on this floor which will require two new registers. I could splice off this 10 inch flex after the controller before the existing tee, and not have to deal with another controller. But if I did this this would lead to a line with three tees in it (would need to tee again to go to each register). I assume this is not a good idea? Rather add a new line with another controller and just run both to same thermostat?


    • Bob Jackson January 7, 2012 at 1:53 pm - Reply

      > Rather add a new line with another controller and just run both to same thermostat?
      That is the best approach to ensure adequate airflow.

  14. Niwashikun April 25, 2013 at 10:05 am - Reply


    Great tutorial, very clear!

    Would the Speedi-Boot/Grille work in a ceiling with 3/4″ shiplap behind (above) the drywall? The drywall is 1/2″, so a total thickness of 1 1/4″ give or take. Will the grille be able to be attached properly given the additional thickness of the ceiling? I’ll have to order through the mail so I can’t go check in person.


    • BobJackson April 25, 2013 at 7:38 pm - Reply

      If you find that the included grille mounting screws are too short for the thickness of the ceiling, then buy a set of #8 x 2 inch long white hex head sheet metal screws. Your local hardware store may only have zinc (shiny finish) #8 x 2 in screws. If so, spray paint the screw heads white before installing to match the grille.

  15. Yash June 17, 2013 at 8:42 am - Reply

    Hello Bob,

    Great turorial with step by step pictures makes me confident that I can do it return air work. Although I have a question for you. On my second story where the bedrooms are it’s like lobby area of around 4′ x 30′ and my thermostat is located in that lobby. I just have return air at this lobby which makes too warm during the summer time and that’s the reason my ac keeps running all the day long. I am planning to put another return air or ac drop at the end of the lobby so it can air circulate and keeps that lobby cool. What do you suggest? Can I just add drop/return air to the existing line or I need to run complete new line?


    • BobJackson June 17, 2013 at 11:49 am - Reply

      To summarize (correct me if I’ve got this wrong): The long central hallway on 2nd floor where the thermostat is located has a single return air vent and the hallway is too warm, causing the A/C to run more than necessary. You’d like to install a new return air vent at the far end of the hallway for better air circulation so the temperature read by the thermostat is representative of the 2nd floor bedrooms.

      > Can I just add drop/return air to the existing line or I need to run complete new line?
      It’s best to run a new branch line duct from the main return air trunk to a new air vent in the ceiling at the opposite end of the hallway from the current return air vent.

      What is the interior diameter of the existing return air duct in the hallway? Another option is if the existing return air duct is undersized, say 6 to 10 inches in diameter, you could upgrade it to a 14 or 16 inch duct if it’ll fit the trunk duct. Remove the existing air register, vent boot and ductwork, then install the larger duct components.

  16. Jon Wymore August 26, 2013 at 6:59 pm - Reply


    I have a question. I have a late 1950’s home and I tried to find where my rigid trunk air duct was but it looks totally different than what you’ve posted. What should I look for to try and ad a new line to a converted garage area?

    • BobJackson August 26, 2013 at 8:01 pm - Reply

      A 1950’s home rarely had central air conditioning, so it was an upgrade added years later. There are three types of supply air duct:
      1. Rigid metal duct – round or rectangular, usually with an external insulation jacket or wrap.
      2. Rigid fiberglass duct – rectangular like my trunk ducts.
      3. Flexible duct – easily identified by the helical wire coil inside.

      Start at the air handler and follow the air supply ducts, it should connect to a main distribution trunk somewhere nearby. However, I’ve seen “octopus” or better “ductopus” configurations where the branch ducts are all connected to the supply air plenum and taken directly to each room… usually an indicator of a poor design and installation.

      Take care that asbestos was used to insulate metal air ducts before it was banned in the United States in 1977 in case your system is really old.

      > What should I look for to try and ad a new line to a converted garage area?
      Converting a garage to air conditioned space has it’s own challenges because the exterior garage walls may not be insulated. You can check if the walls are insulated by drilling a 1/4″ hole in the drywall and insert a hooked wire to poke around. Be careful to stay clear of electrical wiring. Foam insulation can be injected between the wall studs to insulate the garage walls; there are professionals who specialize in this.

