On Thu, 29 Jul 2010 06:45:27 -0400, Herb Eneva wrote:

Yesterday while running hot water it was very hot --almost boiling. I
have a 40 gal. electric water heater. Does this mean a stuck thermostat
and how do I change it?

Possibly... as someone else said, it might just be loose and not pressing
up against the tank (but seems strange that it suddenly happened all on
its own without outside influence).

My tank has two thermostats, upper and lower. You unscrew the cover
plates, unscrew the terminals, unclip the thermostat modules. Replacement
is the reverse operation; it's trivial to do. The upper thermostat also
has an emergency cut-off for the entire heater, the lower one is just the
'stat, but they're standard modules (Home Depot etc. sell them).

If you have a multimeter, you can work out which one is bad and avoid
replacing both (but I think my local farm supply place has replacements
in pairs for about $20 - and if one's gone bad due to burnt contacts then
chances are the other's getting a little long in the tooth, too)

cheers

Jules