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I'm installing a tankless water heater (150lbs static load, 10" deep)

I only have one option for installation and it is a 2x3 framed wall with 24" centers.

The Top bracket is designed to mount on top of a horizontal crossmember with screws penetrating the cross member in four locations and the studs in two. The bottom bracket requires a cross member as well but it just bolts to the cross member.

My question is this. What is the best way to install this right (without rebuilding the wall). Double jack studs would work if the actual dimensions were 2x3, which they are not. Should I just add another stud inside and work off of that?

I would fit two 2x6 (or larger) blocks crossways between your studs at the two mounting locations. Put three 3" construction (not black drywall) screws through the studs into each end of the blocks. Pilot through the studs to avoid splitting. You won't need to pilot into the end-grain of the blocks unless they're very hard wood.

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| |* | |* <-- toenail screws or angle brackets
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| |______________________| |
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| | 2x6 block | |* <-- three screws
| |______________________| |*
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| | | |

This will easily carry your heater. The only weakness at this point is the connection of each stud to the top plate. you might add toenailed screws or angle brackets there.

You could also lay in two 2x6 studs, flatwise, tight to each 2x3 stud in the bay where you'll mount the heater. This would give more horizontal stiffness if you feel like your studs are too flexible. Run screws through the 2x3 studs every foot or so.

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| | | | | |
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| | 2x6 | | 2x6 | |
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The other answer given so far describes a solution where the tankless water heater is being installed in an area where the studs are open and not covered. There are instances where the studs are not open and may be covered with drywall (a.k.a. sheet rock). It this instance the installation of extra studs or cross blocking is not so easy and other methods are called for. I will describe two options:

1. You can cover the entire wall area where the brackets will be attached with a piece of 3/4" plywood. This may actually need to extend beyond the end of the bracket if the existing studs in the wall are farther away from the bracket mounting points. Secure the plywood to the wall surface using screws through the plywood and into the existing studding structure. Then mount the bracketing to the plywood. With proper size pilot holes for the screws the plywood does a really good job of holding the screws.


2. An alternative solution would be to mount two horizontal 2x4s or 2x6s flat on the wall centered vertically where the tankless heater brackets will be located. These cross members are attached with screws at each existing stud location. They may have to extend longer than the width of the bracketing if the next stud is further over from the mounting location. Once the cross members are secured the bracketing can be screwed into the cross members.

Note: Either of these schemes can also be used on a wall that has open studs. In fact applying one of these techniques could even be easier to install than extra studs and blocking between existing studding.