and the opposite of what stamcon said is true too. If you are drawing hot water, the lower element will kick on first. if you start to run low, the upper element turns on and shuts off the lower element at the same time.
How does this work?
the upper element basically has a 3 way switch in it's thermostat. when the upper element is off, the switch is in the position to allow power to the lower element. If the lower element wants to turn on, it can. when the upper element turns on, it's switch flips, sending power to the upper element only, regardless of what the lower element wants.
Why have two elements if only one can run at a time?
this is for faster recovery time. since the upper element is only heating about 1/3 of the tank, you will have hot water faster than if there was only a bottom element.
Why can only one run at a time?
Because thats how they come hooked up. If you only have 100 amp service, two elements running at a time may not leave enough power for an oven, stove, and dryer.
If you have 200 amp service, you can rewire the heater so there are two breakers going to it, one for each element. That is what i did on the heater at my vacation cabin. I needed fast recovery times, because I turn off the heater when i am not there. when i go up for vacation, i want hot water FAST, and two 4500 watt elements gets the job done amazingly well, especially considering the well water is only 40 degrees. the 50 gallon tank is heated to 150 degrees in about 2 hours and i have warm water in less than 10 minutes. Of course, when im also running the baseboard heat and oven, the electric meter looks like it is going to blow up
I have never seen an element with dual wattage settings, but my theory is the 3800 watt setting is for 12 AWG wire and the 2900 watt setting is for 14 AWG wire. 2900 watts at 240 V is exactly 12 amps, which is the maximum continuous load on a 15 amp circuit (3 hour rule, don't know code reference). don't tell anyone, but I only have 12 AWG wire going to my 4500 watt (18.75 amp) heater
maybe i'll fix it if i have time, but it's worked fine for the last 60 years
[This message has been edited by cpalm1 (edited 01-21-2004).]