Installing new Thermador hood over existing electric range w/ 8" round galv. steel pipe with short 5' run: two elbows sloped outdoors between kitchen and laundry room windows. My question is with twin blower unit w/ three speed fan, we have a "limited location" to install duct thru wall allowing a maximum distance from the laundry window opening of 18" - 24" if we install structural header required if existing outside wall stud is modified.

We realize "mechanical code" has minimum distance standards based on air recirculation to prevent fumes entering the home. We like most in our location use central air and never open the window in question. we also know that code is worse condition based on gas fumes we do not use.
I see no alternative to move that duct farther from this window without complete removal of my kitchen ceiling drop to run a safer distance.
Does anyone with experience on this issue have better solution or experience with this code issue?
I know my situation has the best airflow for a short exhaust run of maybe 7' run with two elbows on a 8" round.
So under 15' run?