testing water heater


Hmm, our water heater seems to have become unwell in the last couple of days - taking a long time to reheat, and whilst the hot water it eventually produces is still hot (not just warm) it runs out very quickly.
It's a dual-element design, and having just run the tank cold, with a meter I'm getting 240V at the lower element terminals but nothing at the top one, suggesting the upper thermostat's at fault (I'll rule out wiring too in a few minutes, just to make sure it's not a loose terminal or something silly).
Questions:
1) Just to clarify, should the upper 'stat / element be operating with a fully-cold tank? I assume both elements should be running in that situation, but figured I'd check.
2) Do these thermostats change resistance according to temp, or are they either off or on depending on setting? (I'm just wondering if I can somehow test it in isolation by blasting it with a hairdrier, or something, and measuring resistance across the terminals, but maybe that won't get it hot enough to prove anything)
3) The upper 'stat is a WH-10 7-terminal, but it's a non-adjustable one (says as much on it, along with something like "factory pre-set to hot setting"). I can get a replacement WH-10 easily enough, but I've only seen adjustable ones for sale - presumably I can just drop one of those in, except that I've no idea what the manufacturer's idea of "hot setting" is :-)
cheers
Jules
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Residential electric water heater thermostats are on-off switches, and NEVER send power to both heating elements simultaneously. When the water is cold, the upper thermostat sends power to the upper element and heats the water at the top of the tank. When the upper thermostat is satisfied, it disconnects power to the upper element and sends it down to the lower thermostat, which in turn sends it to the lower element, which remains on and heating, until either the lower thermostat is satisfied, or the upper thermostat calls for heat. Since you have some hot water, and you say that you have 240 volts at the lower element terminals, I will deduce that the lower element is defective. You can easily verify this with an ammeter
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mfrencher had written this in response to http://www.thestuccocompany.com/maintenance/testing-water-heater-394385-.htm :
Jules wrote:

------------------------------------- Most residential elec H2O heaters dont run both elements at once under any circumstance. Water is pulled from the top of the tank and filled w/cold at bottom of tank. Top elements often run first until temp is reached followed by the lower elements heating cycle. Mineral build up at bottom of tank seems to knock the lower elements out more often than the top. Disconnect the power leads to the element and test the resistance across the element usually at 13.5 or so when good and open line when bad. Change both stats at the same time for best results.
Marco
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mfrencher wrote:

Measuring voltage at the element terminal does not really check if it is good or not. Remove power and Ohm it out or use Amprobe. I have a cheap multimeter with Amprobe clamp and temp sensor. It paid for itself many times over.
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On Sun, 13 Sep 2009 19:20:12 -0600, Tony Hwang wrote:

Yes, I had actually done that yesterday (before reading Marco's post, but after posting mine) and got 13.2 ohms on each element.
New upper thermostat appears to have sorted things - I went to look at a 'universal' one in stock at the local parts place which turned out to be a WH-10 anyway (actually an adjustable type). Setting range from 'hot' to 'very hot', so I just left it on 'hot' for now...
If Marco's explanation matches my system (I did trace out the wiring, but don't have it handy right now) then it seems like the upper thermostat had failed in such a way that it was always supplying power to the lower 'stat, so all the work was being done by the lower element.
cheers
Jules
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Jules wrote:

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