Ventless or vent-free heaters that are properly installed and maintained are safe as long as the proper ventilation recommendations are followed. Vent-free heaters designed for home heating have a built-in carbon monoxide detector, which monitors the oxygen level of the air.
Since vent-free heaters have near-perfect combustion of the heating materials, the small amount of carbon dioxide that is produced is considered non-toxic. Adequate fresh air in the room being heated keeps the carbon dioxide levels down.
Ventless heaters produce around a gallon of water every four hours. This humidity, when added to higher humidity already in the air, could cause upper respiratory problems in some people. If the humidity is low to begin with in the room, this added water can help make people feel healthier.