It this the correct install with regards to the combustion air intake?
(The missing pvc pipe)
They can be done like that. Check the manufacturer’s instructions for conditions…unconfined spaces usually.
After reviewing the rest of my photos, I discovered that the burners are exposed at the inlet, so I am recommending a qualified HVAC contractor review the manufacturer’s installation manual and conduct all necessary repairs.
most manufacturers would recommend an elbow fitting to be installed as to prevent it from getting clogged and keeping out combustion air.