New HVAC duct vs. existing old rigid conduit

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LarryFine

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
I have a landlord customer who asked me to "simply relocate two conduits" that were exposed when the HVAC guy cut a hole for a new HVAC install. It's black-iron conduit, original to the building, on the first floor of a three-story apartment building. At least one of the two turns horizontal as it passes through the top plate.

This is what is basically a zig-zag in a wall between the living room and the hallway, against the ceiling. You can see the kitchen wall on the left that turns away from the camera through the hole, and there is another wall to the right that turns toward the camera, so there is no simple task of moving them to an adjacent stud space.

So, rerouting them would involve finding both ends of each conduit, pulling the wires out, make ceiling and wall holes, cutting the pipes, using threadless fittings to go around the hole, pulling in new, longer wires and/or adding exposed J-boxes, and patching the walls. A nightmare with drywall over lath and plaster.

Wouldn't it be code-compliant to leave the conduits in place and have the HVAC guy cut and seal the duct around them, since it would be passing through the duct and not run along within it? I remember this being allowed with NM cable, and I can't imagine doing the same thing with conduit would be a greater hazard.

I'd like to recommend this suggestion to the landlord. I really don't want this job.


IMG_1281[1].JPG
 
You'll need to know more about exactly what/how this duct is constructed and its purpose. 300.22(B) covers ducts specifically formed for air handling purposes. I think you're a no go under that. But 300.22(C) exception covers other spaces for environmental air and it specifically mentions joist and stud spaces in dwellings for return air. It has to go through the shorter dimension, and it looks like these do. But if this is going to be a branch duct or main trunk, you're probably stuck.
 
The only equation I know in air calc's is; (and I think I got this right) take the square area of the total obstruction.
Down line from obstruction it will take an addition 1.667 additional sq areas of that square area size for truck to get back
to desired air flow.
 
I am amazed someone asked to move the conduits! Usually "they" just pan around them and continue with the HVAC install. Unless the HVAC guy is really on the small size of duct and thinks these will obstruct airflow.

What is the highest temperature the HVAC guy thinks will be in that duct? If it is over 140 °F then....
 
LarryFine said:
Right. I don't have access to my '17 NEC right now.

I need to know whether code would allow this like I believe is allowed for NM.
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Larry,
I don't see that this can be compliant with 300.22. The allowance you are thinking is for a space such as stud cavity not a specific fabricated duct such as you will have. The only exception would be if the wiring was required for equipment in the duct.
 
Perhaps the HVAC guy could be consulted whether the open area to the left of the conduits is sufficient by itself for a duct transition to be fabricated so that the duct passes through just on the left side of the conduits.
Or if more cross-sectional area is needed, maybe the opening in the wall could be extended downward somewhat, and a horizontal to vertical transition similar to the following one could be fabricated to put on both sides of the opening:

http://www.ductworkinstallation.com/Portals/0/Guide 102/443tfv.jpg

It would reduce the headroom, particularly closer to the wall. But if it goes no lower than the top of a door opening that might be OK.
 
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I'd open the walls and ceiling more and tell them to patch/ replace the drywall or plaster. That is cheap by comparison to the electrical and duct options. You still have to deal with the electrical or duct modifications, but drywall is cheap.
 
I may have missed it.
Is it a supply or return?
Return leave it.
Supply, either move duct or conduits.
Which has less repair involved.

Have HVAC enlarge hole and use a split duct.
They could also use a rectangle duct with a long dimension going up and down.
 
have HVAC guy wrap those conduits with sheet metal and seal them so that they are no longer in the air flow when he cuts and fits duct around them
 
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