Single vs dual aquastat - water heater

Does anyone know the difference between single and dual aquastats?

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Normally an aquastat is on a larger commercial water heater. But if you mean thermostats on a residential water heater, then the function is the same. The lower one does the majority of the heating so that the tank is kept full of water. If the demand is such that the tank is running out of hot water, then the upper one is activated so that a heating element closer to the outlet of the heater will try to provide some level of warming. The upper one usually operates first to heat the top of the tank and then transfers control to the bottom one to finish filling the tank with hot water. In commercial water heaters they can both operate at the same time. The advantage of a dual unit is that when the lower one burns out the upper one will still provide a level of hot water until the lower one is replaced. With a single one you do not have any hot water until it is replaced.

Thanks for the info HJ. This is an oil fired hot water heater.

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In that case, since there is only one heating source, the second one could be an override or high limit depending on its action, (i.e. NO or NC), and how it is wired into the circuit.