Most heaters of that general shape are, it seems. Usually, the only question is whether or not you can mount it horizontally. If the heater has any kind of auto shut off mechanism to prevent it from running while not in the water, those sensors are in the top and catch the rising (literally) temperature. If you mount the heater horizontally, the sensor will never trip. Of course, if you want to install the heater horizontally, the obvious solution is to mount the heater low in the tank, so it's never exposed to air.