Re: when to replace t stat?
Thermostats can fail (jam) either open or closed. They usually get replaced if there's either an overheat issue (stuck closed) or cold plug fouling (stuck open). Other than that, they are not usually "routine maintenance" items. We usually check them if we're replacing an internal anode, which is often located near the thermostat. They can be tested in a container of water on a stovetop, with a thermometer. The manufacturer will have a "start to open" temperature, and sometimes a "fully open" temperature. If in doubt, test it. If bad, replace it.