      Installing air ducts in the garage can be easy to difficult; easy if there’s an attic over the garage that you can access to install supply and return ducts – or difficult if the garage is below bedroom. Worst case you could build an interior soffit to conceal the ductwork along the corner of a ceiling and wall. It all depends on the layout and construction of your home and A/C system.

      You can e-mail photos to bob (at) for specific recommendations.

  17. Michael January 4, 2014 at 7:39 am - Reply

    Great advice! I live in southeastern TN and I just enclosed my 9×15 front porch. The weather here is typically mild both summer and winter. I want to add heat/air to the new space. I checked the capacity of my existing HVAC unit and it’s already close to capacity. My original idea was to tie into an existing vent about 4 feet from the new space. After reading this I see you typically shy away from that. Any suggestions?

    • BobJackson January 4, 2014 at 11:09 am - Reply

      The problem with tying into an existing branch duct is that duct is probably sized to serve the just the one room. A new takeoff on that branch duct may reduce the airflow such that both the original room and the enclosed front porch are uncomfortable. The best solution is to run new supply and return ducts from the trunk duct. I realize this can be challenge depending on attic or crawlspace access, etc. An interior soffit is useful for concealing ductwork if crawlspace access isn’t possible. Also see this comment about building an interior soffit.

      Supplemental heating for the enclosed front porch is easy with an electric space heater plugged into a wall outlet. Summertime cooling and humidity control requires the central AC system, so I think cooling will be the stress test if the duct size is too small or there’s not enough airflow.

      Take a look at these ductwork design and installation references which may help with ideas:

      * Ductwork Installation Guide

      * This comment talks about duct sizing and has a link to the Residential Air Duct Calculator.

      Let me know if you have questions.


  18. Alex Jones July 25, 2014 at 4:20 pm - Reply

    First off I want to say THANK YOU for this great article, it was VERY helpful. A few comments about my experience. First off I am VERY jealous of all the room you have in the attic, I was not so lucky. I have a sloping roof so it was tough and it was quite hot so I was miserable!

    I would be happier if the Speedi folks made it able to staple to the beam versus forcing that low hanging nail like that. It isnt a strong nail and the area is SUPER tight, a staple would be a better solution. Even a screw is too tight and the wood is so strong that it makes it tough.

    The other comment is placing a non-Speedi register in was a challenge, I have it placed as I should but I could not get the screws to connect to the unit so I will have to deal with that quite soon..digging out the drywall more and try and get the screws to sit properly.

    The tips were a huge help, I had everything I needed from the metal tape to zip ties and that was a big bonus. I was also unlucky to have a metal air handler so I had to deal with cutting the metal and I had to improvise.

    Thanks again

  19. Mike Judge December 15, 2014 at 9:36 am - Reply

    I’ve noticed that you’ve interacted personally with those folks with specific questions. I’m hoping you may do the same for me. If so, thanks in advance.
    I live in a fairly new house in the upper Midwest. We have a forced air furnace/AC that supplies branch lines to all the rooms from a large rectangular sheet metal (main trunk line) duct attached to the ceiling joists in an unfinished area of the basement. We have a finished basement rooms on the other side of the wall adjacent to the furnace area. One is a bedroom that gets a little “cool” in the winter in relation to the rest of the house. It has its own return vent and one heat/AC supply vent in the ceiling. Since this bedroom has only one supply vent, I would like to add another one. The problem is the finished drywall bedroom ceiling. I can easily tap into (install a start collar) on the main supply duct and run the new ductwork through the rafters from the unfinished side of the basement. The problem is that I don’t have access to the framing (inside the bedroom ceiling) to install support for the register boot box. It’s covered with drywall and the main floor is directly above. I can easily locate the proper position to cut the ceiling hole for the boot. How can I install and secure that boot/vent box through the hole in the finished ceiling? Oh yes, additionally, I would have preferred to just install the vent in the common wall between finished and unfinished areas, but there happens to be a bedroom closet there. I’ll have to use the ceiling pocket and install the vent beyond that closet with a run that will be approximately 4 feet.

