Guest
A heating element without water around it burns out, so you can have an air pocket due to improper piping. A heating element that runs all the time is not being turned off by the thermostat, so the thermostat is not sensing heated water (because of an air pocket?) or the thermostat is bad.
Also: http://www.diptubesettlement.com/
http://www.stemdoctor.com/hot_water_heaters.htm
