Electric hot water tanks - thermostat

I have two 75 gallon(US) electric hot water tanks plumbed in parallel. House is new - we moved in 16 months ago and starting about 3 weeks ago there has been a shortage of hot water. We are running out of hot water sooner than we used to with similar use patterns. Water temp is plenty hot to start with and for quite a while. The problem is about a one-third to one-quarter loss of capacity. We are now running out of hot water when everyone is taking a shower, some of us like long showers. If only 2 or 3 people shower there is no problem. For over a year we could never run out of hot water but now we do, fairly often.
I think the dip tubes must be fine or we would have a problem with the first shower, not the fourth.
I tested the elements with a continuity tester and all four lit up the light. I also tested for resistance and got about 14.3 ohms for all 4 elements. That's why I think a thermostat is the problem.
Voltage is 240 and there are 4 thermostats. The make of the tanks is Giant, model 172ETE-3F7M, 4500 watt elements.
The upper wire block has 7 screws - 4 in the left column and 3 in the right column, there is a high limit reset button which has no effect on problem. There is a yellow and a blue wire to the upper element and a red wire to the lower element and a black wire to the lower thermostat. There is a temp setting screw.
The lower wire block has 2 screws - black wire in and a black wire to the element.
I downloaded the manufacturers wiring diagram and the connections match the diagram. Worked fine for over a year so I assume connections are OK.
Can someone explain how to test and determine which thermostat is bad? Or could there be a bad element?
Thanks,
Reinhard
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use a clamp on ampmeter. that will tell you if both tanks are drawing the same current.
you might have a bad thermostat or bad element.
and always use a analog meter, not digital.
the digital meters are so sensitive the readings can be wrong
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Reinhard wrote:

Which tank is cold?
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This is slightly OT, but I have 8-unit apartment complexes that run off of 150 gal electric tanks and no one ever complains about lack of hot water.
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On Sat, 16 Feb 2008 20:28:04 -0800 (PST), Pat

More than one? How many?

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wrote:

This is slightly OT, but I have 8-unit apartment complexes that run off of 150 gal electric tanks and no one ever complains about lack of hot water.
Residential electric water heaters use 2- 4.5 KW (standard) elements, operating one at a time, to heat the water. Commercial electric water heaters can use more and higher KW elements in the same size tanks, so they have much faster recovery
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Stagger your showers have you drained the sediment from the tanks and cut the time save a bundle on Hydro never understood why people waste so much mony on showers.
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wrote:

Stagger your showers have you drained the sediment from the tanks and cut the time save a bundle on Hydro never understood why people waste so much mony on showers.
I don't think people consider it a "waste", to spend their money on things that they enjoy, whether it be long hot showers, steak dinners, or luxury cars.
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i seriously doubt the tanks are sluged up since the house is only 16 months old
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I solved that problem. I cook my steaks on the engine and take my showers in the car.

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Can you close off the output from one tank at a time and see if the other tank is putting out hot water? They might both be, but if not, that would be a big clue. Or close the input to one tank.
Of course there could be a bad element. My impression of my WH is that there was no need to disconnect any wires from the elements to do a good measurement of the resistance, but certainly in some things, like radio where parts are in parallel, that isn't always true. you should re-examine the wiring diagram to make sure there isn't some alternate path that would make an open element look like a working one.
In my case, and yours since like mine, there are also only two wires going to the bottom thermostat/element combo, it will be easier to measure the bottom than the top. If no one has used hot water for an hour, the water should be hot and the lower thermostat should be open. Turn off the power and with the meter on 240AC or higher, measure for AC voltage across the thermostat. Yes, I know I just told you to turn off the power, but some day you will forget to do that. If you attempt to measure resistance when the power is still on, you'll likely burn out your meter, if nothing worse. So always measure for voltage before measuring for resistance.
When there is no voltage, measure the resistance across the thermostat. It should be infinite, meaning that it is open and that means one of the leads from the lower element doesn't connect to anything now. So now you can measure the resistance of the lower element without there being an alternate path, and you will get a correct reading. Anything lower than infinity is likely to be good.
Also measure the resistance from the ground wire or the metal case of the water heater to each of the wires on the heating element. It's possible for the element to short against, I guess, the element cover, and to make the case "hot". If so, the element has to be replaced. If the element is 14 ohms, then whatever the resistance between one of the screws and the case is, unless it is infinite (as it should be) the resistance between the case and the other screw should be 14 ohms higher, or lower.

