To explain the components of a forced air system: a fan draws air from the house into the system through return air vents Image 1. During cold weather, gas burners create heat in the heat exchanger Image 2which in turn heats the air. The hot air goes up into the plenum and from there flows to the supply trunk line Image 3 where it is distributed through vents throughout the rest of the home.
The conditioned air then re-circulates back into the system as needed. During hot weather, you turn on the air conditioner and cooling coils inside the plenum Image 4 then cool the air, which is then distributed throughout the house.
You need a load calculation to make sure the furnace is large enough to supply the new room. Generally speaking, if the room existed when the house was completed, the furnace is probably powerful enough. If it is an addition that was added after the house was built, you may need a new furnace.
A couple other components to a gas forced air system is a source of fresh air from outside the house to aid in combustion, and you'll notice chimney pipes that carry flue gases from the hot water heater and the furnace to the outside of the house. Mark the center point for the takeoff and drill a pilot hole using the cutting bit in the hole cutter. Preset the cutting tool to the correct diameter. Place the center guide in the pilot hole and start the cutting bit along the circumference of the hole.
Use a right-angle drill because you have to work in a tight space, but you can attach the cutting bit to any drill. The cutting bit then swivels around the center guide Image 1forming a perfect circle. Note: Sheet metal hole cutters and right-angle drills are available at most rental centers or specialty stores.
After the takeoff is in place, reach in and bend the tabs over Image 2 to secure it to the duct. When doing any heating or ventilating project, you may need to pull a permit, so check with your local municipality. In some cases, you may even need to hire a pro to help with this kind of project, so check on that as well.
Install the ceiling register boot Image 1 so you will know how long to cut the supply duct. Center it between the joist and screw into place with self-tapping screws.
Note: Use a power nut driver to drive self-tapping sheet metal screws. Measure the length you need for the supply duct. Take the measurement from about 1" inside the takeoff to about 1" into the boot collar.
Cut the duct to length with a round duct cutter. You need to use a round duct cutter because the cutter actually removes a thin strip of metal along the cut line Image 2which makes the cut a lot smoother. It is almost impossible to cut the metal with any other tool. Snap the duct together. It has a special snap lock built in at the factory.
Install the damper into the duct by drilling a hole into the side of the duct and then screw the damper into place. The damper control indicator will show you whether the damper is open or closed. Put the crimped end in first, then put the other end of the duct in. The takeoff rotates to make it easier to fit the duct into place.A household forced-air heating, ventilation and air conditioning system depends on a system of ducts to provide a controlled path for distributing heated or cooled air throughout the home.
Typically, HVAC duct systems are made of sheet metal, but other rigid heatproof materials such as fiberglass or insulated plastic may be used. Regardless of construction material, the heart of an HVAC duct system is the plenum. A plenum is an air-distribution box attached directly to the supply outlet of the HVAC equipment that heats or cools the air to make the house comfortable.
The ductwork that distributes the heated or cooled air to individual rooms of the house connects to the plenum. The HVAC equipment has a blower that forces heated or cooled air into the supply plenum and from the plenum into the rooms through the supply duct system.
Air-cleaning filters may be located in the supply plenum. Forced-air systems also have return ducts that connect to the HVAC equipment through an air-collection box known as a return plenum.
The air from the rooms collects in the return plenum and is drawn back into the HVAC equipment through the return inlet for another round of heating or cooling. Air filters often are located in the return plenum. Herb Kirchhoff has more than three decades of hands-on experience as an avid garden hobbyist and home handyman. Since retiring from the news business inKirchhoff takes care of a acre rural Michigan lakefront property and applies his experience to his vegetable and flower gardens and home repair and renovation projects.
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Results 1 to 7 of 7. Thread: Question about plenum material. Thread Tools Show Printable Version. Question about plenum material Hi, I'm new here, and have a question about a construction material HVAC contractors in the area are wanting to use.
I've got a new central air and evap. I'm trying to avoid ductboard, but finding local HVAC folks that do sheetmetal fab. My question is: has anyone here ever used foil-faced polyisocyanurate foam insulating board to make a plenum?
It's foil-faced on one side like ductboard, but it's foam in the center. Is it becoming common for this to be used? Is it safe? RE: flammability, toxic gases, etc I may have the plenum part incorrect; its the area from the top of the evap.
I think some folks call it a trunk line as well? As with all foam plastics, this product will burn. Johns Manville AP sheathing products must be protected from open flame and kept dry at all times.
Do not leave exposed. AP sheathing is a non-structural product; use acceptable corner bracing. Check local building code for specific requirements. Can't get wet, is flammable, and emits toxic gases when burned not a good idea for you air distribution system.
Never heard of this being used, but I would bet someone on here has seen it done It is polyurethane foam, one of the "better living thru chemistry" products. At first glance I was concerned about its combustion products during a fire, but looking up the safety data sheets on the material, it probably is barely more toxic than wood would be. I wonder what is the reason to avoid getting wet? Thanks -- Pstu. I just posted from what I read on the U.
