This tutorial explains how to add to your existing ductwork by installing a new heating/cooling air vent to a bedroom for increased comfort.
HVAC Problem
My daughter’s bedroom was too warm in the summer and too cold in the winter because it had just a single air duct from the central heating and air system. By comparison, my son’s bedroom is about the same square footage and has two supply air ducts that keeps it comfortable year round. I therefore decided to install a second air vent to the ductwork for my daughter’s bedroom.
HVAC Flexible Air Duct Branch Line
Room Heating and Cooling Factors
The bedroom is located in the front wing of the house and I found access to that part of the attic is difficult. The attic insulated with blown fiberglass – white fluffy stuff that looks like cotton. I found that the attic over my daughter’s bedroom had little or no insulation in large areas because of the limited access; instead the workers just piled the insulation near the entrance to this part of the attic – correctly assuming that no one would see the sloppy work… until now. Proper insulation is critical for climate control.
Speedi-Boot™ – A Faster and Better Way
Please see this newer project for a faster and better way to install a room air vent using Speedi-Boot™, which is available at Home Depot.
The remainder of this article explains how to install an air vent using a conventional vent boot.
Air Duct Installation Materials
Air Duct Branch Line Materials
The materials for the six inch diameter branch line cost about $60 at Home Depot:
- Starting Collar – 6 inch diameter
- Insulated Flexible Duct – 6 inch diameter, 25 feet length
- Register Box – 6 in by 10 in
- Register Grille – 6 in by 10 in
- Metal Foil Tape – one roll
- Nylon Zip Ties – 36 inch
Tools
The tools needed for this project are:
- Handsaw
- Carpenter’s square
- Tape Measure
- Step Ladder
- Cordless Drill/Driver
- 3 inch Wood Screws
- 1/2 inch self-tapping sheet metal screws
- Flashlight
- Cordless Fluorescent Light (very helpful)
- Rotozip
- Sabre Saw (or Jigsaw)
- 15 amp Heavy Extension Cord
- Hammer
- Small Finishing Nail – 4d (penny) size
- Pencil
Duct Work Installation Guide and Tips
GAF Materials Corp. has several helpful duct work Installation Guides, Tips and FAQs. I recommend reviewing these references to customize the duct work layout to fit your particular situation.
Ceiling HVAC Vent Installation
The first task is to locate the new register vent on the ceiling. I measured the existing ceiling register and plotted the new register in an identical position on the right side of the room. A carpenters square is used to draw a perfect 6″ by 10″ outline on the ceiling where the new hole will be cut for the air vent.
I relied on the regular spacing of the ceiling joists such that the register box would be in the gap between the ceiling joists. Before cutting the drywall, I drove a finishing nail into the ceiling to verify there were no joists, making a series of small holes as shown below. If I made a mistake in the position of the register box, it’s easy to patch the small hole made by a finishing nail. A stud finder could be used.
Ceiling Register Outline 6″ x 10″
A Rotozip spiral saw is used to cut the drywall for the ceiling register. Tip: Have a helper hold a vacuum cleaner hose up to the saw to catch most of the dust.
Cutting the Register Box Hole
I left the drywall panel in place to prevent the attic insulation from falling into the room. I’ll remove the panel after the insulation is cleared away in the attic.
Ceiling Drywall – Air Vent Cutout
Now it’s time to go into the attic.
Attic Access for Ductwork
Be extremely careful while working in the attic. Step only on the joists because the drywall under the insulation will not support your body weight – you can fall through the ceiling – resulting in injury or at least a messy drywall repair.
The attic gets very hot in the summer and you can quickly over heat. It’s best to work very early in the morning before it gets hot.
I needed to go into the attic to install the register box and flexible air duct. But I found there was a problem – I couldn’t get to the attic above the bedroom because the roof deck over the main house didn’t leave enough room to squeeze my 215 lb frame through the low gap. Crawling wasn’t an option due to the risk of falling through the ceiling.
Attic Wing above the Bedroom
To solve the problem, I cut the roof deck with a handsaw and sabre saw as shown in the photo below. This allowed me to crouch and slide through with ample handholds while walking on the ceiling joists.
Roof Deck Cutout
View of the roof deck cutout looking back to the main house from inside the L wing.
Roof Deck Cutout – L Wing Side
Here’s the insulation problem I found – mounds of it were on the left, but little or no insulation was over the rear areas of the attic above the bedroom. I redistributed the insulation evenly over the attic.
Mounded Insulation
This project is continued in Part 2.
Take care,
Bob Jackson
Copyright © 2013 HandymanHowTo.com Reproduction strictly prohibited.
