You can see it is too close to the deck, too close to the side wall, too close together, and the vent is too short.
Extending the vent 2 feet would, I expect as someone ignorant on the subject, solve the problem.
IF it is an acceptable solution.
According to the installation instructions, it can be set up as a single pipe, getting the combustion air from the basement. That would imply (again to an ignorant person) that extending the vent 2 feet wouldn't hurt anything.
This is wrong for 2 reasons....under the deck and in a corner. I'm surprised the furnaces runs at all.