Electric water heater problem

My girlfriends water heater goes cold as she runs a bath. (It used to give plenty of hot water for 2 baths.) Twice now as I opened up to get to the thermostats it decided to work and was OK for one bath. It's an old fashioned unit that is in a white porcelain cabinet in the kitchen so I can't leave it open for safety reasons.
When I did open it, the lower element was on (240AC and making the noise they make) and would switch on/off with the upper thermostat.
I'm guessing a bad thermostat (electrical connections are good), any idea which one?
What if one thermostat is off a bit? Can I turn one a hair higher than the other and if it doesn't work put it back and turn the other one a bit higher? Does that make any sense?
Now that I think of it, if the upper thermostat turns on while I'm bumping it as I'm opening it up, that is probably the bad one....?
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The upper thermostat sends power to the upper element, once it's satisfied, it disconnects from the upper element and sends power to the lower thermostat, which in turn sends power to the lower element until it's satisfied, at which point it opens the circuit to the lower element. Without an ammeter, you can't easily tell if the elements are heating to capacity, but it does sound like, from your description that the upper thermostat is faulty
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On 11/10/2010 9:45 AM, RBM wrote:

Thanks for the tutorial on the operation, I never worked on one before, just replaced leaking ones. It sounds much easier to diagnose now. (And I do have an ammeter that goes up to 20 amps if needed.)
Tony
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wrote:
...

From a cold start, the upper element will come on to heat the upper part of the tank because that is where the hot water exits. After the top thermostat has been satisfied, the upper thermostat switches power to the lower thermostat which in turn sends power to the lower element. After the water at the bottom of the tank is hot enough, the lower thermostat cuts power to the lower element.
When you start to draw hot water, it is drawn from the top of the tank, and cold water enters through the fill pipe which is extended inside the tank somewhere near the bottom. The lower thermostat senses the bottom of the tank is getting cold, and activates the lower element and starts to heat the bottom of the tank. If you draw enough hot water and the top of the tank starts to drop in temperature, the upper thermostat will cut out the power feed to the lower thermostat and activate the upper element to try and keep the water at the top of the tank hot so you don't run out of hot water. Hope that clears it up.
                    RON =======================================================Remove the ZZZ from my E-mail address to send me E-mail.
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On 11/10/2010 11:55 AM, Red Green wrote:

Another thanks for the tutorial on the operation, I never worked on one before, just replaced leaking ones. It sounds much easier to diagnose now.
Tony
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On 11/10/2010 4:50 PM, Tony Miklos wrote:

OK, I opened it up and the lower element was on. Ran the hot water and could feel the cold water rising in the tank. As it got to the top the thermostat turned on the top element and turned the lower one off. Still ran out of hot water. Checking individually, each 4800 watt heating element draws about 19.2 amps (meter and test leads resistance probably a factor). I ask her again "how long has it been this way?", she says "as long as I've been here... about 6 years."
After banging my head on the wall a few times I began to wonder how many gallons the bath tub holds. Google tells me 40 to 60, I don't know how high she was filling it. The water heater is 38 gallon. It did seem to take too long until the top thermostat turned on.... so I'm thinking maybe turn the top thermostat up a little higher than the lower one? What about installing a higher wattage element? (I didn't notice what gauge wiring and breaker were in the circuit.)
For now I told her to run the hot in the bath slower than normal and she might get enough hot water.
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wrote:

Another possibility, especially if it's an older tank, is that the dip tube that runs to the bottom of the tank has broken off so the cold water is mixing at the top of the tank where it's being drawn off
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On Fri, 12 Nov 2010 12:00:51 -0500, RBM wrote:

Yes, I thought that too.
I'd say drain the tank and take the top element out (hopefully it won't be so corroded in place that it won't unscrew), then take a look in there with a flashlight. Even if the tube still looks to be in place, maybe turn the water back on just long enough to see if it's spewing cold water around the top; there might be a break rather than it outright having fallen off.
It'd be a good time to check that the bottom of the tank isn't completely full of sediment, too (on my heater - and all the ones I've seen since at the store - the drain valve's a crappy plastic thing designed to get blocked by flakes of sediment, so the tank can be drained but the years of sediment stays trapped in there)
cheers
Jules
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On 11/12/2010 12:22 PM, Jules Richardson wrote:

This is a tank with loose insulation around it, and it's built inside a square white steel cabinet in the kitchen. It's not real old, it's just a really old fashioned model that was often used when people first got hot running water and there was no better place to install it. So with the cover off and pulling the insulation aside, I could feel the tank the whole time the hot water was running. I was very surprised at just how well the cold water stayed separated from the hot. So I'd say the dip tube is fine.
As far as sediment, I really have a hard time thinking that makes any difference in an electric water heater. A gas one, yes that makes sense since the flame is at the bottom, but an electric one... I just don't see what the big deal is. And by the way, it looks like brand new inside there, I'm sure the elements will come out easy. Maybe I'll pull them and check to see if they are built up with calcium and other minerals?
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