This tutorial explains how to add an air heating/cooling branch line to a bedroom for increased comfort.
The 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 duct in my daughter’s bedroom.
Air Duct Branch Line
Other 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.
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 you review these references to customize the duct work layout to fit your particular situation.
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.
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.
Register Grill Outline 6
A Rotozip spiral saw is used to cut the drywall for the register grill. 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.
Register Cutout
Now it’s time to go into the attic.
Attic Work
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
July 12th, 2009 at 11:35 pm
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
July 13th, 2009 at 7:06 am
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.
July 16th, 2009 at 10:45 am
Do I need to check my AC unit to determine if it can handle another air duct???
July 16th, 2009 at 11:37 am
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
July 20th, 2009 at 12:35 pm
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?
July 20th, 2009 at 5:01 pm
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
July 22nd, 2009 at 3:14 pm
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?
July 22nd, 2009 at 6:06 pm
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.
December 5th, 2009 at 8:36 am
Thanks Bob, just what I needed to get my daughter’s new bedroom done this morning!
December 5th, 2009 at 10:08 am
Wonderful! Thanks for letting me know. – Bob Jackson
February 28th, 2010 at 8:27 pm
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?
February 28th, 2010 at 9:26 pm
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?