I dunno, you can find a lot of hot air in the desert and there seems to be not a lot of water in it. It may be able to hold more water but it has to get it from somewhere. You can heat air with a low RH and you won't find it going up in RH value unless you add water to it usually with a humidifier. It is not getting any moisture from the air that is used in combustion as that goes out the stovepipe as part of the flue gases.

Bob