metal heat duct blocking J-box location, how to proceed???

jaansuMarch 20, 2014

In order to center the ceiling fixture, I need to put in a box just where a circular aluminum duct runs. Even moving the fixture 4" to get it next to the duct will be noticeable. Is it code to attach a metal box right to the duct with screws (I would have to bang in the duct a little to make it fit into the ceiling). Or perhaps to cut out the profile of the box and place part of it in the duct? Is heating of the box an issue? Can I insulate it somehow?

My last resort is to bang up enough of the duct to attach it to the adjoining joists via a chandelier hanger junction box. This location is near the end of the longest duct run of the house and won't impact air flow that much.

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joefixit2

Ducts usually lie on top of the ceiling joists, leaving a 3.5" space between the duct and the ceiling. If for some reason yours is laying right on the plaster/drywall you can cut it off and replace it with a section of flex duct.

    Bookmark   March 20, 2014 at 9:52PM
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bus_driver

Is this new construction or remodel?

    Bookmark   March 20, 2014 at 10:02PM
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jaansu

this is remodel. I like the idea of flex duct - I didn't know something like that exists. Still, it would be easier to attach to the duct itself using a shallow box. Anyone know if this is code?

    Bookmark   March 21, 2014 at 12:13PM
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bus_driver

Being unaware of flexible duct identifies you as inexperienced. Not meant as an insult. My suggestion is to contact an HVAC contractor to look at your situation as the next step.
In building a house, the sequence is that the plumber gets first choice of the spaces for utilities-- the DWV system must have suitable fall-- because sewage does not flow uphill. The HVAC gets next dibs on the remaining space. The electrician gets whatever, if any, space that remains after that.
The present challenge that you face is not unusual for electricians. Can lights in a large room and down a hallway can be extremely challenging to get in a straight row because of framing and other utilities.

    Bookmark   March 21, 2014 at 3:13PM
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niick84

If its a ceiling fixture you cannot connect it to the duct it's not rated to hold any weight and the light may fall. You will need a bracket to span the two joists, you will need to use a metal box to attach to the bracket, if the regular box is too deep you can use a pancake box. They do make an old work version that you put in the hole and turn it til it's tight it spreads as it turns.

    Bookmark   March 22, 2014 at 9:53PM
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jaansu

I suspected as much about the duct idea but good to hear from someone that knows his stuff. I've put in dozens of chandelier hanger boxes over the years and will probably need to do so here as well.

    Bookmark   March 23, 2014 at 11:49AM
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jaansu

problem solved. Opened up the ceiling and only had to bump in the vent about an inch to allow a bracket to span the joists and install a pancake box. As the duct is very near the end of a long run anyway, there is almost no effect on the air flow.

    Bookmark   April 5, 2014 at 7:51PM
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alan_s_thefirst

That was probably your best option. Electricians, plumbers, alarm and media installers do that and worse to ductwork all the time.

For future reference, they do make a surface mount electrical box for these cases - it's only just over half an inch deep so it can be mounted directly on a ceiling joist. In a retro application, you'd be cutting the drywall carefully away to expose it, repairing/installing a vapour box over it up in the ceiling.

The downside to these thin boxes is they don't have a lot of space for wires, so you wouldn't want to use it in an instance where you have a bunch of wires entering unless you had room in the fixture to allow them to fit inside, assuming that's permissible by code.

    Bookmark   April 12, 2014 at 1:22AM
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