The outside diameter of the Air Inlet Conduit and maybe an inch more to give you some elbow room and the drop angle to the outside, (3% 0f the thickness of your wall, so they say) to avoid water from traveling inside. *Note page 14* Thicker walls will possibly need a little larger hole. Basically, if you can stick the air inlet conduit through the wall and pour a small amount of water in it on the inside and if it runs outside, you’re good to go. The angle doesn’t need to be extreme.
Just by looking at it, the flue is maybe 3″, like a hot water tank vent, and the O.D. of the air inlet conduit is either 6-8″. I agree that the instructions are not clear.. JMO..
https://www.rural-energy.net/docs/om_homcomfort.pdf
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