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Can I test an immersion heater to see if it's failed

Discussion in 'Plumbing and Central Heating' started by pedr0, 20 Nov 2020.

  1. pedr0

    pedr0

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    Hi,

    I've got a water tank that is meant to heat up overnight with an immersion heater powered by economy 7 electricity. Unfortunately it's stopped heating up. No one's in the property overnight so I can't get a multimeter on the immersion heater to test if it is indeed getting power (although no reason to suspect it isn't).

    I'd rather know if it has blown before I drain the whole system and replace it.

    Is there anything I can do to test?
     
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  3. polesapart

    polesapart

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    resistance of healthy element is around 20Ω across terminals

    stat can be checked for make/break
     
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  4. fixitflav

    fixitflav

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    If you have a multimeter you could isolate the power supply, and test for resistance of the element. Should be about 17ohm for a 3kW immersion. Also resistance of the thermostat switch, which should be zero.
     
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  5. ianmcd

    ianmcd

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    If it is an unvented cylinder the immersion thermostat will have an over heat thermostat if that has activated you will need to press the little button in
     
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  6. pedr0

    pedr0

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    Nice. Thanks both. I ll get on it.
     
  7. pedr0

    pedr0

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    @ianmcd this one I think is a vented system. (you're also helping me on another post which is a totally different property and system).

    This one's got a cold water tank that's just like a bucket with a loose fitting lid. That cold water tank feeds the hot water tank. I will check though for any reset button as well though.
     
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  8. oldbuffer

    oldbuffer

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    1. As ianmcd said.
    2. Given the small tank with loose lid. If this is immediately above the hot water cylinder, and connected to it, you may have a thermal store.
    3. Unvented, thermal store or modern immersion heater - all will have an overheat thermostat which requires manual intervention if triggered.
    4. Sure no one's switched it off? Consumer unit MCB / fuse not tripped / blown?
     
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  9. JohnD

    JohnD

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    if you can get hold of an electric clock, you can wire it in and see if the time changes.
     
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  11. pedr0

    pedr0

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    @oldbuffer I did check that the fuse at the consumer unit was fine, that the RCD was not tripped, and that the switch in the cupboard was on. All were fine.

    The immersion heater has been replaced before. I vaguely remember he got the part from screwfix or toolstation. And it would have been about 10 years ago. Not sure if that classes it as 'modern'.

    Cold water tank is indeed just above the hot water cylinder. Property is all on one floor (and hot water pressure is pathetic)
     
  12. pedr0

    pedr0

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    Another question regarding the reset button. If the overheat protection has been tripped (but the heater is otherwise fine), what should my multimeter reading be _before_ I press the reset button?
     
  13. Hugh Jaleak

    Hugh Jaleak

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    If the Overheat has popped out it shouldn't affect the reading across the element itself. Normally Immersions are wired with Live into the Stat, (which contains the Cutout), then a pre wired supply from the stat to the positive terminal of the element. If the element has blown I'd expect it to give a reading for an open circuit.

    If you're testing across the Stat, I'd expect a reading of 1, (i.e. total resistance as it's open circuit), until the Overheat is reset when the reading should drop to the impedance of the Stat.
     
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  14. pedr0

    pedr0

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    The reset button was indeed popped out! Thank you for possibly saving me a whole load of effort.

    So next question - what might have caused that to pop out?
     
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  15. Terrywookfit

    Terrywookfit

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    They do it for no apparent reason but obviously it has overheated.
     
  16. JohnD

    JohnD

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    The purpose of the pop out is that it will pop out if the main thermostat fails and the immersion heater causes the water to get too hot.

    That would be a reasonable assumption of the cause unless you can prove otherwise.

    It can also happen if the cylinder is overheated by some other source, such as a boiler when the thermostatic control has failed.
     
  17. ianmcd

    ianmcd

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    The immersion thermostats are very cheap items and most are very poor quality, if it activates again change the thermostat first they are very cheap, but you need to fit another with an over heat cut out
     
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