HWC tripping the circuit breaker

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neb

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#319145 27-Mar-2025 12:54

Had noticed the shower water was getting less and less warm over the last few days and, just as I was going to check whether the HWC was getting power, checked the circuit breaker for it and it had tripped. It's never done this before, and resetting it restored normal operations, but I'm wondering whether it's a sign that something is on the way out and/or needs to be investigated.

SomeoneSomewhere

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#3357820 27-Mar-2025 13:24

The fact that it reset (with a cold cylinder, so element should have been on already) means it's probably not a catastrophic failure. I assume it's now heating properly?

Circuit breakers aren't extremely high precision and nor are elements. It's possible that you have e.g. a 3kW/13A element on a 10A breaker. It probably should trip eventually - but only after a few hours, maybe if the switchboard is a bit warmer than usual or the voltage is a bit higher than usual.

Check the breaker size - should be something like C10, C16, or C20. Check the element size, both on the cylinder label and by turning it off/on and seeing what the power meter shows.

HWCs usually aren't on an RCD, so moisture ingress isn't generally going to cause a trip.

It's possible, but unlikely, that there's a conductor that has rubbed through the insulation (or had a nail shot through it) and suffered a short circuit, but has either blown clear, or for whatever reason isn't currently isn't quite touching.

neb

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#3357826 27-Mar-2025 13:42

It reset without any problems and is now heating normally. It's also been like that for the 15 years that we've had the place, and who knows how long before that, without any problems. MCB is a C16, just checked the power readings and they're very odd:

The spike was when I reset the MCB. Power throughout the week has been:

Is this symptomatic of a failed heating element? The cylinder is very well insulated so hard to tell how warm it is, but you can't feel much warmth ATM.

neb

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#3357828 27-Mar-2025 13:46

Just sanity-checked the readings, this is the overall power draw for everything not on the solar ESS, the spike to ~1kW at 9pm is the dehumidifier running, then the spike and ongoing draw at around 11am is the HWC MCB reset. So my guess is a failed element, since there should be a spike to 1.5kW followed by a drop to nothing once it's heated.

neb

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#3357831 27-Mar-2025 13:51

And here's the trace back to last-known-good:

So you can see the standard 1.5kW cycling, then a half-hearted attempt to 300W with a long tail before the MCB trips.

larknz

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#3357836 27-Mar-2025 14:05

1500 watts is what would be expected from an older element. The fact that is now only drawing 500 watts would indicate that the element is failing. The only way to be sure is to perform a phase to earth insulation resistance test which can only be done by someone that has a suitable tester. I would plan on doing this sooner rather than later.

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#3357839 27-Mar-2025 14:13

Dump some water using the TPR valve (pull handle at top of cylinder). Stick a thermometer in it and see how hot it is. Should be 60-65C ish, maybe a few degrees lower.

1.5kW is unusually low for an HWC unless you've specifically had the element replaced to be lower power for better solar behaviour, or it's a very old or very small cylinder. Are you sure it's not on the emergency supply and you're actually measuring it?

With a reasonable-size cylinder on a 1.5kW element you should be looking at 4h or more to reheat.

Could well be failed element.

C16 MCB won't trip on 300VA; you need 4kVA sustained for an hour or much more current for a shorter time.

neb

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#3357842 27-Mar-2025 14:17

I'm wondering whether it's going to be easier, and cheaper in the long run, to just replace the HWC. It's a low-pressure Rheem 135L from 1984 so has probably done its bit.

It looks like the current replacement, with a view to it lasting for awhile, would be mains-pressure SS or VE, or for a like-for-like replacement low-pressure VE.

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neb

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#3357845 27-Mar-2025 14:18

SomeoneSomewhere:

1.5kW is unusually low for an HWC unless you've specifically had the element replaced to be lower power for better solar behaviour, or it's a very old or very small cylinder.

It's a 40-year-old 135L, so definitely fits the description.

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#3357847 27-Mar-2025 14:25

Yeah, almost certainly time for a new element if not new cylinder. I doubt it'll heat fully.

larknz

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#3357850 27-Mar-2025 14:58

1500 watts was pretty standard 40 years ago

neb

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#3357860 27-Mar-2025 15:07

And it's actually pretty decent given that we're on a solar ESS, makes more sense to use less power over a longer period than a giant pulse of power that drains the batteries.

I've started a new thread for further discussion on a replacement HWC since the "is this a problem" seems to be resolved to "yes".

Unless someone has comments about how soon I need to be worrying about legionella given the current situation.

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#3357862 27-Mar-2025 15:13

I think that you will find the outer sheath of the element has failed. This will have let water into the insulating material , which means that the element could fail completely at any time.

neb

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#3357865 27-Mar-2025 15:27

Yeah, have definitely decided it needs a replacement, just wondering whether it's should-have-been-done-yesterday or whether there's a few days to explore options.

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#3357884 27-Mar-2025 16:00

neb:

Yeah, have definitely decided it needs a replacement, just wondering whether it's should-have-been-done-yesterday or whether there's a few days to explore options.

It will have arced over to the outside which tripped the breaker, and now your heating will be getting done by the wet stuff in the element having some conductivity to the ground of the element. It will be dead in days if not hours of run time.

Richard rich.ms

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#3357889 27-Mar-2025 16:05

"Made a note in my diary on the way here. Simply says, 'bugger'".