I’m building a 1000 and a 500 gallon offset. Not sure how far below the grate should I start the exhaust collector at. Grate level? 2 inches or 3 inches below? I was thinking of running it 3 inches below and 3-5 above. So 8 inches tall.
For what it is worth, I locate the plenum such that the bottom of the plenum is 1-3” above the lower cooking grate and run it vertically 1-3” taller than the diameter of the exhaust tube. I found make it 1-2” deeper and wider than the pipe diameter leaves enough material on top of the plenum to securely weld on the pipe. Best to mock the plenum up with cardboard (in the old days I would use cheap old 1/2 inch plywood) before cutting steel.