Testing a faulty Thermistor

5 Jun 2008
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United Kingdom
Just a quicke - what's the best way to check for a faulty thermistor inside a boiler - as you have to check resistance with power off !!!...
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Yeah I know that - but what reading proves its faulty - as sometimes they only go fauly when the boiler is switched on! :D
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ok ! I know the resistance shoukd change with temperature change, wasn't sure though the best way to test - as some go faulty and not just on open circuit..

So basically - fire boiler up - turn off power, take temperature reading of pipe close to thermstor and check resistance reading.......
yes sounds good!

some only go out of calibration when they heat up, so the boiler might work ok until say it gets to 55c then the boiler might go off because your thermister might read 1k say, even tho it should read 8.5 k for instance

hope that helps
ok thanks for that - is a type K thermococuple (with clip on temepartrue sensor) suitable ?

so when testing, if not open circuit, you need to watch the resistance reading doesn't go too low to prove it is faulty?

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