Originally Posted by gp_wa
There would be if the heating element was in the cold area, creating hot water that wants to go "up"...
Any other reason to have two elements?
No, because the water on top is already hot.
If the entire tank is cold, only the top element will turn on. When it is satified, the lower element will turn on. When you run hot water, the water is tapped off the top and fresh water is fed into the bottom (still hot on top, cold on bottom). The lower thermostat will call for heat using the lower element but since there is already hot water on top, you not have convection currents.
If you run enough hot water so the top thermostat calls for heat, the lower element will be turned off (we are back at the start with the entire tank cold).
You always have hot water above the cold water or you are heating the top of the tank so you will end up with hot water on top of the cold water. Then the lower portion gets heated.
Now if you did something such as setting the lower thermostat higher than the upper t-stat, you could cause such currents but you don't set the t-stats like that just do you do not have this situation.
Understand why you will not have convection currents now?
So, now that that should be settled, lets go to why 2 elements;
you know the order in which they run. I just explained it. So, not what happens is you have a 50 gallon tank (just an arbitrary number). The heating does not split 50/50 between the upper and lower elements but for discussion, lets assume they do.
So, when you run hot water, you could theororetically run out 50 gallons and that would be it except, as you deplete the hot water, the cold water coming in makes the lower t-stat call for heat. The element heats up and starts heating the cold water. If you run the water slow enough, the lower element could nearly keep up with the flow and you would have constinuous hot water. If you run water faster, you will end up with tepid water after you have depleted the initial 50 gallons and then, since the top t-stat would be calling for heat, it would try to heat the water you are using right now and the lower element would not be on at all so when the tepid water runs out, you have just cold water.
The bigger reason for the 2 elements is it allows you to use a limited amount of hot water and due to the order of running, it allows for a faster recovery. As long as you do not run the water more than the 50 gallons and you run it slow enough for the lower element to actually heat the water to some degree, this means the top element only has to heat it the rest of the way to the setting so viola` faster recovery.
the numbers used are for examples sake. They will vary.