Ventless gas heaters work on many of the same principles as other heaters do. The main difference between ventless gas heaters and normal gas heaters are the fact that ventless heaters take the air that is used for the combustion to take place from the room that they are in rather than from an outside vent.
Because of this, the heated air is then blown directly back into the room. Without a vent to the outside, this means that there is less chance for heat to be lost during the reaction which creates a much more efficient machine. This does not mean, however, that there are no issues to be found with venltess gas heaters.
These heaters burn air for fuel, so there is no smoke emission into the air. The main safety concern with this type of heater, as with any type of heater that burns air, is carbon monoxide. This cannot be smelled and it is deadly for humans.
It is almost a requirement, then, that if you have this type of heater in an indoor area and do not vent the air somehow, to have a carbon monoxide meter handy so that you will know the safe levels. Some states, such as California and Massachusetts, do not allow ventless gas heaters for this very reason.
Another issue that can be caused by the burning of oxygen through ventless gas heaters is the creation of water vapor in the air. This water vapor will raise the humidity in a room if it is not properly vented out and will allow mold and mildew to grow inside the room. This is certainly less of a health risk than the carbon monoxide levels.
Conclusively, ventless gas heaters are one of the best options available for indoor heating. The problems with carbon monoxide are less than those of more traditional methods such as fireplaces. In addition, the benefits as far as efficiency are concerned almost cannot be measured. Without the problems of having to vent exhaust into the atmosphere, almost no heat is lost to the air, leading to a very high efficiency.
Natural gas and propane, two of the main gases used in ventless gas heaters, have very efficient and safe exhaust fumes and are good choices for consistent burning through a gas heater. The main types of exhaust vented through these are water vapor, carbon dioxide, and carbon monoxide.