Robert, you have three basic concerns with these terminations:
1) Function--it has to breathe for the appliance to work properly. It you restrict the airflow too much, such as with metal deflectors or plants, it may experience operational problems including but not limited to: sooting, lazy flame, delayed ignition, shattered glass and deoxygenation.
2) Exhaust- the hot exhaust needs to go somewhere that it doesn't cause problems. They can easily melt siding and soffits or nearby patio furniture. That rack thing over the vent in your pic is in for trouble.
3) Thermal burns- These things get hot! That's why we call them FIRE PLACES. There is FIRE in that PLACE! ;-) You wouldn't believe how many homeowners don't get that point. I've measured flue gases in excess of 600F on some units.

The gas codes have charts that are default clearances in lieu of the mfrs. listed instructions. The main clearance they show that would apply is the 7ft. clearance above a Public walkway--private walks don't count. Apparently, its ok to bonk your own head as long as you don't do it to neighbors.

Little boys love these teminations. They bounce balls off them smashing the deflectors causing the above noted problems, stuff their sister's dolls into them, tennis balls, weeds--you name it. I've also seen them covered with vines to where they can't breathe. Then there is everybody's favorites--building combustible decks and fences in direct contact with them or directly over them. A clear violation of the 6 'P's: Prior/ proper planning prevents P*** poor performance.

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disgusted with some people on this forum. Out of here!