Hubbell Model E water heaters with 30 gallon storage or more have both upper and lower heating elements in the tank, and models 20 gallon storage and less have one heating element in the tank. The purpose of having two heating elements in a tank is to improve the recovery rating in a water heater of significant storage capacity. Sometimes referred to as a “Quick Recovery” water heater, this configuration allows the upper element to heat a much smaller volume of water (about 25% of the tank's capacity) before the lower element takes over. This provides a small amount of usable hot water quickly while you wait for the bulk of the water to heat. It is worth noting that in the standard dual element design, the upper and lower heating elements are interlocked so that only one can heat at any time, with priority to the upper element. Once the thermostat controlling the upper element is satisfied (i.e. at set point temperature) the lower element is allowed to heat. As an option, you may order your Hubbell water heater with the optional “simultaneous operation” feature which allows each element to operate independently, potentially allowing both to be on at the same time in order to increase the recovery rate of the heater.