Can this be done with a first story room? Our Master bedroom is big and only has two air vents on one end and it stays hot during the summer and cold in winter since the air doesn’t circulate. Would it be possible to do this? can we run the air from the other two ducts?
Thank you,
Lucy
I need to know more about your house and the arrangement of the central air system to say with certainty.
Are the A/C registers on the floor or ceiling? If you have a basement and the ducts are routed under the floor, you should be able to run a new branch line and install a register. You’ll have to cut a hole in the floor of course for the register.
Lucy wrote:
> can we run the air from the other two ducts?
I don’t think you’d be happy splitting an existing branch line as it won’t increase the overall air volume significantly. A new branch line from the main trunk will increase the volume of air and make a real difference in comfort.
Do I need to check my AC unit to determine if it can handle another air duct???
Central A/C units are sized for the *square footage* of the house and adding a new duct shouldn’t be a problem. The important thing is to run the new branch extension from the main trunk line as I’ve shown here.
Take a look at the GAF Installation Tips at this link (also referenced in the main article above): http://www.gaf.com/Content/GAF/METALDUCT/LLBP_Duct_Installation.html
I bought a home where the previous owners put an addition over the garage which is now a large extra bedroom in the house. They had a wall unit heater installed which is extremely ancient, and running it increased our electricity bill by alot. There are no existing vents going to this room, but rather than replacing the wall unit, I would like to install air duct vents into this room, is that possible?
You should be able to work something out. It’s difficult to give specific advice without knowing the details of the house construction. Do you have an attic over this new addition? Some pictures of your A/C system ductwork and attic areas would help – post them on http://tinypic.com
I have a room similar to yours in the l wing of my house in Florida, this room seems to be much warmer (5-10 degrees) especially if there is no fans running. I have been told that a return air vent would work better than adding a register. I have also heard that reducing the size of the duct running to that room with help as it is a thicker diameter than other bedrooms in the house. Supposedly the smaller size keeps the air flow higher and thus cooler the room better. What do you suppose is the best solution to getting this room as cool as the rest of the house?
To determine if the room needs a new return vent, open the door about 1/2 inch. Is the room still warm after 30 minutes? If it’s warm, you don’t need an additional return vent (assuming you already have at least one), but a new cold air supply vent.
The effect of opening the door about 1/2 inch is to mimic the effect of a new return vent by giving the air an escape route; at the same time the door is *almost* closed such that cold air from the main house doesn’t spill into the room.
> I have also heard that reducing the size of the duct
> running to that room with help as it is a thicker
> diameter than other bedrooms in the house.
Sounds like the installer knew the room needed a larger air supply and therefore used a larger diameter (“thicker”) duct.
> Supposedly the smaller size keeps the air flow higher and
> thus cooler the room better.
You don’t want to do this – a smaller duct means lower air volume. The air velocity may be a bit faster through a small duct pipe, but friction losses in a small duct will defeat your goal. Bernoulli’s Principle explains the physics of air flow. See the pipe animation at http://home.earthlink.net/~mmc1919/venturi.html
To get more air use the same or larger size flexible duct pipe and be sure to tap off the main trunk line. It wouldn’t hurt to consult with a HVAC technician.
Living in Florida means hot summers and lots of solar heating of the roof over this room. Have you checked the attic insulation? Adding the R-30 fiberglass insulation as I’ve shown here really improved the thermal stability of the room. Less heat coming in means less A/C is needed to keep it comfortable.
Thanks Bob, just what I needed to get my daughter’s new bedroom done this morning!
Wonderful! Thanks for letting me know. – Bob Jackson
If installing a flexible duct, can I squeeze it to get it through a tight space and still get adequate airflow? I need to get a 6″ diameter through at 4″ opening and an 8 inch diameter through a 5 1/2″ opening?
That’s not a good idea because a 6 inch duct has 28.27 square inches of capacity versus a 12.5 square inches for a 4 inch duct – that’s more than a 50% reduction in duct capacity. Same issue for pinching an 8″ duct through a 5-1/2″ opening. The area of a circle (the duct capacity) varies with the square of the radius by the formula: Area = 3.14 * Radius^2.
What is the obstacle that requires the smaller opening? Is there no way to route around the obstacle or modify the opening to accommodate the larger duct?
I am buying a home with central air and a heat pump. it is three bedrooms and i am converting a family room into the fourth bedroom we need on the first floor. The issue is when I put up the wall I will be closing off the heating and air source. Can I cut into the duct in the roof/floor between the first and second floor? If so can you show me a blow by blow demo on that?
