How big a heat sink is it attached to? Without a fan you need a pretty big sink to get rid of 100W, if you want things to stay cool. As for maximum temperature, that should be part of the device's spec. Some of the all ceramic designs can be run like an electric fire, with the resistive element glowing red. It reduces the lifetime, which may or may not matter to you. The resistance will also change quite a bit, which also may or may not matter to you.