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I am in the process of replacing an old bathroom fan in a guest bathroom with a new Nutone ARN110LKVV. After running some new electrical and getting the old fan out, I finally got to the step where I attach the new fan metal enclosure to the joist. I am following the instructions, which tell me to use "appropriate fasteners." I have no idea what an appropriate fastener would be.

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Moreover, the metal box being attached to the joist does not have any actual "holes" to screw through. They seem to be metal indentations, that don't seem like they're supposed to be screwed through.

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This is a close-up of what one of those "identations" look like on the inside of the box, and this is a close-up of that same indentation from the outside.

Inside:
enter image description here Outside:
enter image description here Is there some sort of screw that's intended to go through the metal and into the joist? Or should I use a drill bit to drill through those indentations and into the joist, then use some sort of screw? The instructions have no guidance on what type of fastener to use.

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  • Pre-drill the holes and use any corrosion-resistant screw you see fit. Typically sheet-metal is affixed using self-tapping screws if you were curious.
    – MonkeyZeus
    Sep 28 at 19:49
  • Unless you're using a finder driver (a sheath) you need self tapping or luck. Push harder....
    – Mazura
    Sep 29 at 4:24
  • That indentation looks like could help to seat the box in place during mounting. Place the box where you want against the joist and give it a good smack (with a fist or mallet or such). Now the box should be seated more stably into the joist so you can drill more easily without it sliding around.
    – thehole
    Sep 29 at 15:04

2 Answers 2

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Almost any wood screw.

It takes only say 3/4" of penetration into a joist to secure such a housing. Thread type doesn't much matter. Diameter doesn't, either, though thinner screws will go in easier. Common drywall screws or similar work well, as their flat heads pull in mostly flush. Sheet metal screws would also work fine. You can punch them through anywhere that doesn't interfere with other components. Any sharp-pointed screw will go through with some force, or you can pilot for them or just punch a hole with a hammer and nail.

However...

Fastening a housing directly to a joist transfers a lot of sound to the home's framing. A/C motors have a distinct 50/60Hz hum, which you're likely to hear in adjacent rooms. Consider mounting with dampening standoffs, bracket arms, or other vibration-isolating means, if possible.

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There exist "self drilling" screws that have a hardened drill point for piloting their own holes through metal, but these tend to have threads that are too fine for grabbing onto wood solidly (they're typically designed for attaching metal to metal).

I would first try driving a normal wood screw through those indentations with a screw gun (I always use drywall screws for fans because they're cheap wood screws that I always have lying around). The indentations are helpful because they prevent the point of your screws from skating across the metal surface, but if the indentations are inconveniently located, then don't worry about punching a hole somewhere else.

If the metal box defeats the screws, then you can drill a little pilot hole through the box for each screw. The sheet metal is so flimsy that the screws will widen your pilot holes as necessary, so there's no need to obsess over choosing the perfect drill bit size, just choose one that looks a little too small.

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  • Thanks for the reply! I did indeed try drilling a normal wood screw (and tried a self-tapping drywall screw) using a drill through the metal, but the metal definitely defeated me. I can try again tomorrow, and should I fail, move to use a pilot hole with a smaller drill bit. Thanks for the help!
    – ZbadhabitZ
    Sep 28 at 7:17
  • "appropriate fasteners" are exterior screws, used inside. They don't say drywall on the box or rust out in 3y. And if it says combination Philips square drive, lemme know where you got 'em; killing me with this star drive crap. I can't drive screws with a gun anymore either; totally spoiled on impact drivers, which would laugh at this (already center punched?...) no matter what screw you use. - But what did they intend by saying that and what's to be found in the field? That they used self tapping sheet metal screws because they're the duct jockeys and that's what they carry anyway.
    – Mazura
    Sep 29 at 4:20

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