Hi Nick,
Without know more details about your ductwork, vents and floorplan, it’s difficult to give a definitive answer. The surest solution is to tap into the main trunk and run a new line as I’ve shown in the article.
> Can I cut into the duct in the roof/floor between the first and second floor?
> If so can you show me a blow by blow demo on that?
If I understand your situation, the ductwork for the family room is between the ceiling/floor of the 1st and 2nd levels of the home. This would make it expensive and messy to tear out the ceiling drywall to install new ductwork; I don’t have a demo for this.
You might consider a different approach – running new a supply duct down from the attic, hopefully through a closet on the 2nd floor, to the 1st floor family room.
I think your situation is challenging enough to seek on an on-site assessment/estimate by a professional HVAC technician. Check Kudzu.com for professionals in your area.
Thanks for reading,
Bob
I wonder if just redistributing the insulation might have gone a long way to fixing this problem, without adding the second duct. I think I would have given that a try first.
Also, where you advise having someone hold a vacuum cleaner to catch the falling dust as you cut the hole in the drywall, I’ve found that fine dust quickly destroys vacuum cleaner motor bearings. They still work, but become unbearably noisy. I use an old one for jobs like that.
Improper insulation was a definitely contributing factor, however when I compared the air flow to my daughter’s room at the end of that long run of duct to the other bedrooms by holding my hand next to the ceiling vent, the air flow was significantly less strong. That bedroom above the also garage receives a lot of sun loading from the east and south roof faces. In the summer, the garage gets very hot from outdoor temperatures in the 90′s F to low 100′s F plus the heat from the car engines which doesn’t help.
I grabbed the house vacuum cleaner for small jobs like this. I use the yellow shop-vac for real messes and big jobs. Thanks for the tip!
I’ve got a persisting problem with my computer room upstairs, esp. being too hot in summer. The attic AC/HT unit sits between the upstairs bedrooms (3). Theo BR on my side of house is the coldest room in house and the BR next to it is functioning very well. I’ve partically closed the other 2 BR vents and opened my BR (home office) all the way and it makes no difference. They’re still cold and I’m still hot.
The AC service we have says the connection of the main return does not allow room to replace with a connector that allows 2 returns. My thought is to place a small return, the size of AC vents and ducts, right above the computers in the ceiling and run that return duct into the duct work of the main return just a couple of feet before it enters the unit.
Does this sound reasonable solution? If so, can I just cut into the main duct and tape and AC putty the smaller duct into it – making sure it seals good? Or is there a “T” connection to allow the main large return to flow unimpeded and a entry for smaller duct to fit into the side?
What may help to know is that the AC/H unit has no room to move or add any larger attachments due to how the attic is built, BUT there is room to install a small return vent and its duct work to run into the main return.
Advice is greatly appreciated!
Glenn
Instead an AC return duct, have you thought about installing a new fresh air supply duct and vent? This will be much more effective compared to an air return duct. How is the office bedroom temperature when you leave the door open? Leaving the door open is about the same as installing a new return duct.
You’ve already tried to balance the AC by closing vents/registers in the other bedrooms. What’s needed is more cold supply air to make the office bedroom comfortable. Is there something in the system layout that prevents running a new fresh air duct to the ceiling? A couple of photos of the attic air handler and ductwork would help. You can send pics to me at this address: bob (at sign) handymanhowto.com.
Hi, I have a similar project we are working on: We have an upstairs that does not have any ventilation what so ever. Plenty of windows, but no venting to push heat or cool air from the central unit (in basement) into the upstairs. Need to know how to go about getting this done to make this into the kids’ room. We do have three openings that lead into the attic, looks like the same as your pics posted, with the joists and insulation all over the place (although ours looks like old newspapers, should we replace?). We are unsure how to get started with this: where to put the vents, how many vents…
Thanks!
The first thing to do is verify your AC unit can handle the extra cooling load of the 2nd floor. See this central AC sizing chart for guidance. If your AC unit is too small to handle the 2nd floor living space, it’ll run all the time and you won’t be comfortable. My AC unit was properly sized to handle the total living area, so my problem was limited to improving the air supply.
The other question is ductwork sizing, which is not a simple question that I can answer here because it requires specialized knowledge, room measurements, design and calculations. You might try this online Ductwork Design Service or better, hire a local HVAC company for an onsite assessment.
Hi,
I am finishing my basement and the contractor added a piece of wood to seal the space between the metal box (the vent) and the drywall.
Should i be worried that the wood will get moldy due to the AC?
thank you!
