Thank you all for the inputs! Mark, thanks. Here is details:
1) model is Bradford White
2) just replaced the pilot thermal couple. Now the pilot can light up to 12 hours without problem.
3) If turn thermal knob to on, and rotote the thermal controller to warm/hot, the burn will start burning. After 20-25 minutes, the water get fairly hot, then the system shut down itself, both burner and pilot at the same time. Once shut down,
a) I need to wait a few hours, seem wait the tank water to get cool off
b) seems to need turn the thermal controller to off and warm a couple times, during turning, I can heat a switch clicked once when turning to cooler side, and once when turning to hot side.
c) then I can light the pilot and things repeat from step 1.
I feel it is like there is a thermal sensor in the water, which shuts off the system when water gets fairly hot. But the manual book ndoesn't mentioin about the part. May be this sensor is integrated with the thermal control unit. If so, the malfunctioning of this part may force me to replace the control unit, or replace whole tank since the cost is probably similar. Please advise.
Finally, the tank is 10 years old. I have never inspected/changed anode. Is this related? Also where is the anode, unless screw off the top cover, I don't see it anywhere. But openning the top cover is not easy. Also, I didn't do "montly drain off a gallon of water to remove silt and sediment", can this cause this problem? Thanks for avdance.
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