Originally Posted by
brian schmitt
should be within the upper and lower 12" per section 702 of the uniform mech code.
They are.
Jeff was asking if the entire vent had to be.
The answer is 'No.'.
As long as at least the top of the top vent is at or within 12" of the ceiling, that is okay.
Likewise, as long as at least the bottom of the bottom vent is at or within 12" of the floor, that is okay.
The entire vent opening does not have to be within that 12".
One could conceivably make a 12" square opening, turn it on the diamond with one corner up, and, as long as that one corner up was at or within 12" of the ceiling, that installation would be acceptable.
The idea between the two vents is to get air circulation in the room, the appliance then draws its combustion air from that circulating air.