Clearances will vary, please download the installation manual for your unit at https://support.noritz.com/technical-literature for the most accurate and up to date info.
Example of 4″ N-Vent from the NR98SV (GQ-2857WX-F US) manual:
Example of 5″ N-Vent from the NC380-SV-ASME manual:
Noritz Concentric Vent (for use with DVC models only) is approved for zero clearance to combustibles.