A plenum box is a compartment, air space or chamber that is filled with air that is then distributed elsewhere.
A furnace plenum then, is such an air chamber that gets filled directly (and quickly) by a large blower that is above, below or adjacent to it.
Air ducts connected to a furnace or heater plenum carry this air to various registers around a house or building.
Where is an intake plenum on my furnace?
Since furnaces come in countless models and styles, there is not one consistent place to point to when identifying your furnace plenum.
If you remove the front panels from your furnace, you won’t ever see the furnace plenum (it truly is a closed air chamber), but you can deduce it’s location following air’s path as it comes in from your return air vent, through your air filter, past the blower, then up through the heat exchanger (area where the burners are and the air is warmed).
When it leaves this last area, it enters the plenum. You can also locate your heater’s air plenum by following your home’s air ducts back to the furnace (although, this may be hard to see depending on install).
Think of the furnace plenum as the last place air is at your furnace, at it’s highest pressure, before traveling throughout your house. From the outside, it’s really nothing more than a metal box, in simple terms.
From a home repair or maintenance perspective, you don’t need to worry about cleaning or inspecting your furnace plenum.
This is due largely to it being made of sturdy metal (unlike most ductwork) and of course, the fact that you can’t actually “get to” your furnace plenum without disassembling things.
Just ensure any seams around the plenum are sealed properly.
Quick fact: the word plenum comes from the word plenus, which means to be filled or become plump.
This is exactly what your furnace plenum does before air is released and delivered. Other terms used for furnace plenum, are “supply plenum” or “intake plenum.”