Is this a PLENUM area I think so.

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AC\DC

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Location
Florence,Oregon,Lane
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EC
This building had the garage catch fire then two days latter the Main panel had a Line to Ground fault on one of the Main lugs.
Went to look about fixing the Panel, noticed that they have the cold air return for the Furnace directly under the panel. Never seen this before. I Usually stay away from Returns and never had to work in a Plenum area before. To me this is a Plenum areas, am I wrong in my thinking?
Furnace is right on the other side of this grill

Another question. The garage is attached by foundation and roof line, It has a 10 foot breeze way. The meter is out on a remote post and feeds the Manufactured home and then a 100 feed to the garage. I think when they were installed they were not connected. Now they are, so now its one building with two feeders, feeding one building. Am I looking at this wrong. I am going to call the building inspector to make sure its not considered a single building. Hope not cause its going to be a pain to fix.

Thank you in advanced.
 

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If the area onto which the air return opens is not separated in any way from the rest of the interior of the structure, it cannot be a plenum.
If there is some sort of utility room surrounding the panel and grate, it still may not be a plenum unless the room collects air from various parts of the structure by ducts ending in the room.

To some extent the designation of plenum hinges on whether the space is deliberately designed as an air directing component or just is incidentally in the air path.
 
This is the only return air for the house. All rooms feed into this room by just air flow. On the other side is the furnace were this grate feeds the entire closed off closet.
I would say this is Air directing Since This is the only deliberate spot, were new air comes into the Furnace.
Thank you for the reply.
 
My mom's townhouse was built like that. The updraft air handler was inside a utility/storage room, with the breaker panel and water heater. (All electric) The air filter was just propped against a return opening cut into one side of the base of the unit; there was no separate return plenum.

On the opposite wall of the closet, there was just a plain grill over a hole that had been cut through both sides of the drywall into the closet. As my mom was always warm, she used a lot of talcum powder, and everything inside the closet was blanketed in a layer of powder and dust.

I don't see how such a space would not be considered a plenum, as it was definitely being used to carry living-space air. Plenum or not, I imagine there was a real fire hazard with the dust, and she couldn't stand it, even though she was inadvertently responsible for a lot of it.

I got a pedestal kit to raise the water heater, extending the pipes and wire, cut holes in two sheets of metal, put a 16" takeoff boot in each one, attached one over the hole in the air handler, the other over the backside of the hole in the wall, and ran 16" flexible duct under the pedestal.
 
A Plenum should be dedicated space for supply or return air only and not contain any other objects that would restrict airflow. Also, it is against the building code to use the building structure as a return.
 
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Building spaces such as cavities between walls, support platforms for air handlers, and plenums defined or constructed with materials other than sealed sheet metal, duct board, or flexible duct must not be used for conveying conditioned air, including return air and supply air. Using drywall materials as the interior surface of a return plenum is not allowed. Building cavities and support platforms may contain ducts. Ducts installed in cavities and support platforms must not be compressed to cause reductions in the cross-sectional area of the ducts. Although a HERS Rater may examine this as a part of his or her responsibilities when involved in a project, the enforcement of these minimum standards for ducts is the responsibility of the building official.
 
Remember that in addition to true plenums, the NEC recognizes "other environmental air" spaces, and requires the use of plenum-rated wiring methods within them.
 
readydave8 said:
hvac installers used to nail a piece of sheet metal to bottom of floor joists to make return plenum, is this illegal now?
and/or the space above suspended ceilings?
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This is exactly what was done in my current home.
 
So if I put them in a metallic raceway like fmc then they are correct then.
What about a fmc chase from floor joist up to panel. Since it’s a chase it’s not correct?
 
AC\DC said:
So if I put them in a metallic raceway like fmc then they are correct then.
What about a fmc chase from floor joist up to panel. Since it’s a chase it’s not correct?
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If it is a chase, (dedicated structure). it must be a sheet metal fabricated structure inside of it, or just flex duct free from a restrictions.
 
I guess as #7 has a good point. 300.22 c "shall apply to spaces not specially fabricated for Air-handling, but is used for air-handling.
Then could I just use exception under this section?
 
As for the NEC is concerned with, it is control wiring for HVAC. But for using building structures as return air, it is the California Energy Commission requirements under Title 24, that forbids it.
 
Clarification ... Using the building structure for return air, applies to residential construction only. Using Section 300.22 refers to environmental air returns in commercial applications.
 
mtnelect said:
A Plenum should be dedicated space for supply or return air only and not contain any other objects that would restrict airflow. Also, it is against the building code to use the building structure as a return.
Click to expand...

What about the space above a suspended ceiling.
 
mtnelect said:
Clarification ... Using the building structure for return air, applies to residential construction only. Using Section 300.22 refers to environmental air returns in commercial applications.
Click to expand...
And yet, apparently in NJ they will let you do that.
 
Yes ... It's because of the CEC Title 24. California doesn't want to build anymore power plants. They want residential structures to be "Airtight", and by using building structures as air returns, air can leak out.
 
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