Please help diagnose water heater problem

Was wondering if anyone with some experience or knowledge in the area of water heaters might be able to provide some advice.
Until a few days ago, my 40-gallon, electric water heater provided about 20 minutes of hot water for showers. Now, all of a sudden, it consistently provides only about 5 minutes of hot water. I checked the circuit breaker and neither of the 30amp fuses has tripped. I would like to diagnose the problem myself if it isn't too involved. So what would be the next step? (I have standard tools and a multimeter at my disposal)
I don't know much about water heaters, but I'm pretty sure most have two heating elements, so could it be that one of the two elements has gone bad in this case? I have also heard about thermostats going bad, so could that be it?
Anyway, would appreciate any advice. Thank you! Peter
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Ugh!
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This is turtle.
Buy what you have said the bottom element or the bottom thermostat is out. You have two thermostats and two elements and it sounds like one of the two at the bottom element or thermostat has went.
Take your ohm meter tell which is not any good.
TURTLE
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Thanks. I appreciate your reply. How can you tell that it's the bottom one?

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This is Turtle.
The Hot water tank has 2 thermostats and the top thermostat is the master thermostat. When you have a cold water tank of water the top Thermostat will over ride the bottom and heat the top first. When the top area of water or the top thermostat is satisfied. The top thermostat will cut off and turn the power over to the botton thermostat to heat the bottom. If the bottom does not work it will just not heat the bottom 2/3 of the tank and just leave the top area hot and the bottom 2/3 will not be hot at all. When you use up the top 1/3 of the water in the tank. the top thermostat will over ride the bottom thermostat andstart heating the top 1/3 of the tank again. You had said that you had 15 minutes of good hot water and then after the trouble started. you had only about 5 minutes of hot water. This told me by process of deduction that you only had 1/3 the water that you did have before the problem. then you must have the bottom thermostat or element out and not heating the bottom area of water. now if it was the top thermostat or element that went out. You would not get any hot water at all or you would have to wait over night to get a full 15 minutes of hot water but it would be rare to even do this. Another item here is if it was the top going out usely you get no hot water at all. The top or master thermostat will not give up the control to the bottom to heat the bottom and then not heat the top area along with it.
Now you said you checked the bottom and top elements and got 14 and 13 ohms. You got the bottom themostat out and in rare cases it can be the top thermostat just not turning over control to the bottom when the top gets hot enough.
Now you ask about where to get the thermostats and elements. Well just about every hardware store Lumber yards, Lows, Home Cheap-0 , and Southernlands has these thermostats and elements. The element will have the watt rating written on the head of the element where the screws are to tie the electric wire on it. They come in 2,000 watts to 4,000 watts ratings and you get the watt rating and go to the hardware store and get that matching watt rating and the type head it has on it to attach back to or in the tank. They only have about 3 type and you will see the head of the old one to tell what type but most all screw in . Now to the thermostats. They have only about one kind that everybody uses and you have a top or master thermostat and a bottom or slave thermostat. If it has 5 or 6 terminals on it. It is a master. If it has 2 or 3 terminals on it . it is the bottom or slave. Now to how it wires up. On every new element you look on the back of the package of the thermostat and it will have a wiring diagram explaining how it wires up. 95% of all electric hot water tanks wire up the same.
Now like others has said here about changing both thermostats. Well i don't see a problem changing both to not have to relive this even again in about 6 months to a year when the other on want to act up. Now let me give you a tip here. Change only one thermostat at a time for you may be back here asking how does it wire up?
TURTLE
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Thank you. I am grateful for your patient explanation. I will probably do as you suggest and replace both thermostats and see what happens.

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Turtle's right, but I'd go one step further....replace BOTH elements at the same time and save yourself from doing it again 6 months or a year from now. Replace top and bottom elements and see how it goes...chances are there's not a problem with the thermostat(s). Elements are not expensive.

