My daughter has a conventional Giant UG40 natural gas hot water heater 5 years old. The hi limit sensor keeps tripping. I have:
- cleaned out the burner area
- replaced the hi limit sensor
- put on a new thermal couple.
- today I put in a new gas control valve (the old one had calcium build up on the water temperature probe.

Still tripping. Arghhhh!

I seem to have good draft, the chimney is not blocked, neither is the flue to the chimney. I even opened the window in the utility room. Have a nice blue flame. When unit is off and I stuck a bbq ligher in the hood cap and there is an updraft.

Any suggestions?

I think the hi-limit switch trips at 250. Problem is it kicks off the pilot so you have to manually restart the puppy.



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It almost seems like the burner is operating at too high a rate. The solution may be to limit the burner pressure a little bit and reduce the input. The control should have an adjustment regulator. Look for a cap screw about 1/4 to 1/3 inch across. Turn the gas supply valve off first. Usually, if you remove that cap, there is an adjustment screw in there. By turning the adjustment counter clock wise, or outwards you will reduce the burner supply pressure. Ideally, you should have a pressure gauge to measure it first before adjusting. However, if you don't, try adjusting it a quarter turn at a time. Once adjusted, put the outer cap back on, open the supply valve and relight the heater. Try it and see if that doesn't eliminate the problem. If it still trips out, give another 1/4 turn outwards doing so until it solves the problem. Notice the intensity of the burner before you make first adjustment. It should reduce a little bit each time you adjust it. I don't know where you have the water temperature set at? Perhaps it is a bit high? If so, you could reduce it a little bit and see if that cures it before doing the above procedure.

Thanks ACT, I will give that a shot -- short of buying a new HWHTR! I don't have gages -- I'm an IT guy with mechanical inclinations!

I did adjust the temperature on the low side suspecting maybe it was too high but same result -- trips.

FYI, I unscrewed the high limit sensor so it wasn't up against the metal and it heated up and shut off fine. Put the sensor back on, consumed some hot water, the unit fired up to bring water up to temperature and tripped.

I am wondering if the screened grate at the bottom of the burner area is clogged, I did vaccumn in there while I had the burner out but maybe not good enough or enough crud fell down there it is choking it. I am not exactly sure were the burner pulls it's combustion air from.

Regards Mac

Water heaters vary as to how the air enters the combustion chamber. A symptom of poor air supply would be a yellow/orange flame. If the flame is blue or nearly transparent, air supply should be adequate.

You have to make sure the screened air inlet is completely clean. Do you have access to compressed air, to blow the screen clean?

I blew out the combustion area and the screen on the bottom the best I could then vacuumed again.

This unit's exhaust was feeding into a 6" liner, so I also replaced that with a 4" liner. Still tripping!.

I am trying to get access to a thermal gun, but I tested the metal with a digital meat thermometer (please do not laugh) and the temperature says it is kicking out at is 180F. If I bypass the hi limit and the temperature stays constant at around 202 to 204. Does that sound right?

When I bypass the hi limit HWHTR functions normally, but I do not want to leave it in that state.

Maybe I have a 2 faulty hi limit sensors!



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I believe you are referring the FVIR sensor, NOT the high limit water temperature, and if so that is a function of the combustion chamber temperature which has little to do with the flame size and everything to do with the flow of air through the heater.

Correct I am talking about the hi-limit heat sensor that is screwed onto the access plate to the combustion chamber that is what is tripping.