Yes, you can use a propane grill on a screened-in porch if there is plenty of ventilation and a safe distance from combustible materials. Keep doors or windows open, install a CO detector, allow for 5 feet of clearance above and on all sides, place on concrete or brick, check for gas leaks, clean out grease buildup, and don't leave it unattended while lit. A propane grill can be safely used in screened porch areas with proper precautions.