This depends on the type of fireplace, stove or insert you’re choosing.
Zero clearance, direct-vent gas fireplaces & freestanding stoves use co-axial venting systems. The outer pipe brings combustion air in from outside while the inner pipe exhausts flue gases.
Direct vent gas inserts use co-linear venting systems. This system uses two separate aluminum liners that are run through the existing chimney system. One brings combustion air from outside while the other exhausts flue gases.
Zero clearance wood burning fireplaces use an insulated stainless-steel chimney system. Type, size and brand of chimney is determined by the fireplace manufacturer and listed in the appliance manual.
Wood-burning inserts use a stainless-steel liner system, which lines the existing chimney system.
Free-standing wood stoves use a combination of smoke pipe and stainless-steel chimney system. All wood stoves require a stainless-steel chimney that is tested to 2100º F. Size & brand is determined by the fireplace manufacturer and listed in the appliance manual.
All fireplaces & stoves are tested and approved as a complete system with their appropriate chimney or venting system. Approved chimney & venting brands & sizes are always listed in the appliance manuals.