The heating part of a heater is a coil of hot wire. As long as the heat can escape into the water it should not be a problem. If you can see the heating coil, like you can in most older heaters, just keep that part where there is water all around it. Sand around the very bottom where there is no heating coil should be alright. I am always too cautious and keep mine completely out of the substarte but I like to lay mine quite flat near the bottom so that when I do water changes, the heater doesn't get uncovered at all. Like I said, I'm a bit overcautious.