Testing triac resistance - Bosch Water Heaters

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The left canister probably has a grounded element in it, which will keep power going after the water is off. Either that or you have a bad triac on your control unit

Use an ohmmeter to compare the resistance of the two. Remove power first of course. And you will also have to remove at least one lead from the heating element before you can get a good resistance measurement. Depends on what you have, but you should get a resistance of somewhere between 8 and maybe up to 50 ohms. If you have a completely failed heating element it is going to have a VERY high resistance, but if it is marginal it might be in the hundreds. If you do have one that is bad or marginal it's going to have something like 10 times (or more) resistance than the good one. You might measure a good comparable one at Home Depot for reference. At difference of maybe 25% (not 25 ohms) is probably OK. Good luck.

9:10 times the control board is bad. You are wise to suspect as much.

1) Here's step-by step for checking electric water heater circuit:
Testing assumes water heater is fully functional (although your water heater might have a short).
http://waterheatertimer.org/Test-electricity-to-water-heater.html

2) Once circuit breaker and line are eliminated, take 30 minutes and test internal parts of water heater as shown on following link.
Ordinary tank-type water heater is fully repairable by homeowner using off-the-shelf generic parts from hardware store.
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html

3) Testing requires cheap multimeter from home center.
When testing continuity or elements, rotate multimeter dial to ohms.
When testing voltage, rotate dial to 240-250-277 VAC

4) Add a comment for more free help.
Also take advantage of fixya phone service.
For a price, fixya expert speaks with you over phone while you work on water heater or any do-it-yourself project.

Remove upper and lower access panels
in upper access, use voltage tester to test voltage of on the uppermost terminals above the red reset button. should be ~220V between terminals (110 to ground)
If not: breaker off or bad,
If 220V proceed:
Test the two terminals immediately below this reset button should be 220V
If not: reset tripped or bad
If 220V present, proceed.
Turn off breaker to heater.

Use ohm meter or continuity tester.
test resistance across upper element terminals -should be 10 to15 ohms
check between each terminal and ground -should be open circuit .
If not replace element
Repeat for lower element.

test across lower element thermostat should be no resistance when stat turned up so it clicks on.
(To test that lower stat turns off, allow tank to heat up completely before conducting this test) Turn sta temp down so it clicks off then test across stat terminals -should be open circuit

The upper thermostat is a 3 way switch
The simplest way to test it is to turn the power back on turn the upper stat all the way up so it clicks on and use voltage tester across the UPPER element terminals, should be ~220V
Now turn the upper stat down so it clicks off- check upper element should be 0 V between upper element terminals -(Caution ! terminals will still be live 110V to ground)
The upper stat has now sent power to lower stat.
Turn lower stat up so it clicks on, check Voltage across lower element -should be ~220V turn lower stat down so clicks off, should be 0 V across lower element terminals.

NOTE If tank is completely cold the upper stat will not switch off and wont send power to lower stat.
If lower tank is cold lower stat may not click off even when turned down.
normal stat setting is 125 to 130
The hotter you you set the stat, the faster your tank corrodes.

You need to check the resistance of the Element... and check that the Thermostat is actually switching the power through, when it gets to temperature. Do a continuity test from the P & N, to the Element. usually the Phase comes in to the Thermostat, then through to the Contacts, to the "Hot" side, of the Heating Element, & back from the "Other"s side, the "Cold" side of the Element, to neutral. When thermostat is Operated, ON, When it gets to selected temperature it switches OFF, you should have a Resistance, measurable from P & N at mains lead in.
You can figure this resistance out by Dividing the Wattage of unit, by the Voltage this gets the Current, Now we divide that Current, into the Voltage and we get the "Resistance" of the "Load" or Element. This is what you should "See" with your OHMS Meter when you look "In" to the circuit. +/-10% is OK

You seem to have a burned out heater elemt. Most water heaters have 2 e.ements. If the upper one bruns out, it will not heat at all. If you have a cheap heater built by State, you probably have a bad lower thermostat. It will over heat and trip the protection device. If you reset the red button on the upper thermostat and it gets too hot (check in about 2 hours) then replace the lower therm. You could still have a burned out upper element.

If the boiler uses oil (#2 fuel diesel); there are several factors that could cause this problem:
* Lack of oil due to clogged oil filter or restricted oil line.
* Cad resistance greater than 1500 ohms.
* Defective Primary control.
* Shoot accumulated on the retention head.
* En pinching flame in combustion chamber.
* Collapsed combustion chamber into firing range.
* There are other reason not listed here.
* Some problems require expensive testing equipment to make an accurate diagnosis.
* Get in touch with an oil burner technician with several years of experience. I have a license to practice in the state of Massachusetts. My number is 978-5443783

I would try a continuety test on the element.  You should get no resistance.  If the water heater has been sitting for any real length of time, then it is probably better to just replace, instead of tryin to reuse, especially since you don't want to shower/wash is stagnant water...
Hope this helps