I too almost had a disaster the other night... one of my heaters had become exposed to the air, and heated up so much that when it touched the water it sizzled and boiled the water for a second. It also burned all of the paint/printing off of the outside of the heater
I thought it was my heater to the house, but I knew that was odd since it usually only smells the first time or two of the cold season...
A few hours later I found the heater... glad it didn;t melt a hole in my sump!