We already briefly explained differences between plenum and riser cables in this article. The plenum is defined by construction contractors, building engineers and air-conditioning technicians as the space between the structural ceiling and a false drop-down ceiling, when this space is used for air- conditioning HVACheating ventilation and air circulation. The space between the structural floor and a false floor, e.
The plenum is typically used for returning air to HVAC equipment. Raised floors and ceilings are convenient spaces where voice and data cables can be run, but national codes require that plenum cable should specifically be used in plenum spaces. This could be an expensive mistake as plenum cables cost up to twice their non-plenum equivalent. Cable designers refer to plenum as the type of cable that is rated for use in plenum spaces.
When working with contractors, cabling professionals and building engineers, be aware of when the term is used for cable and when it refers to the air space.
Building management and local authorities might at times require that plenum-rated cable be used in non-plenum spaces. You should be aware of the requirements in your area. There are numerous commonly held, but incorrect, beliefs about plenum rated cable.
Not only do these misconceptions make discussions about LAN cabling difficult, but they are especially annoying when UTP is discussed.
What Is a Plenum in an HVAC System?
Building maintenance personnel and the company that installed the HVAC will know for certain. If it is not a plenum space, there is no need to spend extra on plenum-rated cable. The fact is that just about all four-pair UTP cables used in the U. Both in the U. There are therefore many tests that measure levels of corrosive and toxic elements in smoke, but for general residential and commercial use in the U.
The first is to give people enough time to evacuate a building, while the second revolves around keeping exits clear and using signs that indicate where exits are located.
These tests are based on the intended use of the cable. The first part of the philosophy is satisfied as plenum cables delays the fire spreading. As all plenum cables are installed in areas not accessible to the public, the second part is not applicable.
As plenum cables are however installed in air-handling space where smoke from burning cables could spread via HVAC fans, the generation of visible smoke is measured by the plenum test. Visible smoke could prevent people from finding exits or cause suffocation. HDPE high-density polyethylene is used in Category 5e and higher non-plenum cables as conductor insulation. HDPE however generates lots of visible smoke and burns like a candle.
The bottom line is that if a cable is rated as Category 5e or better, it is Category 5e or better, irrespective of the flame rating. This is not true for two reasons. It will most likely be fastened to wooden studs that will burn hot, fast and with poisonous, black smoke.Many people haven't because most HVAC installers don't bother with the bottom plenum.
But if you have seen one, it's usually a sign of a smart, conscientious installation. It's true in all 5-ton systems, but several 4-ton systems also require the 6" plenum. And if there's enough space to install one or we need to reduce static pressure in the system, we'll even install the 6" plenum in a 3-ton system. Airflow matters for several reasons. For starters, it affects the comfort of a home's occupants. With too little airflow, your system might not effectively distribute air throughout the home.
That's bad. Inadequate airflow also has a negative impact on system performance. Think poor efficiencyhigh operating costs, and premature equipment failure. But when return air enters the furnace from both the side and the bottom or both sidesairflow increases.
The system operates more efficiently, and you enjoy more consistent air distribution. It's better. More return air can also improve indoor air quality. You pull it out to change it, and there's 4" of clean filter at the top.
Because air was only moving through 16" of the filter media. Without the extra plenum, there's no air for the rest of the air filter to, you know… filter. Money, time, and knowledge. When you don't install the 6" plenum, homeowners save money on labor and contractors save time on the installation.
In some cases, the contractor might not even realize it's a problem to neglect the 6" plenum. But you know who doesn't beat around the bush when it comes to the 6" plenum on a 5-ton system? HVAC manufacturers. Installation manuals say you need to install one.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.
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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.
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. Other Related Ducting Pages.How to Assemble Starter Ductwork
Stoddard, Detroit, Mich. Please feel free to link to this page from your website.
