Air returns/ NM cable

[nizak]

I know that NM cable installed in a joist space can run perpendicular to the joist but not the long dimension due to toxic fume spread if burning.

How is it that plumbers and central vac guys can run their pvc piping inside the panned off returns. Residential job I was on vac guy shows up and installs at least 10' of 1.5"pvc directly in two returns.

HVAC / plumbing rough inspection completed today and approved for cover.

Makes no sense to me.

 

[Sierrasparky]

Just because it was passed does not make it legit.

 

[J.P.]

Maybe because their piping isn't apt to actually start the fire.

For their stuff to release toxic fumes the chase would have to be on fire already.

 

[mgookin]

OP does not indicate what type of job this is or where it is.

If it's in an air stream for an HVAC system which supplies air to a means of egress, it's not allowed under most codes.

I've never heard of the perpendicular exemption but maybe some inspectors let it slide (without authority to do so).

It's a mechanical code issue.

 

[GoldDigger]

OP does not indicate what type of job this is or where it is.

If it's in an air stream for an HVAC system which supplies air to a means of egress, it's not allowed under most codes.

I've never heard of the perpendicular exemption but maybe some inspectors let it slide (without authority to do so).

It's a mechanical code issue.

There is also an NEC issue: It is found in 300.22 [2011]. Subsection (C) describes wiring in environmental air spaces not specifically constructed for air handling (i.e. covered stud or joist bays). But there is an exception to the requirements of that subsection:

Exception: 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.

For anything which is specifically constructed as an air duct (e.g. 100% metal ducts or transition boxes, flexible or rigid round duct, etc., as described in subsection (B)), the only wiring allowed is for smoke, flow or temperature sensors and the like. There is no perpendicular exception there at all.

 

[kwired]

There is also an NEC issue: It is found in 300.22 [2011]. Subsection (C) describes wiring in environmental air spaces not specifically constructed for air handling (i.e. covered stud or joist bays). But there is an exception to the requirements of that subsection:


For anything which is specifically constructed as an air duct (e.g. 100% metal ducts or transition boxes, flexible or rigid round duct, etc., as described in subsection (B)), the only wiring allowed is for smoke, flow or temperature sensors and the like. There is no perpendicular exception there at all.

Is there similar requirements in other codes that would apply to say a PVC sewer pipe?

I can see the sewer pipe not being allowed in a space specifically intended for air handling, but could be in a covered stud or joist bay. Then comes why can a PVC sewer pipe run parallel with the joist bay but non metallic wiring methods typically can't? Fire likely to start in wiring but not in the plumbing - only slightly makes any sense. Chance of a fire starting in an unbroken length of conductor with proper overcurrent protectino is so slim the risk is almost non existent IMO. It is much more likely for a fire to start at/near a splice or other termination point, so if anything don't put any junction/outlet boxes in the joist bay that is also used for environmental air, but then I am not on any code making panels...

 

[JFletcher]

Sewer/water pipe is a lot thicker than NM insulation, and short of flushing lava or cesium metal down the drain, it's not going to catch fire.

When is a joist bay used as an air return?

 

[nizak]

It is used as an air return all the time in wood framed residential construction.

Warm air is metal ducting usually oval shape.Return air is simply the wall cavity created by vertical 2x4's ,and the drywall on each side forms the containment.

 

[kwired]

Sewer/water pipe is a lot thicker than NM insulation, and short of flushing lava or cesium metal down the drain, it's not going to catch fire.

When is a joist bay used as an air return?

So is PVC conduit, which also must follow same rules - not allowed in environmental air spaces except when run perpendicular across a joist bay or similar space used for environmental air. I never see PVC conduits or non metallic cables inexplicably catch fire, there has to be some other condition that is a code violation involved, like improper conductor sizing or improper overcurrent protection. Failed terminations can cause fires - but you don't have those in the middle of the run, you put them in junction boxes, cabinets, etc.

 

[JFletcher]

It is used as an air return all the time in wood framed residential construction.

Warm air is metal ducting usually oval shape.Return air is simply the wall cavity created by vertical 2x4's ,and the drywall on each side forms the containment.

Ive never seen this. Regardless, wall cavities are stud bays, not joist bays. How am I supposed to know if I can run electrical thru certain wall bays? Wouldnt using the stud bay be dependent upon insulation?

eta: I've FELT this (air blowing). Using a stud bay seems to me like an extremely lazy way for the HVAC guy to get his work done... or is there a better than "I want to be lazy as hell so I'll use the bay as a duct" reason not to run flex/hard ductwork in a wall?

 

[JFletcher]

So is PVC conduit, which also must follow same rules - not allowed in environmental air spaces except when run perpendicular across a joist bay or similar space used for environmental air. I never see PVC conduits or non metallic cables inexplicably catch fire, there has to be some other condition that is a code violation involved, like improper conductor sizing or improper overcurrent protection. Failed terminations can cause fires - but you don't have those in the middle of the run, you put them in junction boxes, cabinets, etc.

I've seen a few incidents of overdriven staples causing conductor problems and subsequent fires. Call it "unintentional reduced ampacity", but it happens.

 

[kwired]

Ive never seen this. Regardless, wall cavities are stud bays, not joist bays. How am I supposed to know if I can run electrical thru certain wall bays? Wouldnt using the stud bay be dependent upon insulation?

eta: I've FELT this (air blowing). Using a stud bay seems to me like an extremely lazy way for the HVAC guy to get his work done... or is there a better than "I want to be lazy as hell so I'll use the bay as a duct" reason not to run flex/hard ductwork in a wall?

Maybe they don't do that in your area for some reason, but it has been a standard practice for at very long time in many places. I've seen panned off joist bays for air returns in 100+ year old homes that originally had convection (no blowers) type heating systems for old coal burning central heating systems.

There is also return pulled through the space above suspended ceiling tiles quite frequently in commercial spaces - no ducting to return grilles they are just open to the space above the ceiling and return duct is run from air handler to space above ceiling and it is just open to that space. Could still have fire dampers in some places but is open once in a space that needs no additional fire barriers.

 

[JFletcher]

There is also return pulled through the space above suspended ceiling tiles quite frequently in commercial spaces - no ducting to return grilles they are just open to the space above the ceiling and return duct is run from air handler to space above ceiling and it is just open to that space..

That I have seen often, in hotels.

 

[readydave8]

eta: I've FELT this (air blowing). Using a stud bay seems to me like an extremely lazy way for the HVAC guy to get his work done... or is there a better than "I want to be lazy as hell so I'll use the bay as a duct" reason not to run flex/hard ductwork in a wall?

sometimes it makes a huge difference in missing obstacles, making turns, and avoiding transitions in size