It’s normal to build a wood frame behind the drywall to mount the vent boot. The wood shouldn’t get moldy because as the air conditioner cools the air, the excess water vapor condenses on the evaporator coils inside the air handler and drains away… so you’ve got cool dry air blowing out the vent. This is a key purpose of air conditioning, to both cool and dry the air to lower the relative humidity.
Hi Bob,
I have a slab garden home with a storage room off the back inside the screened in patio. The storage room is insulated therefore I put a dog door in the door so my dog can go in and out to get out of the heat. All duct work is in the attic with vents in the ceiling. I would like to run a vent into that room to keep him cool during the hot summers. The master bathroom shares the wall with the storage room. I have two options. Tap into the vent that feeds into the masterbath and run at best five more feet to the storage room. OR Come off the main trunk which is at the front of the house in the attic and make a long run across the entire attic. The storage room is tiny and wont take much to cool but I am extremely concerned with messing up the balance of the rest of the house. What would your advice be?
Regards,
Blake
I normally don’t recommend splitting a branch line, but since it’s a “tiny” storage room go ahead and do it. Take care to install a ceiling register with adjustable louvers that will fully close. Open the louvers just enough to make the storage room comfortable and limit the impact on the master bathroom. During the winter, close the vent louvers if you don’t want to heat the storage room.
Have you thought about a return air duct/vent? Otherwise the air will be forced outdoors around the dog door. If there’s an interior door to the storage room without a bottom weatherstrip or sweep, that could be your air return. (If it has a bottom weatherstrip/sweep, leave it alone.) Might need to shorten the door so there’s a 5/8″ clearance above the floor for the return air. Or you could install a pass-through vent in the common wall shared by the master bedroom and storage room; this might not be desirable though because the dog will hear you and bark and whine.
Should splitting the branch line prove unsatisfactory, you can always run a 3″ duct line from the main trunk at the front of the house.
Bob, It appears that you used R6 flex. I am getting ready to replace all of my attic flex and was told that R8 had to be used in attics (any area without climate control) due to a code change. I was told that I would have to wrap or blow insulation over all R6. Please help.
Simply buy R8 rated duct. Home Depot has Master Flow 6 in. x 25 ft. Insulated Flexible Duct R8 Silver Jacket, Store SKU # 810991.
i added an extra duct but the air flow has stop where i tapped into and both have stop working any suggestions?
I need the particulars of how to you installed the branch line. Did you tap into the rigid trunk ductwork or wyed into a flexible line? Are you sure you tapped into an air supply line and not an air return line? Pictures will help a great deal – send them to bob (at) handymanhowto.com.
Thanks – very nicely done.
GAF web link has changed to: http://www.gaf.com/Other-Building-Products/Ductwork/Ductwork.aspx
Dan
I updated the link. Thanks!
is a 12 inch duct too big for a standard room
Hi Bill,
The short answer is “Yes – a 12 inch duct is probably too large for a standard size room.’, but determining the correct duct size requires some thought and engineering as discussed in my reply dated June 21, 2011 at 9:31 pm in the comments section of this project. Also see my discussion of AC Sizing in the comment dated December 14, 2011 at 11:00 am in this related project.
Let’s assume a ‘standard size room’ is 150 square feet. According to “How Contractors Really Size Air Conditioning Systems” the typical cubic feet per minute (CFM) of airflow required is 1.04 CFM/square foot. You will therefore need at least 156 CFM of airflow and possibly more (1.5 CFM/square foot = 225 CFM) if the room is not well insulated or you live in a very hot or cold climate.
Let’s assume 1.04 CFM will be sufficient air flow. Plugging the following values into the ResDuct™ – Residential Air Duct Calculator:
* Duct Type for Supply & Return: Flexible
* Airflow CFM for Supply & Return: 156
Results in a 9 inch diameter flexible duct.
Re-running the Air Duct Calculator with 1.5 CFM/square foot for 225 CFM total airflow estimates the flexible duct size at 10 inches in diameter.
Please run the duct size calculator with actual square footage and duct type (sheet metal, fiber board, flexible) for your room to get a more accurate estimate.
If in doubt, you could install a 10 inch diameter duct with a louvered vent to regulate and better balance the airflow.
am i able to cut a hole in my air duct in my basement to heat and keep a section of my water pipes warm when the temp drops well below freezing
You want to install a new air vent in the main trunk line to keep the basement water pipes from freezing? If you have a rigid board or sheet metal trunk line, a Speedi-Collar with built-in damper to regular the air flow should do well. It’s fast and easy to install. The damper has a rachet system to lock it in place and closes air tight when you don’t need it in warmer weather.
