Due to another (as yet unresolved) issue the immersion heater for the hot water tank was needed during a recent heatwave (≥34oC for a few days). However it had been previously noted as not working.
These are not complex pieces of equipment, with only marginally more parts than a kettle, consisting of a heating element and two temperature controlled switches (a thermostat and an overtemperature cut-out) bolted into the hot water tank and powered from the mains electricity. Generally they are used as back-up heating devices and therefore only sporadically.
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| Hot water tank diagram. The immersion heating element is in red and the thermostat in grey - Source. |
Our system is slightly different to that shown above in that we have an unvented cylinder with the immersion heater mounted on the side of the tank.
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| Baulky immersion heater |
Immersion heaters are an industry standard item and ours is from an Italian company, Thermowatt. I couldn't find the exact item on their website, but given their generic nature I got close enough to get a circuit diagram.
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| Thermowatt circuit diagram for immersion heaters with unipolar thermostat and safety disconnection. |
The circuit diagram is pretty straightforward. So what is wrong with it? All I had from The Major is that 'it isn't working', so some general troubleshooting is required. As is commonly the case, YouTube came up with a good guide in the shape of John Ward of Flameport Electrical.
| Immersion heater fault finding guide |
Out with the multimeter for a bit of testing then; no issues with the fuse in the plug or the lead to the heater.
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| Internals of immersion heater. The thermostat control is in red and the overtemperature cut-out is in the hole to the left of the lower brass screw. |
If the unit is below its' set temperature the resistance across the live and neutral pins of the plug should be around 20 Ω, which it was,. So next, under the guise of checking that the thermostat was working correctly, I did the usual thing of twiddling the knobs and pressing the buttons and everything appeared to be working normally. Somewhat perplexed I put the cover back on and switched it back on. And lo and behold it was working.
Had it been working all the time? Had altering the thermostat set point or pressing the cut-out helped? No idea, but…
JOB DONE
Time taken: 20 minutes
Cost: £0.00
