I use the same sort of horse trough water heater. I fill my mash tun keggle with 15 gallons the night before and I also make sure to have the March pump left open so that when the timer goes off, it automatically recirculates. I find this helps maintain an even temperature, and with a five hour heat up, I achieve 15 gallons of 170oF water. I transfer whatever i don't need iover to the HLT. This typically drops the temp in the HLT from 170o to 130o because my setup is in a cold garage, but that beats starting at tap water temperature.