This sort of vibration isolation is primarily useful to reduce sound transmission into the building structure and then inside, it won't do much to reduce sound transmission through the outside air.
Can someone please link the product? There are so many models listed on this page:
https://novibration.com/products/bonded-tube-mounts-22000-series/
As for which product, possibly the smallest size listed on that page. Look at the load ratings link, and then:
Assuming you've got 4 bolts holding the water heater in, what you need to do is determine the vertical load (which is radial on a horizontal bolt) and horizontal load (which is axial on such a bolt) on each bolt. If the water heater weighs 60 lbs, then the vertical (radial) load is 15 lbs on each bolt. As to the horizontal (axial) load, say the center of mass of the unit is 4" in front of the plane of the mounting brackets, and say the mounting brackets are 24" apart vertically. Then the moment is 4*60 = 240 in-lbs, and each pair of vertically space bolts (left and right) would need to resist 120 in-lbs. At 24" apart, those bolts would only see 120/24 = 5 lbs each.
So do that calculation using the actual values for your water heater, and select a mount that is rated somewhat above, but not way above, the axial and radial loads. You also need the natural frequency to be quite a bit lower than the frequency of the sound you are trying to isolate, so figure out that frequency (e.g. if it is the fan, look up the fan speed, or maybe just use an app on your phone to analyze the spectrum of the sound the unit is making). As you won't be loading the mounts to the full rating, the natural frequency of your configuration won't match the rated natural frequency on the spec sheet, but as long as the loading is close, the natural frequency is close. So that's one reason you want a natural frequency much lower than the frequency you want to isolate, to allow for that error.
[If it were desirable to more closely match the rated axial and radial loads, and the radial load is quite a bit lower than the rating, you could increase the radial load by cantilevering the unit farther off the rigid portion of your mounting system, i.e. using a spacer block and longer bolts. Just something to consider and maybe figure out if your first attempt is not producing good results and you know the actual axial load is much less than rated.]
Same for snubbing washer, which one should I buy?
https://novibration.com/products/washers/
The snubbing washer just needs to be a sufficiently thick fender washer whose ID will pass the bolt you use, but not the bolt head, and whose OD is large enough that it won't pass through the hole through which the rubber passes.
Also, I'm a novice. Can someone please explain how I'd use this? I have a tankless water heater mounted on a block of wood which is mounted on the wall.
If you want to use a tube mount like those shown, what you need to do is rigidly mount a solid (probably metal) bar on standoffs to your structure. The bar thickness would be dictated by the tube mount you use (or rather the tube mount selected to match your bar thickness), and you'd drill a fairly large hole in the bar at each bolt location, again of the proper size for your tube mount. Then IIRC each tube mount is two pieces, rubber on the outside with a solid metal cylinder down the middle. The two rubber pieces go into the hole, one from each side, and the bolt goes down the metal cylinder in the middle. The item mounting bracket is under the bolt head against one end of the rubber tube (which is skinnier in the middle in the part inside the hole in the bar), and the end of the bolt receives the snubber washer and a nut (hence the need for the space behind the bar, so those components don't contact the building). The result is that the bracket, bolt, snubber washer, nut, and tube mount metal cylinder are all in contact, but that assembly is not directly in contact with the rigidly mounted metal bar anywhere, they are separated by all the rubber components which provide the springiness that leads to isolation.
Hope this helps, that's what I remember from doing this just a couple times in the last 20 years.
Cheers, Wayne
