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Lag screws to mount tankless heater on exterior wall

1818 Views 3 Replies 2 Participants Last post by  carpdad
I've got a tankless water heater that needs to be mounted on an exterior wall. Manufacturer suggests mounting to studs. I think I've managed to locate the studs, but I am worried about the length of the fasteners I need. I was thinking about going with 3" long, 1/4" diameter lag bolts. That should be long enough to get through the stucco and wood sheathing and into the stud. House is single story 1940s pier and beam construction. A few questions:
• Are 1/4" lag bolts thick enough? The heater weighs about 60 lbs.
• Should I use stainless steel bolts outside? The mounting bracket for the heater is galvanized, so I'm concerned about corrosion. I can use nylon washers under the stainless bolts to separate the metals, although the bracket will still be slightly contacting the stainless as it will be resting on the bolts.
• Can I safely assume the floor joists line up with the studs? After a cursory inspection under the house, that looks to be the case but I'm not 100%. I want to get this right the first time and not drill a ton of holes into the side of the house.
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First check from inside if there may be electric cables or plumbing pipes?
Where the mount will go, check from inside for studs. Look for electric outlets, remove the cover and try to see if it's fastened to a stud. Most studs are 16" apart. Once you find the center of that stud (drill more holes if must or stud finder), measure over 16" and confirm that mark is the stud center. Inside holes can be patched and spot painted.
Then you can measure from common spot, such as a window or door and make the approximate mark outside. Or get long drill bit (16" or so x 3/16) and drill through the wall to outside. First prove with a wire and make sure you hit nothing but the sheathing and there's no wire or pipe outside.

If you know about where the studs are, you can confirm the centers by drilling from outside as well. Make a mark where the heater mount will go and drill small holes below it. Caulk over later and all the prove holes will be hidden by the heater.

There may be stud finders that can sense through the wood siding and even tell you if there are cables and pipes, but I like to confirm physically.
Electrical wires are usually centered in 2x4 studs and you can use 1.25" screws only when installing 1/2" drywall. Keep that in mind for your fastener lengths. For 60 lbs, and the heater probably doesn't move or vibrate (?), so even #14 or so wood screws should be enough. Even galvanized, I would use ss screws.

When proving with drill bit or wire, don't punch through. Go as much as the depth of the drywall, example, then prove with a wire for obstruction.
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How about 2x pressure treated lumber to span the studs? Or galvanized metal angles? Sorry but no experience with hanging heaters. One thought is does it need a fire block? You can ask this question in the hvac or plumbing sections.
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