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Makes sense to me - thanks for the advice. Are these elements fairly interchangeable from heater to heater or are they proprietary? Also, can they be easily obtained from places like HD and Lowes or do I need to find a specialty store?
Finally, do you have any specific advice (besides switching off the circuit breaker) about changing the heating elements?
Thanks again.

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Most water heaters use one of 2 or 3 styles in a couple of different wattages.
Best to take the old element in with you when you buy a new one. However, if you check the plate rating (make sure you take the "installed wattage" number, _not_ the "max wattage" one), and write down the manufacturer and model number, you should be okay.
Probably best to go to a dedicated plumbing store.

Some elements have a "fold back" arrangement that if you snag the "fold back" on the inside of the tank, they'll bend and become a b**ch to get out. So do things gently.
Pay special attention to not scarring the mounting area on the tank, and make sure it's cleaned off before installing the new one. Nothing like a piece of grit destroying the seal.
And finally, while you're at it, check how much crud is in the bottom of the tank. That may have been what killed the element. A wet-dry shopvac with a piece of garden hose taped on the end is good at getting the gunk out.
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Chris Lewis, Una confibula non set est
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Thanks for your kind advice. I'll be sure to follow it if I indeed need to replace the elements. In the meantime, though, I just checked both elements with an ohm meter and it turns out that the upper element reading was 13.4 ohms, while the lower element is 14.7 ohms. Does that mean that the elements are actually fine? And if so, what is my next recourse?
Incidentally, I also checked both terminals on each elements against the metal of the element itself to make sure there weren't any shorts. None seem to be present.
What's next?
Thanks again for the info. Strangely, I can't seem to find any manufacturer's markings on the water heater, although the serial number and model number are readily apparent.

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(Correction: 13.4 KiloOhms and 14.7 KiloOhms respectively for the upper and lower elements.)
I just checked the resistance of the thermostats: Upper: infinity Lower: 500 ohms
Does this suggest that one of the two thermostats is bad?
Also, the manufacturer is US Craftmaster.
Would appreciate any further guidance.

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If that was the correct value, they'd both be defective. But you are getting some heat...

If that were the correct value, I'd be worried about the lower one.
When cold, a water heater element should have a resistance in the 5-15 ohm range.
When at full temperature, both thermostats should read infinity. A bit below full temperature, one of the thermostats should read very low ohms (considerably less than 1) and the other infinity. Farther below full temperature, the open and closed one reverse.
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Thanks. My multimeter may automatically switch between KOhms and Ohms in the readout depending upon the order of magnitude of resistance present, so multiply or divide by 1000 to get the figure that makes sense.
Based upon what you and Turtle have written, it sounds likely that the lower thermostat needs to be replaced. I will proceed in that direction (and possibly also replace the upper while I'm at it).
Thanks again for your help!

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Once you factor in the "divide by 1000", the thermostat reading may be okay. I suggest a different set of tests.
Make sure the covers are off, and the insulation pulled back.
Turn _on_ the water heater power. Be vewy vewy careful.
Switch your voltmeter to volts range, and measure the voltage on the elements.
When the HWT is up to heat, neither element should have voltage. When you start drawing hot water, the bottom element should go on (show voltage). After drawing a lot of hot water, the top element should go on (show voltage) and the bottom element go off.
If it does that, both thermostats are okay.
Check that the voltage on the elements matches the voltage rating on the elements (it's probably 240V).
During the latter two stages, listen very carefully. You can generally hear the elements in operation (microbubble formation from the heat).
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Chris Lewis, Una confibula non set est
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I've had problems with HD water heaters (Rheem I recall) in which the thermostats did not properly sense the temperature. They were poorly mounted against the outside of the tank and relied on what seemed like a flawed method for correctly sensing the temperature. I cleaned up the opposing surfaces and applied a thermally conductive "grease" which corrected the problem. I had this on two different, but identical water heaters.
Peter Michelson wrote:

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