HVAC contractor concerns

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kwired said:
That 3-1/2" of fir was specifically fabricated to be structural support, the HVAC installer happened to use a portion of it as a duct wall.

If they would have used some additional horizontal distance of the joist space as duct we would all probably agree it is "other space for environmental air", but since it is only 3-1/2" inches long some see it as something else, but it isn't. The NM cables are still crossing perpendicular to the air flow just like they would be if the "duct" were horizontal in the joist space, effects of air flowing over the cables is not really any different in either case. Biggest issue with the install is the ugliness of being able to see the cables through the register.
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Yup, NEC violation or not, it is just wrong!
 
kwired said:
That 3-1/2" of fir was specifically fabricated to be structural support, the HVAC installer happened to use a portion of it as a duct wall.

If they would have used some additional horizontal distance of the joist space as duct we would all probably agree it is "other space for environmental air", but since it is only 3-1/2" inches long some see it as something else, but it isn't. The NM cables are still crossing perpendicular to the air flow just like they would be if the "duct" were horizontal in the joist space, effects of air flowing over the cables is not really any different in either case. Biggest issue with the install is the ugliness of being able to see the cables through the register.
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The reason you don't want cable in your duct work is that in a fire the products of combustion get pumped around the house, especially in a residential setting where you rarely see duct smoke detectors installed for unit shut down. The joists have become part of the fabricated duct work by function if not by design.
 
gadfly56 said:
The reason you don't want cable in your duct work is that in a fire the products of combustion get pumped around the house, especially in a residential setting where you rarely see duct smoke detectors installed for unit shut down. The joists have become part of the fabricated duct work by function if not by design.
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Absolutely, but the allowance to run them across a return air in a panned off joist space leaves you nearly exactly the same risk of that happening as what the OP has.
 
kwired said:
Absolutely, but the allowance to run them across a return air in a panned off joist space leaves you nearly exactly the same risk of that happening as what the OP has.
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OK, maybe I don't get it. Where are you allowed to run perpendicular through a duct?
 
gadfly56 said:
OK, maybe I don't get it. Where are you allowed to run perpendicular through a duct?
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You are allowed to run perpendicular to the joists, studs, or rafters, in a return air plenum made by closing off the space between those structural members. You are not allowed to run parallel to the plenum even for short distances. I think that the idea is that if you pop in and come back out the same side you should have been able to run outside the plenum instead, but if you are crossing it there may be no good way around.

You are never allowed to run wires inside a supply air duct except for a limited distance to connect to sensors in the duct.
 
gadfly56 said:
OK, maybe I don't get it. Where are you allowed to run perpendicular through a duct?
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Unless you work in dwelling units it is not something you would run into.

Here is the exception to 300.22(C) that allows it.

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.
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GoldDigger said:
You are allowed to run perpendicular to the joists, studs, or rafters, in a return air plenum made by closing off the space between those structural members. You are not allowed to run parallel to the plenum even for short distances. I think that the idea is that if you pop in and come back out the same side you should have been able to run outside the plenum instead, but if you are crossing it there may be no good way around.

You are never allowed to run wires inside a supply air duct except for a limited distance to connect to sensors in the duct.
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OK, now I get it. However, IIRC the current ICC Mechanical Code no longer allows slapping tin over joists and such to turn them into ducts. So the HVAC installer may have dodged one violation but he has a different one to contend with.
 
gadfly56 said:
OK, now I get it. However, IIRC the current ICC Mechanical Code no longer allows slapping tin over joists and such to turn them into ducts. So the HVAC installer may have dodged one violation but he has a different one to contend with.
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I wonder why that is. It is a pretty cost effective way to make a return air plenum. I would not want to do it for a supply plenum due to leakiness but for a return plenum it seems like a pretty decent choice.
 
