Q. Is it permitted during Shabbos to open or increase the water dripping into a humidifier connected to the heating furnace. The warm air rushes by a metal net or mesh, that spreads the droplets of water into the air exiting the furnace and it evaporates into it. The temperature of the air at that point is less than 40 C. Is this allowed?
Is it permitted to add water to the tank of a cold-water room humidifier, that has some water but needs more to keep working the whole day?
A. Horav Shlomo Miller’s Shlit’a opinion is that in regard to adding water to the tank of a cold-water room humidifier, in case of need it is permitted, since it is already working and no bishul or cooking of the water is involved.
However, regarding the humidifier units connected and feeding moisture to a hot air furnace, although some furnaces at some points of their heating cycle may indeed be forcing air that is bellow 40 C at the area next to the humidifier, most heating units are way above that temperature when close to the ignited area and usually the humidifier unit is adjacent. Therefore, the Rov maintains that during Shabbos no changes should be done to the amount of water entering the unit. Nevertheless, it is permitted during Yom Tov.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a