I'm nowhere near a pro, but aiui, in a some wheaters the upper and lower elements might run at the same time, but I think in most, the upper element runs first to provide some wh at the top of the tank and when it heats up a little, then the top thermostat sends the current to the lower element which does most of the heating. The lower thermostat turns that one off when the whole tank is hot enough.
Doesn't it say on the manu. wiring diagram something to indicate which kind of system you have? The kind you have will determine what you should look for.
My last wh had an LED connected across the upper element, so that one could easily tell when it was on. (at least when there was power to it. That didn't mean the element was working, but I never had doubts that it was.) I was thinking of putting an LED across the bottom one too this time, because I've had some of the same questions you have now**. You probably don't have room for these when the covers are on, but you could, when the power is off, connect lights like this temporarily and see when power is going to each element. LEDs for AC may be different from those for DC, i don't remenber. But the rating will include 240 volts AC.
Or you can very carefully use a meter. I would, when the power is off, make sure that the insulation isn't in the way of making measurements, and for the lower element, I would sit on the floor so I didn't fall over and fall into the waterheater. I would even consider using jumper wires with alligator clips on both ends and connecting these to the meter probes and the screws when the power is off, then turning the power back on. I assume you are well acquainted with the precautions you need for 240 volts. It's much more dangerous tan 110.
**But the new styrofoam? insulation around the thermostat left no easy room for even the one LED, so instead now I have none.
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ahh LEDs are generally run on just a few volts max.
what your probably want are neon lamps, they can run on 120 or 240 volts
they use nearly no power, and last nearly forever
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On Sun, 17 Feb 2008 14:50:57 -0800 (PST), " snipped-for-privacy@aol.com"

Well, some are made for higher and come with resistors to lower the voltage to that, but I think you are still absolutely right wrt water heaters. Thanks for the correction.

Yes, and they do fine with AC current. I've only seen led's on direct current. I thought about that before I posted but then thought, "Heck, it's a diode. It will just rectify the AC." But now I don't know.

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When you're out of water, feel the pipes and see if one WH is still delivering hot water.
As to the WH delivering cold, it still could be a bad dip tube.
As I understand WH, only one element is powered at a particular time. I'd expect the lower element to show power / current for a while, and then the upper one later. An infared thermometer is also a good idea, to see what temp the thermostat is "seeing" in the tank.
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On Feb 18, 4:52�pm, "Stormin Mormon"

use clamp on amp meter check current draw of both heaters after drawing a lot of water, both should match
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On Mon, 18 Feb 2008 14:30:12 -0800 (PST), " snipped-for-privacy@aol.com"

Two people have recommended that, but neither mentioned the difficulty of using one, iiuc.
Don't you have to separate the two power leads to use a clamp-on, so that it goes around only one of them? Otherwise don't the current in each of the two wires going opposite directions cancel each other out?
You could open the top and pull out one of the wires. Is that what is intended?
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Two people have recommended that, but neither mentioned the difficulty of using one, iiuc.
CY: What's iiuc?
Don't you have to separate the two power leads to use a clamp-on, so that it goes around only one of them? Otherwise don't the current in each of the two wires going opposite directions cancel each other out?
CY: Correct.
You could open the top and pull out one of the wires. Is that what is intended?
CY: Got to be a connection box some where. Or you could open the circuit breaker box.
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If I Understand Correctly
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