Thank you for posting your question here on Fixya.com.

Hi limit switched are embedded in the header and trip when the water temperature is too high. Most of the time the cause is either a faulty sensor or poor flow thru the heater causing the exchanger to overheat. If you have a bypass plumbed into the water lines at the heater close it and see if the problem goes away. If not, you'll need to check the limit switches to see if they are faulty or if a condition exists causing them to trip.

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John