Gas appliances that have no need of a flue.
A hearth product, such as a free-standing stove or fireplace, that operates without an exterior venting system.
Gas appliance systems where all combustion byproducts (mostly water vapor and carbon dioxide) are released into the same room environment as the appliance.
gas appliance that has no need for a flue.
Certain gas stoves or fireplaces are not required to be vented, therefore, they are called "vent-free".