Our water temp rose substantially overnight and I can't figure out what is going on. Our heater was set at 74 as always. We had warmer weather here yesterday (85) but our tank is in the bsmt and it doesn't get warm down here - the air temp down here was 70 this morning. Why would the tank have gotten so hot? I just came down for the morning, and water is 89.5, but now starting to drop - I unplugged heater and opened windows (it is chilly outside this morning) and opened all tank cabinet doors. We have not been having heating issues at all, so I am thinking something must be malfunctioning, probably the heater? It is a ProHeat 300W heater. WE have an RO system and holding tank with auto topoff to the sump, and that heater is also set at 74. I checked the water temp of the holding tank and it was at 80, so I turned the heater down a bit on it. Any suggestion as to what else I should do? Thanks.