Site Build It! Site Navigation. Copyright - perfect-home-hvac-design.The air conditioning compressor is designed to pump vapor only! Tight tolerances within the compressor do not allow for the presence of non-compressible liquid. The heat exchanger is designed to separate combustion gases from the indoor air.
The sensor is not designed to operate on a continual basis only for emergencies. Eventual failure of the safety device may result in a runaway condition where the unit will not shut off or will shut off and not turn back on, requiring a service call to repair the equipment.
When an air duct system is attached to HVAC equipment, it operates under different pressure characteristics unpredicted by the equipment manufacturer. This is controlled by the size of the air duct system, its length and number of fittings incorporated in their design. This sucks the filter media against the air duct further restricting airflow several hundred percent more than the simple design flaw pressures. Under these conditions equipment failure is eminent. Very often the first floor HVAC system on a multi-story house is located below the stair system.
This is a very large void that is generally not used for any other purpose. There is no excuse or reason to construct an undersized return Plenum when there is sufficient space available such as below a stair system or in the attic. The amount of air leakage is based upon pressure differentials between the interior of the duct and the ambient atmospheric pressure.
The greater the pressure from restriction of its design the greater duct leakage will occur.
Many return duct systems are located in the crawlspace or basement below the house. Excessive pressure leakage in the return system pulls in greater amounts of radon gas which is distributed throughout the house through the air delivery system. The greater the air leakage, the greater the concentration of these allergens within the system.
If the return Plenum is not appropriately designed, these devices cannot be utilized without significant negative effect. Improper air duct design and pressure control results in a change in the sensible heat ratio away from the equipment design.
This is commonly related with failure to control moisture conditions within the house. This prevents things from drying out after activities such as cooking, bathing and simply the presence of the occupants residing within the building who perspire and transpire moisture into the space in significant amounts. We are responding to a homeowner complaint of comfort, indoor air quality etc. If there is indication of previous equipment failure or actual testing measurements accompanying your report, the severity of the issue is elevated.
If the construction is nonstandard but the equipment is operating within measurement parameters, you should be aware that how you operate the equipment such as simply increasing the efficiency of the air filter may have detrimental effects on your equipment and your indoor air quality.
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What in the world is a plenum, you ask? The return plenum is a box that connects the return ducts in your ductwork back to your HVAC system. The air is then sent back into your HVAC system through a return inlet to be put through another cycle of cooling or heating. In any case, it is well worth your penny for peace of mind. Especially with ductworkwe recommend getting your installation from a seasoned expert. Your supply and return plenums must be properly fitted in order to effectively distribute air throughout your home.
Also, proper fitting will help to reduce risks of air leaks and other potential problems within your system. If you have any ductwork needs, whether they involve your supply plenum, return plenum, or other aspects, look for a reputable company for help. At A 1 Air, we put our focus on quality and satisfaction for every step of any installation we do. Contact us today. You must be logged in to post a comment.
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Low Inventory Alert! This adjustable plenum is a larger more square duct which sits on top of the air handles unit or evaporator coil. The top end is sealed with an end cap. This product is proudly made in the USA using galvanized steel construction for durability and rust prevention.
Email Item. To: From:. Send Cancel. View Shipping Options Shipping Options. Delivery dates and costs shown are estimates. See exact dates and costs when you select your shipping option during the checkout process. Features The plenum is a large, square duct that sits on top of the air handler unit or evaparator coil. The top end is sealed with an end cap Galvanized steel construction for durability and rust prevention 26 gauge. Optional Accessories. Overall Depth.
Overall Width. Product Type. View Return Policy. Please Note: Prices, promotions, styles and availability may vary by store and online. Inventory is sold and received continuously throughout the day; therefore, the quantity shown may not be available when you get to the store.
This inventory may include a store display unit. Online orders and products purchased in-store qualify for rebate redemption.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.
Skip to main content. Home Guides Home Home Improvement. About the Author Herb Kirchhoff has more than three decades of hands-on experience as an avid garden hobbyist and home handyman. Customer Service Newsroom Contacts.