Jupiter Hvac Equipments

Plenum box

Product deatils


The plenum is the section of duct immediately beyond the furnace or air handler. It is typically the same width and depth as the discharge opening of the furnace or air handler, and extends approximately two and a half to three feet. It may be connected using a flexible connector of canvas or similar material to reduce transmission of vibration. It typically has a number of separate ducts connected to it by the use of “take-offs” of different types and sizes. This may be called a plenum “box”. A “plenum box” is the cardboard box the plenum came in.

Large plenums are used in commercial work to combine incoming fresh air with the air stream. There are also smaller plenum “boxes” sometimes used in commercial work, typically where a duct connects to a diffuser.

Finally, when the space between the suspended ceiling and the floor above is used as a return air means, that space is referred to as a plenum space, but not as a plenum box.

How important is the construction of the plenum box to the integrity and longevity of the air handler and HVAC system?

In most air conditioning systems the plenum is the most important part of the ductwork because it is where all supply or return ducts come together. It is where air pressures and velocities are greatest. It is important to construct it in such a way that there is no leakage, over time and each downstream duct gets its full share of the supply air.

What is the difference between a plenum and a drop ceiling?

Dropped ceilings or suspended ceilings are often installed so that the space between them and the floor above or the roof deck can be used as a return air plenum. This is most common in commercial buildings, schools, hospitals, or libraries. The air conditioning system supplies heated or cooled (conditioned) air through ducts in the ‘’above ceiling’’ or attic space and this air comes into the room through a device called a register, vent, or diffuser. The air coming into the room pushes air out ‘’return air’’ grills, where it flows back to the inlet side of the air handler (the machine that pumps the air through heating and cooling coils to go back to the conditioned spaces. Other air conditioning systems use ducted return air, so the above ceiling space isn’t a plenum.

One concern when the return air isn’t ducted, where the above ceiling is considered a return air plenum, is the installation of certain flammable materials in that space. Plumbing pipes, electrical wires, and other items that are potential producers of toxic fumes or smoke when they burn are usually not permitted. PVC vent pipes, for instance, are changed over to cast iron, telephone, data, and thermostat wires must be “plenum rated”, and electrical wiring is usually installed in metal conduit, instead of plastic jacketed Romex cable.