    • Bob Jackson December 15, 2014 at 4:31 pm - Reply

      A couple of ideas for installing the new branch line come to mind:

      Option 1: Route the new duct through the common wall between the unfinished and finished sides of the basement through the bedroom closet. Build an interior soffit along the closet wall & ceiling to conceal the duct. A larger version of an interior soffit is shown in this project.

      The advantages are easier access, common building materials and no complicated drywall measuring and sawing. An interior soffit for 6 inch insulated duct wouldn’t be overly large.

      Option 2: Install a drywall access panel in the bedroom ceiling so you can mount and connect new vent boot. I installed an access panel in the finished basement drywall ceiling to reach a concealed natural gas fireplace valve.

      The disadvantage is more measuring and sawing the drywall. The benefits are the drywall access panel will be useful for future maintenance activities such as installing new electrical wiring and plumbing repairs.


  20. Anton January 10, 2017 at 11:02 pm - Reply

    What a great post !!!
    I just recently moved into a house which was built in 1945. It is a 2 floor house. Recent owners just added two new rooms on second floor. They haven’t installed yet HVAC Heating, A/C.(No duct work)

    Smaller room size:
    20ft length by 12 ft width = 240sq feet
    240/1.25 = 192 CFM

    Larger room size:
    20ft length by 18 feet width = 360 sq. feet.
    360/1.25=288 CFM

    Those two rooms are located next to each other. It is very cold in there. Those 2 rooms built on top of unfinished garage.
    Furnace located in the basement. I do have window in the basement, which I can take off and run the duct work from furnace to those rooms.

    Do I need to install 1 register and 1 air return in each room? Or I need to install 1 register in smaller room and 1 register and 1 air return in larger room?

    What is the best way to run the supply registar and air return ducts?
    How do I find from where do I have to run the supply air for hot air and how do I find from where do I need to run the air return duct work?

    Please help,

    Thanks !!!!

    • Bob Jackson January 11, 2017 at 7:02 pm - Reply

      Is the proposed new duct for heating only or heating & cooling? If for heating only, is the furnace large enough to handle load of the two new rooms over the garage? See the Furnace Sizing Calculator. Also see the Air Conditioning Sizing Calculator if you have a central HVAC system.

      If the existing furnace is too small in terms of BTU capacity, it may be easier and less expensive to run new electrical branch circuits to baseboard heaters. Ceiling fans would help circulate the air.

      Let’s assume the furnace has sufficient BTU capacity:
      Why did you divide the room square footage by 1.25? It’s better to assume 1 CFM air flow per square foot of floor space and throttle the air flow if needed with louvered registers than to have too little airflow. Have you tried the Residential Air Duct Sizer and Velocity Calculator?

      The 20×12 ft room could get by with well placed single supply and return ducts/vents (see Figure 1 – Ceiling Register Locations on page 16), but two would be better. The 20×18 ft room should have two supply and return vents/vents to prevent stagnant areas.

      Does the attic over the garage connect to the attic over the main house? Is there a trunk duct in main attic? If so, what size is the trunk duct? Compare the size of the existing trunk duct to the additional air flow capacity estimated by the Residential Air Duct Sizer calculator to determine if it’s large enough to support branch ducts to the new rooms over the garage.

      Let me the particulars for more specific advice.

  21. Steve prebyl February 20, 2017 at 6:59 pm - Reply

    Hi bob, i have 2 heat runs to a second floor bedroom with no return air. Can I use one of those runs as a return air? I have no way to install return because of kitchen directly below bedroom!

    • Bob Jackson February 20, 2017 at 7:46 pm - Reply

      Missing return ducts are an all too common problem. I don’t recommend repurposing a supply duct as a return because the room will be too cold in the winter and too hot in the summer.

      Your return air options are:
      * Undercut the door
      * a transfer grille in the wall
      * a jump duct in the bedroom ceiling to the hallway
      * or a Tamarack Perfect Balance In-Door Return Air Pathway.

      See the Tamarack link above for details.

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