Catalytic heaters consume oxygen, produce very little CO, and result in an increase of CO2. Hence, in a closed environment they pose a threat of hypoxia exacerbated by a moderate CO concentration and by an increase in the carbon dioxide concentration that accompanies the depletion of oxygen. In addition, unvented catalytic heater combustion produces excess H2O. Unvented systems cause condensation on windows vents and walls. A vented catalytic heater removes CO and H2O produced by combustion and therefore only adequate oxygen supply is required, which the vented system helps replenish. With a unvented catalytic heater, it is necessary to also allow CO and CO2 to escape as well as replenish oxygen. That is why manufacturers of unvented catalytic heaters suggest opening two vents or windows, one low and one high. Vented catalytic heaters have no such requirement and are therefore more effective because they do not also have to compensate for cold ventilation air as does an unvented heater.