Smash said:
I'm an electrician rewiring a older home. I cut a 18" trench in the attic floor gable to gable drilled all joists to enable access to all of 2nd floor. Had drilled 3 holes in each and neatly completed the 2nd floor. The HVAC contractor uses my cut floor trench and cuts his return vent into the 2nd floor ceiling. He makes a cardboard square installs his return duct and tapes around the opening. Now underneath there is a vent cover and all my wires are running through. All he had to do was move over 2' and cut his own attic floor hole. How legal is this ? Any danger ? Can I be liable ? Does he have to remove it and cut his own. Just want to get my facts right before I approach him. It was obviously my trench installed by yours truly. Thx for any clarification on this subject.
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My question would be is this job getting inspected by the AHJ ?
 
petersonra said:
I wonder why that is. It is a pretty cost effective way to make a return air plenum. I would not want to do it for a supply plenum due to leakiness but for a return plenum it seems like a pretty decent choice.
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For the same reason I pointed to regarding cables in the plenum above. When the fire starts eating through the wood, smoke and possibly fire get sucked into the return plenum. I actually have this in my 70's era house. They didn't encase the wood, just slapped a "cover" over the joists. Still wood on three sides.
 
gadfly56 said:
For the same reason I pointed to regarding cables in the plenum above. When the fire starts eating through the wood, smoke and possibly fire get sucked into the return plenum. I actually have this in my 70's era house. They didn't encase the wood, just slapped a "cover" over the joists. Still wood on three sides.
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But a fire in the room the register is in wouldn't ever suck smoke into the plenum, right?

This is one rule that has always been a little puzzling to me. Can't put wiring methods into plenum (with some exceptions) because of the smoke they might produce, yet we are supposed to select sufficient ampacity conductors and provide proper overcurrent protection so that they don't make any smoke in the first place. Sure they have plastics that give off toxic fumes when burned. The average house has much more items to possibly burn and give off similar toxins then it has electrical insulation. Then they also want us to install smoke alarms that should also warn you of such things before it gets really bad.

In applications that have a fire alarm system, there usually is automatic shut down of environmental air systems if the alarm is activated as well.
 
jumper said:
Section 602.3 of the 2012 IMC still allows wall stud and floor joist plenums for return air, there are conditions to be met.

Edit: No changes in the 2015. Just checked.
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Learn something new every day.
 
kwired said:
But a fire in the room the register is in wouldn't ever suck smoke into the plenum, right?

This is one rule that has always been a little puzzling to me. Can't put wiring methods into plenum (with some exceptions) because of the smoke they might produce, yet we are supposed to select sufficient ampacity conductors and provide proper overcurrent protection so that they don't make any smoke in the first place. Sure they have plastics that give off toxic fumes when burned. The average house has much more items to possibly burn and give off similar toxins then it has electrical insulation. Then they also want us to install smoke alarms that should also warn you of such things before it gets really bad.

In applications that have a fire alarm system, there usually is automatic shut down of environmental air systems if the alarm is activated as well.
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I'd surmise a lot of those rules were the result of this:

https://en.wikipedia.org/wiki/MGM_Grand_fire

Since I started out doing hotel wiring, we learned about CMR (vertical burn characteristics are better and UL-approved) and CMP (teflon jacket gives off less toxic smoke than PVC). You dont want hundreds of pounds of highly toxic (if it catches fire) wiring of any type in air returns. Yes, the carpet, furniture, drapes, etc will all produce wayy more smoke in a fire, unless that fire happens to start in an air handling space.

Though I cheerfully fire-stop all my vertical penetrations and use CMR/P when necessary, those are really quarternary on the grand scale. Building rated assemblies, compartmentalization (5/8" or double layer drywall, HVAC dampers, rated doors in stairwells), suppression (sprinklers) and notification (strobes, horns) are much more important, and if you get a fire inside a wall or above a drop ceiling, imho the building is already in serious condition and no amount of firestop or CMR/P cable is going to help at that point.
 
JFletcher said:
I'd surmise a lot of those rules were the result of this:

https://en.wikipedia.org/wiki/MGM_Grand_fire

Since I started out doing hotel wiring, we learned about CMR (vertical burn characteristics are better and UL-approved) and CMP (teflon jacket gives off less toxic smoke than PVC). You dont want hundreds of pounds of highly toxic (if it catches fire) wiring of any type in air returns. Yes, the carpet, furniture, drapes, etc will all produce wayy more smoke in a fire, unless that fire happens to start in an air handling space.

Though I cheerfully fire-stop all my vertical penetrations and use CMR/P when necessary, those are really quarternary on the grand scale. Building rated assemblies, compartmentalization (5/8" or double layer drywall, HVAC dampers, rated doors in stairwells), suppression (sprinklers) and notification (strobes, horns) are much more important, and if you get a fire inside a wall or above a drop ceiling, imho the building is already in serious condition and no amount of firestop or CMR/P cable is going to help at that point.
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I was only about 11-12 years old then, but would imagine wiring methods and construction methods in single family dwellings didn't change all that much because of it.
 
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