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The attached photo is the lower thermostat on my electric hot water heater. We noticed a burning odor yesterday so I shut off the power and looked behind the panels to investigate. How is this problem repaired? Thank you. If it helps, I believe the problem is described by someone with more knowledge that me in this answer: https://diy.stackexchange.com/a/130763/56424.

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Update: It looks like the thermostat got fried - it was all burned out on the inside and stunk something awful. I replaced it and all seems well. The burned screw is leading to the heating element, which I tested for resistence as described by other members and concluded was fine.

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  • It looks like corrosion. And the corrosion might have bridged the contacts. How about cleaning it up with a wire brush? FIRST turn off the power. Apr 9 '20 at 15:28
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So the first step is to find out why this terminal started burning in the first place. Chances are you have a failed or failing element that is drawing too much power, but not quite enough to trip the breaker or you have a failed thermostat.

It looks like the main damage is the insulation of the wire that I assume goes to the supply. You will need to replace or, if possible, repair the wire's insulation. If the conductors are also burned you will need to replace. Some heat-shrink tubing that has the same heat rating at the original wire, which is probably at least 90C could be used. But check that as it should be marked on the undamaged wire.

A water heater element is a 240V connection that has no connection to ground. Often these fail internally and one of the conductors will connect to the steel tube that contains the element itself. So if you measure one with a VOM (that's a Volt-Ohm-Meter) with the POWER OFF!!! and the wires disconnected from the element, you should get a reasonable resistance (in OHMS mode) between the two terminals and a very high or infinite resistance from either terminal to ground.

What is reasonable? Well you need to check the label on the unit where the element power outputs are listed. Often they are in the range of 4-5 KW (that's kilo-Watts). Ohm's Law and the Power Equation will give you the expected resistance:

P (in Watts) = V (in Volts) * I (in Amps)

So 4000 = 240 * I => I = 16.6 Amps, at 5000 W you have I = 20.8 A

Ohm's Law: V = IR which can be re-written as:

R = V/I

Again at 4000 we have R = 240/16.6 = ~14.5 Ohms and at 5000 we have 240/20.8 = 11.5 Ohms. So you should see an R value on your VOM between the two terminals of the element in that range of 11.5 to 14.5 Ohms. Much different than that (again assuming you have one that is 4000-5000 W) and you have a problem.

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  • Thank you. How can I determine if the problem is with the element or the thermostat? Would it be possible to simply clip the damaged part of the wire off, assuming it is still long enough to reach the contacts?
    – MarcGuay
    Apr 9 '20 at 15:41
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    @MarcGuay, check the resistance of the element with a meter. That will tell you if the element is ok. The thermostat really either works or it doesn't so there's not a special test other than making sure it is supplying voltage to the element when it needs to heat. The elements and thermostats are common replaceable parts you can find at home improvement stores. They are pretty standard.
    – JPhi1618
    Apr 9 '20 at 18:00
  • Thank you both. I verified that the resistance of the element is okay using the math above and replaced the thermostat with a generic model that is identical in appearance except for the markings which say 59T 4000 instead of 59T 4095. I suspect that this is a difference in the branding only and have written the manufacturer to find out, but perhaps someone here can answer faster than them? I gave it a test run and it worked fine but have switched off the breaker for now just in case.
    – MarcGuay
    Apr 10 '20 at 0:21
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You need to pull the component and see where the damage started / is worst.

I think the problem started right at the screw terminal. This is probably due to incorrect torque on the screw. And this is why torque screwdrivers were recently made mandatory for this kind of thing.

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