Replies

1. 
   ms | Dec 12, 2009 01:43pm | #1

   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

2. 
   BillHartmann | Dec 12, 2009 04:28pm | #2

   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.

1. 
   Gunner | Dec 12, 2009 06:53pm | #3

   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.

1. 
   rich1 | Dec 12, 2009 10:04pm | #4

   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.

1. 
   Gunner | Dec 12, 2009 10:33pm | #5

   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.

1. 
   rich1 | Dec 13, 2009 06:31pm | #8

   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?

2. 
   rich1 | Dec 13, 2009 11:51pm | #13

   reread your question. occupancy type is different from numbers of people. ie; office/retail/warehouse/parking

3. 
   Clewless1 | Dec 14, 2009 08:58am | #16

   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.

2. 
   DanH | Dec 13, 2009 03:12pm | #7

   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).

1. 
   rich1 | Dec 13, 2009 06:38pm | #9

   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.

2. 
   DanH | Dec 13, 2009 10:36pm | #12

   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.

3. 
   Clewless1 | Dec 14, 2009 09:07am | #18

   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.

2. 
   DanH | Dec 13, 2009 03:11pm | #6

   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.

1. 
   Clewless1 | Dec 14, 2009 09:02am | #17

   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.

3. 
   Howard_Burt | Dec 13, 2009 08:31pm | #10

   Another space to consider is that sometimes a plenum is the underfloor space in a computer/server/data room area.

1. 
   DanH | Dec 13, 2009 10:31pm | #11

   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.

1. 
   rich1 | Dec 13, 2009 11:54pm | #14

   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?

2. 
   DanH | Dec 14, 2009 07:26am | #15

   Just don't start flaming --
   Just don't start flaming -- the fire dampers will come crashing down.

4. 
   HuskiesHowls | Dec 19, 2009 09:36pm | #20

   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

1. 
   Clewless1 | Dec 20, 2009 02:40pm | #21

   Interesting point, for sure. Bet they need to be e.g. copper.

2. 
   DanH | Dec 20, 2009 05:54pm | #22

   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.

3. 
   Tim | Dec 23, 2009 09:02am | #23

   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.

3. 
   rdesigns | Dec 14, 2009 09:54am | #19

   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.