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Thread: Voltage at elements & other questions

  1. #1
    DIY Junior Member JohnEdgewater's Avatar
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    Default Voltage at elements & other questions

    I installed new heating elements in my 12yo AO Smith 40 gal. water heater today. I got the elements at Home D. The elements I removed were: top, 3500w 208v shorter; bottom, 4500w 240v, longer. The new elements were both the same length and both 4500w, 240v.

    Quest. 1: Is it okay that I installed 2 of the same type elements?

    Quest 2: Is it okay that the new element I installed in the top differs in watts and voltage than the element removed?

    Quest 3: When I put a multimeter on the two screws that connect the power wires to the element in the top, the meter registers 240v. When I touch the probes of the meter to the screws on the bottom element, nothing. If I touch the meter probes to one of the screws connecting power to the bottom element and touch the other probe to the tank, I get 115v reading on the meter. Is everything okay here? Why no 240 at the screws on the bottom element? Why no reading at all at the bottom element when I touch the probes to both screws?

    Quest. 4: do I need to do anything to the expansion tank on top after I've drained and refilled the water from the water heater tank?

    Thx in advance for any help

  2. #2
    Plumber jimbo's Avatar
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    1. Your old elements were the same watts. A given element, which would be approx. 12.5 ohms, would produce 3800 watts on 208 volts, and 4500 watts on 240 volts. Many water heaters are rated at 3800/4500, and that is the deal.

    2. NEVER trouble shoot a water heater by measuring from one terminal to ground.....that will lead you on a wild goose chase every time. The only measurement that matters is directly across the two terminals of the element. Either it has 240 or zero. Note that on a residential water heater, the two elements are never energized at the same time. That would pop the breaker, and the thermostats prevent that from happening.

  3. #3
    DIY Senior Member JerryR's Avatar
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    Answer to quest 3:

    Your readings are normal for a tank that is heating up.

    One terminal on each element will always have 120 vac as measured to ground.

    Upper element runs first. (240vac across both screws)

    Once the top of the tank is hot the upper thermostat redirects one leg to the lower thermostat. If the lower portion of the tank is not up to temp at that time the lower themostat will connect the second leg to the lower element snd you should read ~240vac across both terminals on the lower element.

    Once the tank is completely at temp you will read 0 across the 2 terminals on either element and you still should read 120 vac from one terminal to ground on either element.

    FYI, One terminal on each element always has one of the 120 vac legs connected at all times. The thermostats switch the other 120 v leg as needed

    Jerry
    Last edited by JerryR; 02-02-2012 at 08:04 AM.
    JR

  4. #4
    General Engineering Contractor ballvalve's Avatar
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    Either it has 240 or zero
    I get a whole range of readings depending on how the element blew. Some keep running at a reduced rate.

  5. #5
    Retired Defense Industry Engineer jadnashua's Avatar
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    Quote Originally Posted by ballvalve View Post
    I get a whole range of readings depending on how the element blew. Some keep running at a reduced rate.
    The implication was 'when operating correctly', not when there was a fault.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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