Department of Energy: Ensuring Fire Protection Foam insulation is relatively hard to ignite, but when it is ignited, it burns readily and emits a dense smoke containing many toxic gases. The combustion characteristics of foam insulation products vary with the combustion temperatures, chemical formulation, and available air. Because of these characteristics, foams used for construction require a covering as a fire barrier.
One half-inch thick 1. Some building codes, however, do not require an additional fire barrier for certain metal-faced, laminated foam products. I am not motivated by issues of money or emotion, but rather on giving advice on the safety of said materials.
If you wold like to perform a test on your own home to find the safe levels of burning foam board, please feel free and post results. I myself have a 2 year old daughter and don't need to worry if the money I save could kill her due to an attic fire spreading smoke through the duct system of my home.Custom Made per your measurements in 1-Day Sheet Metal Duct Plenum Boxes are used on all brands of heating and air conditioning equipment Plenum and Return Air boxes are the source to attach the ductwork connections to.
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Mountain Ave. All Rights Reserved.Duct board consists of compressed fiberglass backed with a foil vapor barrier. The insulating qualities of compressed fiberglass reduces the temperature loss from air moving through the plenum and stops condensation from forming on the exterior surface of the duct plenum.
Excessive condensation will damage finished materials surrounding the plenum and cause mold to grow in dampened wooden framing components. Properly cutting and seaming the duct board plenum will ensure minimal damage from condensation. Set a piece of duct board on a firm surface with the foil vapor barrier sitting on the work surface.
Mark the duct board. Pull the tape measure along the piece of duct board.
What Is a Plenum in an HVAC System?
Set the short side of the T-square against the side of the duct board. Align the long edge with a cut mark. Pull the soapstone along the edge of the T-square to mark the line. Slide the duct board knife along the width of the duct board.
Return Air Support Box w/ Filter Rack
Pull the V-groove tool along each marked cut line. Keep the flat section of the V-groove tool flat against the top of the duct board to ensure the notch is at the proper depth. Remove the notched insulation from each groove. Fold the foil flap over and staple the flap to the duct board. Seal the connection with a length of foil tape. Place soapstone marks 3 inches in from each edge of the cut end pieces. Extend each mark with the T-square.
Remove the insulation from around the edges of the pieces with the duct board knife as described earlier. Secure the end caps to the plenum. Push the cut pieces in each open end of the plenum. Slit the foil vapor barrier extending beyond the edges of the duct board with a duct board knife. Fold the ends of the foil onto the outside of the duct board.
Staple the foil to the duct board. Tape the flaps down with foil tape to complete the duct board plenum. Reasebased in Texas, has been a professional construction and outdoor writer since Rease served a four year apprenticeship to become a union sheet metal journeyman and earned a construction management degree from Florida State University. Hunker may earn compensation through affiliate links in this story.
Step 1 Set a piece of duct board on a firm surface with the foil vapor barrier sitting on the work surface. Step 2 Mark the duct board.
Step 4 Pull the V-groove tool along each marked cut line. Step 6 Fold the foil flap over and staple the flap to the duct board. Step 7 Cut two pieces of duct board 6 inches longer and wider than the inside opening of the plenum. Step 8 Place soapstone marks 3 inches in from each edge of the cut end pieces. Step 9 Remove the insulation from around the edges of the pieces with the duct board knife as described earlier.
Step 10 Secure the end caps to the plenum. Tip Wear long clothing and a respirator to avoid irritation from the fiberglass fibers. Share this article.
Show Comments.Here in our hvac beginners ductwork section we will look at some common ductwork fittings and configurations. As discussed in our Hvac for Beginners page, each component of a forced air comfort system adds a certain amount of restriction to the airflow of the system.
This restriction friction loss adds to the TEL total equivalent duct length of the system. It is not our intentions to train designers, but to emphasize the importance of a good design. For the Do-It-Yourselfer, we hope this hvac beginners ductwork section will aid you in installing a problem-free forced air system that you can brag to the neighbors about.
Square ells, return or supply, are a bad idea. Each one in a system can add as much as 80' to the TEL. Rounded throats on ells drastically improve airflow, especially a rounded inside throat. This is the best configuration, and mandatory if the unit has a 5-ton blower. These two examples are very common for two-way duct systems.
Example 1 with straight take-offs act the same as any square or right angle fitting. Each straight take-off adds 80' to the TEL. Each flared take-off adds only 25' to the TEL. The plenum ell is the best approach to one-way duct systems. A one-way duct system can be where the equipment is placed at one end of the structure, or when the ducting feeds one-way out from a mechanical room to a tee which feeds two ways.
What Is a Plenum in an HVAC System?
The output end of this plenum ell must be sized to deliver the full cfm cubic feet per minute called for by your Manual D calculations. Proper air flow will usually entail flaring the plenum out to a larger width or depth to provide the needed cfm output.
This can be accomplished with a standard reducer flipped upside down and used as an increaser. Rounded throats are a must in this configuration. A straight plenum with a flared take-off is not a good idea here.
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Reduces ventilation system noise and condensation for improved system performance. Can be cut to size and used in other insulating applications. Product availability is subject to change without notice.
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