Electric water heater keeps tripping high limit.

Discussion in 'Water Heater Forum, Tanks' started by bldn10, Nov 9, 2009.

  1. bldn10

    bldn10 New Member

    Messages:
    2
    Location:
    Memphis TN
    This is in a rental house and the tenant will have hot water and then it will quit. Every time, the high limit is tripped. I have replaced both elements, the upper thermostat, and even the old fuse box w/ a new breaker panel. After I put the thermostat in it seemed to be working fine - elements were heating independently. I guess I'll replace the lower thermostat but I'm not optimistic. Is there something esle that could cause this?

    Bill/Memphis
  2. jay_wat

    jay_wat Plumber

    Messages:
    24
    Location:
    Skagit Valley
    could be a short in the tank itself,,and also when you change the thermostats,,best to replace upper/lower at the same time
  3. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    How many gallons?
    4500w elements?
    How hot does the water get?
    How long does it take to trip?
  4. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,519
    Location:
    Cave Creek, Arizona
    heater

    You may have done a lot of stuff that had nothing to do with curing the problem. A loose screw on the upper thermostat will also trip the high limit switch. I would have tested everything to determine where the actual problem was. A fused lower thermostat could also cause the problem, as well as the other parts you changed. But since you still have the problem, they obviously were not the cause.
  5. bldn10

    bldn10 New Member

    Messages:
    2
    Location:
    Memphis TN
    Fixed

    The problem was the lower thermostat, that last and cheapest part I replaced.

    For the benefit of other DIYers I offer the following:

    In a double element heater the lower element is primary; it does the bulk of the heating, and hot water rises to the top and the cold goes to the bottom in a convection cycle. I.e. although the bottom element is primary, the water heats from the top down. The circuitry is such that only 1 element is on at a time; the bottom element heats the water up to the temp its thermostat is set on, then shuts off. If the water temp at the upper thermostat is under its setting the upper will come on until its temp is reached. And so on. If it gets too hot the upper limit switch will trip and power will be shut off until the switch is manually reset.

    That is what was happening in my case. Now I know that the tipoff was that after resetting, my tenant would get really hot water then have none the next morning. What was happening was that the lower thermostat never shut the lower element off and the temp would rise until the limit switch tripped sometime during the night. I figured this out when I saw that w/ the lower thermostat set at its lowest temp the water at the upper was so hot that the upper thermostat would not kick in even when set on its highest temp.
  6. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,519
    Location:
    Cave Creek, Arizona
    t'stat

    quote; I figured this out when I saw that w/ the lower thermostat set at its lowest temp the water at the upper was so hot that the upper thermostat would not kick in even when set on its highest temp.

    You lucked out, then, because the same symptoms would occur with a fused upper thermostat. When a thermostat is fused, it makes no difference what the setting is because it is not controlling the temperature.
  7. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    So a clamp-on ammeter would have a shown a continuous 19A draw and your electric energy usage would have gone up by 3000 kwh/month.
    Or, use your elec. meter and a clock to find this problem.
    Last edited: Nov 17, 2009
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