Gas heaters burn gas to produce heat called combustion which will make pollutants (carbon monoxide and nitrogen dioxide) and water vapour. A flued gas heater has a flue or chimney to carry the combustion pollutants and water vapour to the air outside the home. An unflued gas heater has no flue and releases the combustion pollutants and water vapour directly into the room. When your unflued gas heaters are used correctly and are well maintained, they only make very small amounts of these pollutants.