Caveats: 1st post.  I'm a homeowner.  This may be a really stupid question. 
I'm getting close to starting construction on 6000 sq ft home in Lakeland, Fl.  Probably going to install a geothermal system but that really doesn't have an impact on this question.  My question is, if my attic space is unvented/conditioned, and is continous, and my air handlers will all be located in the attic space, are there reasons I couldn't just use the entire attic space as the return air "plenum," and simply place return air grilles in all the rooms that need them (essentially just connecting each room to the attic) and avoid all the return air ducting?  It would seem to me to be a simpler, self equalizing system that could allow easier whole house dehumidification, ventilation, etc.  ?
Clark

Good idea except for one possible problem. The problem you will have is if you have any PVC jacketed wiring passing through the attic. The National Electric Code in particular doesn't allow regular electrical wiring in air plenums except under limited circumstances. The concern with PVC jacketed wiring is the toxic gases it produces if it burns and if in a plenum, distribution of that toxic smoke throughout the structure. There is plenum rated wire & cable available, but it is more expensive than your standard Romex house wire.

I wonder if the NEC definition of the word 'plenum' would actually apply to the entire attic space.
Think about the volume of space normally associated with return air ducts vs the volume of space involved in an attic space. The velocity of the overall air movement within the two spaces would be substantially different at the same CFM.

My house has return ducts that use the unfinished wall cavities. But I'd ask your inspector.

Thanks all for your replies.

Refer to NEC section 300.22 "Wiring in Ducts, Plenums, and other Air-Handling Spaces" for more detailed info. Using the attic as a plenum is somewhat of a gray area. Sub-section (C) "Other Space Used for Environmental Air" states in part "It does not include habitable rooms or areas of buildings, the prime purpose of which is not air handling." It could be argued that the "prime purpose" of the attic is structural support, not air handling. But this sub-section also has the following statement "The space over a hung ceiling used for environmental air-handling purposes is an example of the type of other space to which this section applies." If you have to get your home inspected, it would be a good idea to get this issue settled before construction rather than after.

My situation is similar. I will have 18" deep open-web floor trusses supporting my main floor over my basement. I intend to use this open space to circulate condition air for my basement rooms. There will be some wiring passing through this area, but I intend to limit it. I consider the prime purpose of this area to be structural support, not air handling. But I am building my home in an area that does not require home inspections, so I won't have to deal with an inspector that might interpret the code differently than I do.

Note that sealed crawlspace areas are often used for air handling, but wiring is generally limited in this area.

For more internet discussion on this subject, google [NEC 300.22].

Posted By arkie6 on 06 Jul 2010 11:12 AM
." It could be argued that the "prime purpose" of the attic is structural support, not air handling. 

Note that sealed crawlspace areas are often used for air handling, but wiring is generally limited in this area.

For more internet discussion on this subject, google [NEC 300.22].

an attic is a "space" not a structural component,  the space cannot support anything but air.  If the space is conditioned it no longer is an attic and needs to be called conditioned space
wiring within an area used as a  plenum has to be in conduit

According to every dictionary I looked at, the area below the roof of a home and above the ceiling joists is considered an attic, regardless if it is conditioned or not. In this case it would be a conditioned attic.

Wiring within a plenum does not have to be in conduit. Per the NEC section 300.22 (B) "Ducts or Plenums Used for Environmental Air" , type MI cable and type MC cable (metal jacketed wiring) can also be used. But note that (B) applies to "ducts or plenums specifically fabricated to transport environmental air." I don't believe that the attic discussed above would fall under section (B), but could potentially fall under section (C) "Other Space Used for Environmental Air." Section (C) states in part "This section applies to space used for environmental air-handling purposes other than ducts or plenums as specified in 300.22(A) and (B)." Section (C) also states "It does not include habitable rooms or areas of buildings, the prime purpose of which is not air handling.". Again, I would argue that the "prime purpose" of the attic is not air handling. But a building inspector might argue otherwise.

Posted By jonr on 06 Jul 2010 08:26 AM
My house has return ducts that use the unfinished wall cavities. But I'd ask your inspector.

That falls under NEC 300.22(C) "Other Space Used for Environmental Air."  This section has a specific exception as such:  "This section shall not apply to the joist or stud spaces of dwelling units where the wiring passes through such spaces perpendicular to the long dimension of such spaces."

In other words, if your return air shaft runs vertical between the studs, then you can route a horizontal standard plastic jacketed cable across the space (i.e. wiring for receptacles), and remain in compliance with the NEC.  The NEC handbook shows a specific figure where this exception applies.

Regardless of code interpretations, if you do intend to use a large space with lots of combustible materials present such as an attic, crawlspace, or space between floor-ceiling as an air handling medium, it would be a good idea to install a smoke detector in that area with a relay contact output that can be used to disable the blower motor if smoke is detected.

Another problem I thought of with using the conditioned attic as an air return is, assuming you will be using spray foam to insulate between the rafters under the roof deck, is the flammability issues with the foam. Generally, the foam would need to be covered with a thermal barrier or ignition barrier per code if directly exposed to the interior of the home, which would be the case if using the attic as a return air path.

Posted By arkie6 on 06 Jul 2010 02:52 PM
According to every dictionary I looked at, the area below the roof of a home and above the ceiling joists is considered an attic,

By that definition (proably from the 1600s), lofts and second story living areas would be attics? Which we both know is not the case. Attics are uninsulated spaces primarily used to ventilate roofs. If an attic is converted to conditioned space it is no longer an attic. If it meets the minimum reqirements for rooms, then it is habitable.

the MC cable would be an acceptable substitute for conduit

Posted By arkie6 on 06 Jul 2010 03:16 PM
Regardless of code interpretations, if you do intend to use a large space with lots of combustible materials present such as an attic, crawlspace, or space between floor-ceiling as an air handling medium, it would be a good idea to install a smoke detector in that area with a relay contact output that can be used to disable the blower motor if smoke is detected.

a fire stat works off temperature, not sure if a smoke detector works the same or is code compliant to controll airhandler

Posted By cmkavala on 06 Jul 2010 03:54 PM

Attics are uninsulated spaces primarily used to ventilate roofs. If an attic is converted to conditioned space it is no longer an attic. If it meets the minimum reqirements for rooms, then it is habitable.

Not according to the International Residential Code.  Reference R806.4 "Conditioned Attic Assemblies".  Also, the 2009 IRC added the definition "Habitable Attic" to R202.

Arkie6;

as I said primarily , refer to R806.1,  the states I operate in have not yet adapted the 2009 IRC, "Habitable Attic" sounds like a "oxymoron" , but not the first I have seen in code books

Posted By arkie6 on 06 Jul 2010 12:09 AM
Good idea except for one possible problem. The problem you will have is if you have any PVC jacketed wiring passing through the attic. The National Electric Code in particular doesn't allow regular electrical wiring in air plenums except under limited circumstances. The concern with PVC jacketed wiring is the toxic gases it produces if it burns and if in a plenum, distribution of that toxic smoke throughout the structure. There is plenum rated wire & cable available, but it is more expensive than your standard Romex house wire.

How about using a wire mesh type of material for a project like this?

Try these guys

http://www.bwire.com/


Located in the United States, and I believe that they should have the exact alloy that you need.  I used wire mesh during the isulation of my attic, and they helped me out a lot, while still delivering a pretty good price for the material.  Has help up 5 years so far.