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We have a wood-frame house with 2x4 studs and half-inch plywood exterior walls. What is the generally accepted best practices for mounting a 80lb mini split condenser on a wall bracket to this type of wall?

The advice on the Internet appears to center around getting it on the studs. Unfortunately, due to installation location constraints, it turns out we couldn't get the unit to sit directly on studs.

There are three bolts on each bracket, and they're 3/8" diameter. They were screwed in tightly into the plywood and hold the machine up. Based on charts of plywood pull out strengths online, it appears that there is enough safety margin with this setup.

The question I have is whether or not this is generally considered an acceptable install, and if not, what steps can be taken to upgrade the mounting. Perhaps extra wood supports to bridge the unit over to studs? Worst comes to worst maybe the unit may need to be remounted, but this is difficult.

What is the best course of action in this case?

Thanks!

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  • How high up from the ground is this unit mounted? Do people and animals walk below it? Is this located in an earthquake area? My initial reaction is that the plywood will hold this unit.
    – Jim Stewart
    Commented Aug 16, 2019 at 10:43
  • The unit is about 7 feet off the ground. It's mounted right next to a chimney, so nobody is going to walk UNDER it. With that said, it's located on the side of the house where people walk past to go to access the alley. This is supposed to be an earthquake area, but we've fortunately not had one yet. The unit is mounted quite close to the wall (doesn't affect airflow much, it's a small unit) so there's not an undue amount of torque being generated.
    – user99078
    Commented Aug 16, 2019 at 20:23

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If the brackets are lag screwed into plywood, I wouldn't trust it. There isn't much meat for threads to work with in 1/2" plywood, and there is going to be some vibration from the compressor.

lag screw

If it's held in with toggle bolts,

toggle bolt

it will be very strong, I would not be worried about it pulling out. But I'd still rather not have a vibrating piece of equipment of this size and weight hanging on the plywood sheath. I'd be concerned that the weight would warp or otherwise deform the plywood over time.

To make an attachment like this, you could bridge across studs with three horizontal pieces. You'd anchor the horizontal pieces into the studs with long lag screws, and anchor the brackets to those pieces.

You could use 2x4's for the horizontal members, and lag screw the brackets into the 2x4's. The 2x4 will hold the threads much stronger than the plywood. You could paint the wood and it might look pretty good.

I would probably use 7/8" tall strut channel for the horizontal pieces lagged into the studs:

unistrut channel

and strut channel nuts to attach the brackets to the channel:

strut nut

Some consider strut a little too industrial looking for home use, but I think it might look fine in this case. It will make such a strong mount you could get by with just two horizontal pieces bridging the studs. After all air conditioning compressors are not invisible, you are going to have to look at something here. The strut is quite a bit smaller than a 2x4 and once it's painted I think it will look good.

This is a slightly harder install than using 2x4's. The vertical spacing between the two pieces must match the spacing between the holes on the brackets, and needs to be pretty precise. The easy way to do that is attach the brackets to the strut FIRST, then screw the strut into the studs.

The hardest part here is going to be disconnecting and reconnecting the unit.

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  • newer inverter systems are incredibly quite (not much vibration) , I would keep an eye on it and if any movement of the brackets is noted get some strut and tie into a couple of studs with the strut then the 3 lag bolts can be converted to machine bolts and lock the unit in. Strut comes in a bunch of different brands all of them will support this load even aluminum. Brands like kendorf, unistrut, super strut, these are all the same basic stuff, I go with whoever is cheaper and I use at least 500’ per year. I would not use toggle bolts out side but they are a cheaper option than strut.+
    – Ed Beal
    Commented May 7, 2020 at 13:49
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I would leave it as is, but of course check it on a schedule. The plywood is probably attached to the studs with siding nails, but plenty of them and this achieves support by friction.

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