On Sun, 13 Sep 2009 19:20:12 -0600, Tony Hwang wrote:
Most residential elec H2O heaters dont run both elements at once under any
circumstance. Water is pulled from the top of the tank and filled w/cold
at bottom of tank. Top elements often run first until temp is reached
followed by the lower elements heating cycle. Mineral build up at bottom
of tank seems to knock the lower elements out more often than the top.
Disconnect the power leads to the element and test the resistance across
the element usually at 13.5 or so when good and open line when bad. Change
both stats at the same time for best results.

Hi,
Measuring voltage at the element terminal does not really check if it is
good or not. Remove power and Ohm it out or use Amprobe. I have a cheap
multimeter with Amprobe clamp and temp sensor. It paid for itself many
times over.

Yes, I had actually done that yesterday (before reading Marco's post, but
after posting mine) and got 13.2 ohms on each element.

New upper thermostat appears to have sorted things - I went to look at a
'universal' one in stock at the local parts place which turned out to be
a WH-10 anyway (actually an adjustable type). Setting range from 'hot' to
'very hot', so I just left it on 'hot' for now...

If Marco's explanation matches my system (I did trace out the wiring,
but don't have it handy right now) then it seems like the upper thermostat
had failed in such a way that it was always supplying power to the lower
'stat, so all the work was being done by the lower element.

cheers

Jules