In my particular trough the water heater is screwed into the filter. Most other troughs use the plug to insert their water heaters. It works nicely as well. David's trough is this way.
The other day I was headed up to shoe Dani away from David's hay once again and noticed either smoke or steam coming from his trough. I noticed as I approached that the heater was just above the water and creating that steam. Yikes! I quickly added water and texted David. He was on his way. I'm sure that thing got pretty hot and would have been unkind to little horse muzzles. Just something to be aware of, don't let the tank level get too low even if you are about to drain and clean the tank soon.