Screwed some boards to the wall as a rotozip guide.
Moved the upright guide to the right side and finished cutting the 3/8" interior osb. "NOTE" Should have cut the right side first.
When following a guide, cutting clockwise works best.
Cleaned out some insulation and cut a notch out so that my dog dish would be in the right position. DOG DISH? Whaaat?
Made this board as a backup/cutting guide/spacer. Screwed a 1/4" piece of mdf on to make it exactly 1"
Slid it nicely into place.
Screwed the dog dish to a board. Used a 3 1/2" hole saw in the drill press and after a lot of noise I had a hole.
Even thin stainless is not fun to cut.
The hole needed to be just a little bigger than 3 1/2" so I rounded off the edge of this aluminum bar to use as an anvil,
then rounded the edge of the hole with a ball peen hammer.
Gooped it up with silicone and screwed it in the hole. Sorry I didn't get a pic of the hole in the outside skin,
I cut it from the inside with the rotozip.
I needed a line that sloped 3 deg up from the top of the hole. So I printed one and extended it with the tape you see.
And that is how I am venting my hot water heater!!
The pipe needed to slope 3 deg so rainwater doesn't run into the heater.https://www.aquahstore.com/catalog/prod ... l3k1jmf177
View from the outside. The dog dish helped keep it from sticking out too far.
This particular tankless WH sucks outside air for combustion.
Air is sucked in through the outside pipe and exhaust comes out the inside pipe.
. 
. I used plumbers helper as a temp- still haven't finished it.