A vent-free gas heater is a heater that uses room air and gas to provide heat through combustion without the need for a vent. Vent-free heaters are compact and easy to install and are typically used for supplemental heat in basements, utility rooms and other hard-to-heat areas.
As of 2015, the combustion process of vent-free products is controversial. Supporters of vent-free products claim they are 99.9 percent energy efficient and safe to install. Several states however, limit the use of vent-free products. Vent-free products can also raise humidity inside rooms due to the water vapor produced in the combustion process.