I have a resmed with a humidifier. I spoke with resmed and they told me that running the machine for the whole shabbos even if the water runs out will not affect the machine other than normal wear and tear. I also looked into filling up the water on shabbos and the model I have has 5 heat setting which are also effected by the atmospheric conditions as to how hot they get, but each setting has a max temperature, For Shabbos I leave mine on number 3 of 5 which will not reach yad soledes bo and then fill up the reservoir as needed. (All this was done in consultation with my Rav.) I also tuned off the auto on/off. I know there are Rabbanim out there that say the auto on/off is ok on shabbos. My Rav said no.