ohm - thermostat

Old 02-06-03, 07:15 AM
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ohm - thermostat

How do you check the upper thermostat of an electric hot water heater with an ohm meter? I have been given conflicting info on how to check it and what the reading should be. (Whirlpool electric). Help!
Old 02-06-03, 08:25 AM
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Turn off the power to the unit. Disconnect the element(s) from the supply lines. You don't need to remove the elements from the tank.

Set your meter to "ohms" (tens scale).

First, place the meter's black lead on a metal part of the tank. Touch the red lead to both sides of the element. The meter should read "open" (infinite resistance, as if the leads weren't touching anything), or the element is shorted to the tank. Replace it.

Now touch the two leads to the two sides of the element. A good element will read between 10 and 16 ohms (depending upon rated wattage). A bad element will read "open".
Old 02-06-03, 09:11 AM
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Rick...I checked the elements and determined that the lower element was bad and the upper element was good.

But I need to know how to check the Thermostat. One guy tells me it's good and another guy says it's gone bad. What is the correct way to determine this with an ohm meter?
Old 02-06-03, 11:44 AM
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If the thermostat is electromechanical, then, with power off:
1)Set the thermostat to a temp below the present water temp.
you should measure open between the point where power comes in and the heater lead connection on the thermostat.
2) Set the thermostat to a temperature higher than the current water temp. You should then measure a short (0 Ohms) across the same points. Mechanical contacts are prone to oxidation and erosion from arcs drawn during opening/closing. If there is any significant resistance seen during this step, you may want to clean the contacts or replace the thermostat.

You will usually hear a click as you cycle past the current operating temperature in either direction- this should coincide with change in your resistance reading.

If the thermostat is electronic, using transistors or triacs to switch the current, then a power off ohmeter check is not very useful. It will tell you if there is a shorted triac( in which case you'd be boiling your water) but not if there is a different problem.
Instead, a power on check of voltage between the heater lead and neutral is used.
1) case 1 above reads 0 V at the heater lead
2) case 2 above reads 115V AC or whatever your source V is.

BE CAREFUL! Water, Pipes, Basements and Electricity present a very real, lethal hazard.
Old 02-06-03, 12:32 PM
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You can check with the power on. Use a 250 vAC scale. Measure across the thermostat. When it is off you will measure 220 vAC. When it turns on you should measure 0 volts. If you get something greater than 0 volts the points are bad.
Old 02-07-03, 09:09 AM
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I see that now, diy novice. I have to learn to read. Sorry about the useless info.
Old 02-10-03, 11:46 AM
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Thanks for all your help! I replaced the element and the thermostat and it appears to be working great!

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