Originally Posted by Bobby_M
Put the fermenter into a larger bin first, fill that space with water and drop the heater in there.
Yep, that's what I do. I put the fermenter into a big cooler, add water and then put the heater in there. I float a thermometer to check the temperature. I know some brewers will circulate the heated water to ensure the temperature is uniform, but I never have and it worked great. I don't heat too many fermentations, as my house is normally perfect for ales in the winter at 62-64 degrees but occasionally I want to ferment up in the 70s and use the aquarium heater.