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I've just finished up wiring an old heater in my sauna. It has laid around in a shed for a decade or so and am wondering if it's spent. It powered on fine and heated for about 5 minutes until the GFCI tripped. I tried to reset the GFCI and it instantly trips.

I pulled the lid from the panel below the heater and with the supply wires disconnected, tested for continuity around the element, and then from each end to ground. The element tests ok at about 15Ω which is what I'd expect (4000W element at 240V). My concern is I have found a varying resistance between the element and ground. I checked it at 3 different points in time, first it was 150KΩ and increasing, the second it was 140KΩ and decreasing, the last just now was down to 10KΩ.

Does anybody know what this is a symptom of? Could it be a case of contamination on the element conducting to ground or is there a fault? I'm sure the element leaking current onto ground and the heater chassis is a big problem and the GFCI was right to trip.

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Replace the element, it's shot

Most electric (immersion or otherwise) heaters are constructed as "calrods" -- a coil of fine nichrome wire in a metal tube that has been packed with magnesium oxide as an insulator, and had its ends sealed against the ingress of moisture and contamination.

However, if water does enter the element (say through a pinhole in the tube, or a faulty seal at one end), it will cause the magnesium oxide insulator to expand as per this document from US water heater maker Rheem, which can worsen seal faults and splits. Since the insulator material is hygroscopic, the failed seal will also attract water actively, resulting in rapid deterioration of the insulation (Source).

With industrial heaters that have field-replaceable seals, it is possible to "bake" the element to drive out moisture, replace the seals with fresh ones, and return the element to service; such a procedure isn't usable for consumer-type calrods though, because their seals are designed to be permanent; it won't help a pinholed or split element for long, anyway.

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  • Might be hard finding a replacement element for this heater, nothing on the internet about the make or model. I do have another heater in storage I just dragged it out but the design looks a little suspect to me. The panel isn't sealed at all it is basically a metal box on the side with a metal cover, I can see gaps between the case and lid. Is this really safe to be used in a room full of steam...?
    – JoshNZ
    May 13 '15 at 0:54
  • @JoshNZ -- the seal is at the ends of the element's tube. Since you have a spare heater, you can measure that and see what readings you get element-to-ground...
    – ThreePhaseEel
    May 13 '15 at 1:01
  • With this spare heater I wasn't referring to the element seal I mean, the case where the supply lead connects to the heater, is not sealed. The bad heater has a sealed case with a gasket. Is the spare heater safe? As you probably guessed from that, the heaters are different the elements would not be interchangeable. And I did test the spare element out of interest. No continuity to ground
    – JoshNZ
    May 13 '15 at 1:18
  • @JoshNZ -- I'd contact the manufacturer at this point
    – ThreePhaseEel
    May 13 '15 at 1:49
  • I doubt they will exist any more (it's that kind of age) but I will give it a go tomorrow. Failing that, as long as the case and chassis is grounded I could seal up that case with silicone? It is supposed to be sealed right...?
    – JoshNZ
    May 13 '15 at 2:32

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