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 Posted: Thu May 11th, 2006 01:54 am
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earbkr
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My water heater is a Rheem 240V / 4500W model with two heating elements, two thermostats, and a high temperature cut-off switch.  The water heater is about 2 years old.

Yesterday, the hot water from our electric water heater was scalding, scalding, hot.   So I manually tripped the circuit breaker at the main electric box.

This morning, we consumed some of the hot water in the tank by taking a couple of showers.

This evening, I checked the heating elements and found the top heating element to be burned-out (open circuit with ohmeter with no wires connected to it).  So I replaced the heating element.  A visual inspection of the burned-out heating element showed that it was indeed burned-out.  And it "appeared" to have been burned-out for some time (not just a recent occurence).

After replacing the upper heating element, I filled the tank with water (I had only drained it a little below the upper element), bled air out of the hot water lines, and turned the circuit breaker back on.

Then for the first few minutes, I measured 240VAC across the top heating element terminals and 0VAC across the lower heating element terminals.  Then apparently the upper thermostat reached its "switching-temperature" and switched the power over to the lower heating element because I then measured 0VAC across the top heating element terminals and 240VAC across the lower heating element terminals.  It remained in this state for about 30 minutes and then the lower thermostat turned off and I measured 0VAC across both the upper and lower heating element terminals.

So from this initial test (after replacing the heating element), it appears that both thermostats are operating.

But I don't understand why the water was scalding hot yesterday.  I don't see how a burned-out upper element could cause scalding hot water?  If the lower thermostat was "stuck" closed, I could understand how the water could get scalding hot.

Also, I don't understand why we have had a 'normal' hot water supply for the past month when it appears that the top heating element has been burned-out for some time?  If the top thermostat detects "cool" water, it would apply voltage to the top heating element, and if the top element is burned out then the water would NEVER heat.  I suppose that if we are a low consumer of hot water, that the upper thermostat could just always stay in the position of applying power to the lower heating element, never needing the upper heating element, and it would appear that everything is okay.

Is there a good way to determine if either heating element is burned out, without disconnecting the wires and making an ohmeter measurement?

In summary, 1) why the scalding hot water when there was a burned-out upper heating element, and 2) how to determine if heating elements are good?

Advise is requested.  Thanks.

 

 

 

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 Posted: Thu May 11th, 2006 06:37 am
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eleent
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Hello:  You surmised correctly that the tank could still heat with a bad upper element.  As long as you didn't use enough water to cause it to switch to the upper element you were fine.

As for the overheating... I'm guessing the lower thermostat is on its way out. Other things it could be would be the lower thermostat hatch was off and the insulation missing, and/or the spring that holds the lower thermostat against the tank was loose. If the overheating happens again with the insulation etc. in place, than the lower thermostat's contacts are likely burned.  Test your relief valve and replace it if it fails the test!

The only good way to check an element is with a meter.  The method I use is to turn off power and remove one wire from the element. Put your meter in the ohms times one scale and read between the two screws. You should see the needle move most of the way across the scale. (yes, I'm analog) That tells you that you have good continuity.  Now set the meter to ohms times 1000 and check from each screw to ground.  You should get no continuity at all with either screw.  If you do, replace the element as it is leaking current to ground and that's considered unsafe :shock:.  If you have a clip on ammeter, you can check current draw when an element is working. 

Yours,  Larry

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 Posted: Thu May 11th, 2006 01:28 pm
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earbkr
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Larry, Thanks for info.

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 Posted: Sun Jul 18th, 2010 12:42 pm
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GOOP
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Water heater over-heating.  Just replaced the upper & lower T-stats.  Checked OHMs across both elements.  13-14 ohms.  Checked ground from each contact on each element.  Reading remains at 1. 

What's the problem here?  I don't get it.  Everything seems to be working fine.  Still over-heating though.

Please reply.

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 Posted: Mon Jul 19th, 2010 03:39 am
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energyexpert
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If an element has a crack it can still read correct ohms when measured screw to screw.  But the crack means there is also a path for electricity to flow to ground and thereby continue adding heat to the water even though the thermostat opened.  The thermostat only opens one leg.  Unless hot water usage is greater than or equal to heat production the ECO (emergency cut out (ie, red button)) will trip.

An element can also be burned in two so it will read open on a 200 ohm scale (but may give a reading on a higher scale if the meter has one).  Again there will exist a path to ground so heating can continue as above.

David

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 Posted: Mon Jul 19th, 2010 04:26 am
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eleent
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Hello:  As I was four years ago when answering the first post I'm still analog :cool:  If there were a leak to ground, my meter needle would swing over when measuring in the 1000 ohms scale between ground and a terminal on an element.  It even moves if I simply touch both probes to my finger, (which must be slightly conductive!)  So, my question is about the reading of "1".  Is this the digital equivalent of ---?---  And what scale is being used?

Yours, Cro Magnon Larry

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 Posted: Mon Jul 19th, 2010 12:47 pm
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GOOP
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Well, since the elements are only $20@, i'll go ahead and replace both.  Already have $30 in the new thermostats.  I would have spent $70 on a plumber just to come out.

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