On Monday, February 18, 2019 at 2:02:13 PM UTC-5, wrote:
This is the best description I have found. Red Green may be talking about a different design but everything I've found about this design jives with the link below.

https://groups.google.com/forum/#!to...ir/hFTH_8rwcXY

That link is right back to this thread. What Red Green said sounds right
and basically all the people I see here are saying the same thing.
One problem is a statement that the OP made about the top element heating
"first". It depends on what you mean by first. In my reply from years ago
I said the upper element gets PRIORITY. If both the top and bottom need
heating, the top gets heated first, the bottom is shut off. But Red is right,
under normal operation, hot water is drawn out the top and if it's not a lot
of water, the cold water entering the bottom will have the bottom element
turn on. It can heat the water enough without the upper coming on because
the upper thermostat never drops low enough.

Consider another example of "first". Suppose the heater has been turned off
for a week. When it's turned on, the upper element will be heating first
because both thermostats are calling for heat.
That's why saying that the upper element has priority, rather than saying
it heats first, is a better way of describing it.