I’ve always thought Plenum was a heating and air situation. Open air return in the ceiling. That’s because the only time I deal with it is pulling low voltage cable in ceilings. My question is. What else if anything is involved in making a ceiling Plenum? I’ve heard some people say that for it to be truly plenum all the construction should be steel. Help a brother out. .
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Gunner,
I'm not an HVAC guy
Gunner,
I'm not an HVAC guy so I could be full of hot air. I've always been told that the plenum was the air distribution system that takes air from a furnace or air handler and when balanced delivers the right amounts of air to each room. I've only seen them made out of tin.
Mark
I always though of it as any enclosed space used to direct the flow of air. And Webster's agrees with me. But of course that is not a technical resource.
But non metal plenums are common. Many older houses have "panned" joist for return air plenum. Also made out of duckboard and similar.
And of course the above ceiling space in some commercial construction.
Now the various codes might treat them differently because of the difference in volume, containing fire, etc. But still all plenums.
The commercial aspect is what I'm asking about. And having an open air return in the ceiling is the only indication that know of to figure out if I'm dealing with a plenum ceiling or not. If it is then I have to use plenum low voltage wire. That is in case there is a fire the covering on the wire isn't toxic and won't poision everyone by releasing toxic fumes back through the open air returns in the ceiling.
Certainly there must be some other rules to havign a plenum ceiling.
Generally speaking, if you use the space above the ceiling for air movement from one space to another, it becomes a plenum. Which means everything in the plenum has to be rated for smoke or fire. Smoke is the big concern. Fire stopping can be a big issue depending on the occupancy.
OK now we are getting where I want to go. So if the ceiling has wooden framing then it's not really a plenum rated ceiling? If the occupancy is high enough.
Gunner, is the wood exposed? Every type of construction has a fire rating. And the type of occupancy determines the type of rating, ie; 1-hour, 3-hour. I don't know your codes, can you talk to your fire marshall?
reread your question. occupancy type is different from numbers of people. ie; office/retail/warehouse/parking
Any space/volume used to flow air is a plenum regardless of what material is in it. Fire rating of that plenum is a different issue. The existence of steel doesn't mean a whole lot (that's why they apply fireproofing to steel in commercial construction). Different applications may require plenums to have a certain fire rating. I've seen above ceiling spaces that are wood and others steel while others have to be fireproof/rated. Some applications require fire dampers.
Bottom line, if it is NOT a duct and INTENDS to flow air, it is a plenum (although there are sheet metal constructed plenums (i.e. CHAMBERS) that help move air from e.g. an air handler to e.g. ducts. Hope that didn't confuse you.
I don't think that a given
I don't think that a given space can simultaneously be a plenum and require fire-stopping (except to define the boundaries of the plenum).
Sure it can Dan. In a multi-use building, there may be a central supply and air return with branches going to each space. Fire and smoke dampers are required in the duct at the wall of each space. The dampers are a type of firestopping. Same with the type of firestop required for electrical and plumbing penetrations.
Wether or not they are enforced is another issue.
Well, I consider firestopping and fire dampers to be different things. The former is a fixed barrier intended to permanently separate/divide two areas, neither of which is necessarily a plenum, while the latter is a device intended to shut down a plenum in case of fire.
But either way is angels on the heads of pins, relative to the current issue.
A damper and fire wall are different things ... one is operable, the other is fixed. But they do the same thing. You penetrate a fire wall w/ e.g. a duct and you need to maintain the integrity of the fire wall ... with a closing damper in the event of a fire. When it shuts, the two elements become/act as one.
I've always understood the
I've always understood the area above a suspended ceiling in to be a plenum, **if** it's used as supply or return for ventilation. In such cases all materials used in the area (in commercial construction) need to be non-flammable and not liable to release toxic gas when heated.
Of course a panned joist bay in residential construction is also a plenum, but is not subject to as strict of regulations as commercial construction.
Dunno if it's legal to use a suspended ceiling area in residential (below wood joists) as a plenum. Certainly not in commercial.
Of course, in final analysis a plenum is what the inspector determines to be a plenum.
I believe I've seen some
I believe I've seen some commercial (albeit small) with wood in ceiling plenums ... e.g. wood framed and plenum return air ceiling. I could be mistaken, though; it's not something I focus on, really.
Another space to consider is that sometimes a plenum is the underfloor space in a computer/server/data room area.
My understanding is that it's
My understanding is that it's gotten much harder to legally use a computer raised floor for both cabling and plenum, since it's hard to guarantee that the cables are plenum rated.
ok, this is interesting. When replying to this thread, it shows that I replied to the thread in energy, even though it was moved from there to here. Maybe a pun about firewalls might be appropiate?
Just don't start flaming --
Just don't start flaming -- the fire dampers will come crashing down.
Here's a good question to ask
Here's a good question to ask the inspector: Why does low voltage wire, in a plenum, have to be plenum rated, (ie: NOT pvc insulated) but drains do NOT (ie: pvc drain lines)??
I asked about that one time, pointing to a pvc drain from an air handler, and asked the fire marshall. He told me that it always has water in it (huh???) and therefore wouldn't burn.
Wanna bet??
Dick
Interesting point, for sure. Bet they need to be e.g. copper.
Just a guess: First off, the pipe isn't going to START a fire, whereas even low-voltage wiring can overheat and start one. And, secondly, the pipe is of sufficiently high mass that it won't get started burning until things are pretty far along -- hotter than things ought to get in a plenum that is supposedly clear of ready combustibles.
In other words, by the time the pipe starts burning things would be so far gone that it wouldn't matter anymore.
You asked the wrong inspector. Many times I have seen plumbing (DWV) in a plenum required to comply with flame and smoke spread criteria. Copper, cast iron or wrapped PVC.
This reply is to Gunner's
This reply is to Gunner's original question.
The IMC code definition of plenum: "An enclosed portion of the building structure, other than an occupiable space being conditioned, that is designed to allow air movement, and thereby serve as part of an air distribution system." The UMC definition is almost the same, except that it specifically includes raised floors of computer centers.
So, any above-ceiling space used for return air would fit the definition. So would any panned joist space. (Keep in mind the above definition is from the IMC, and does not apply to residential construction; the IRC governs residential.)
Your question seems to be concerned most with how this affects the fire rating of materials, like wires, exposed within the plenum. Section 602 of the IMC deals with this, and it says, first of all, the a plenum must be limited to one one fire area--so questions and comments about how to protect the plenum if it penetrates a fire-rated assembly are irrelevant. As for wiring exposed within plenums, the IMC requires it to meet certain limits of flame development and smoke spread. But the bottom line is simple: the wiring, optical cable, or raceways must be listed and labeled for exposure within a plenum. All you have to do as an electrician is read the label. It will specifically say if it's plenum rated.