The two hot water heater thermostats are set at 120 deg. F. Occasionally the high temperature limit switch will trip. What can cause this?

This is typical when they are going bad... and the more they trip, the weaker they get.

Our rule of thumb on apartments we manage: If the tank is under 8 or so years old we replace the part... if older than that we replace the tank.

Our rule of thumb on apartments we manage: If the tank is under 8 or so years old we replace the part... if older than that we replace the tank.

A exception to this rule is my water heater. My house was built in 1954 and I still have the same 30 gallon one element heater. I flush it once a month and in all the years it's been installed have only changed out one element.
This heater is built into the kitchen cabinets and would be a real hassle to replace. But then why replace something that's working? Cheers, Tom