If the exterior basement walls aren’t insulated, then insulating the walls will minimize heat loss and cold intrusion.
I also recommend wrapping the water pipes with foam pipe insulation if you haven’t done that already. Foam pipe insulation is available at home improvement stores.
I want to remove a duct from the main supply trunk and move it back, but there is another branch in the bay I want to use. Can I run the two vents out of the same tap on the supply trunk, using a wye branch?
The best way would be disconnect the old branch line and cap off the trunk tap, then install a new starting collar at the bay trunk line for the new branch line.
I’m guessing there’s no room to install a starting collar on the bay trunk line? What size is the branch line that you want to relocate? In the follow example, less assume it’s a 6 inch branch line.
1. Cap off the old 6 inch branch duct at the main supply trunk starting collar. Remove the old starting collar and install a cap plate.
2. Remove the 6 inch starting collar at bay trunk line that you want to wye.
3. Install a new starting collar that is 2 inches larger in diameter on the bay trunk line. In this example, the new starting collar is 8 inches in diameter.
4. Install an 8 inch inlet to 6 inch outlet wye at the bay trunk line. You may need to run a short section of 8 inch duct to connect the 8 inch starting collar to the 8 inch wye inlet.
5. Connect the old and new 6 inch bay branch lines to the wye.
6. Install louvered air registers in the rooms to balance the air flow.
The reason for an 8 inch inlet on the reducing wye with two 6 inch outlets is to provide sufficient air volume for the two branch lines. An 8 inch duct has 201 square inches of area. A 6 inch duct has 113 sq. in. of area. 201/113*100% = 178%, meaning the 8 inch wye inlet has 78% greater capacity than the 6 inch wye outlets. This is should be enough to make the rooms comfortable assuming no special heating or cooling needs, for example, a room with lot’s of windows and sun exposure. In my home, most of the louvered vent registers are about 2/3rds open, therefore I have more than sufficient HVAC capacity.
If you believe an 8 inch to 6 inch wye won’t be large enough, use a 10 inch inlet wye with 6 inch outlets. A 10 inch duct has 277% greater capacity (314 sq. in.) compared to a 6 inch duct.
Summmary:
* Best method is to install a new starting collar in the trunk duct for the relocated bay branch line.
* If that’s not possible, start with an 8 in to 6 in reducing wye.
* Should the 8 in wye not be sufficient, you can always cut a larger hole in the trunk line for a 10 in wye.
The risk of installing too large a wye is your vent registers could get noisy from too much air flow whistling past the louvers.
Hi,
I am finishing/soundproofing a studio/music room over garage and it will be sealed as airtight as humanly possible. I thought I should install a return vent so we don’t choke on our own CO2. I would install an a/c vent as well but don’t think the system can handle it. Will this help?
Also, and I can’t seem to find this out anywhere, how do you cut a start collar hole in flexible duct? The room is unfinished and the large return air trunk is right there. I would like to tap into it. (Much easier than trying to thread a new duct back to the main unit). Are there special take-off collars for flex? How do I cut a neat hole with that wire running through it?
Thanks!!
> I would install an a/c vent as well but don’t think the system can handle it. Will this help?
A return air vent without a fresh air supply vent will not have much, if any, affect on the rate of air exchange in the room over the garage.
> I can’t seem to find this out anywhere, how do you cut a start collar hole in flexible duct?
> Are there special take-off collars for flex? How do I cut a neat hole with that wire running through it?
Starting collars are for rigid duct board and sheet metal ducts. An air duct wye connector is used to tap into flexible duct. The wye is installed by cutting flex duct in two, slide the two ends on the wye and seal with HVAC tape and a zip tie. Connect the new branch duct to the 3rd leg of the wye the same way.
Take care to correctly orient the wye. For a fresh air supply line, the air flows in the bottom and out the top of the Y in a diverging path. With a return air duct, the air enters the top of the Y and exits the bottom in a converging path.
I should have gotten back to you sooner. I closed off the one branch. I put in an 8″ whye on the top of the main trunk where I wanted to relocate the branch. That location had a branch line already. Then I fed off in two directions. I reduced the branches to 6″. Damper in each(wasn’t a need for them, but they’re there). Worked out great. Thank you. Due to previous renovations I assume, someone found it easier to run flex tube across bays in basement when changing heat register locations, than to change location on the main trunk allowing to run in the bay. I hated having flex tube hanging all over, not to mention that I eliminated over 10 ft of supply. I was just concerned that running two branches off one location on the main trunk would be a problem. It wasn’t, as far as I can tell.
You’re welcome! Thanks for the update.