Please explain Thermistors

[PlumbSafe]

I am essentially an installer that dabbles in service and breakdown, been doing it for nearly 15 years now, and am now moving completely into the breakdown side of things, I can use a multi meter and I understand most things inside and outside the modern boiler, but there are a few things I need to clear up, top of my list is the thermistor.

Can you guys please explain function, correct and in-correct operation and how to test them, what should I be getting at room temp in ohms etc, and does this apply the same for the DHW and FLOW/RETURN.

Cheers

 

[oldplumber]

thermistor are resistors that lose resistance as they heat up so at room temp they may be 11 ohms reducing to 4 ohms as they get to 50 degrees etc. see manufacturers literature for the curves of each thermister they use. each has 2 probes with which u can test them.

 

[Billy Bob Bob]

its easier to gear yourself up on popular boiler thermistors,most are dirt cheap takes seconds to plug one in to verify i rarely check resistances unless its a unusual make

 

[oldplumber]

its easier to gear yourself up on popular boiler thermistors,most are dirt cheap takes seconds to plug one in to verify i rarely check resistances unless its a unusual make

if your anything like me you still end up with loads of them and never the one u want!!!!!!!!!!!!!!!!!!! (cheapest option is to take them out of working boilers that customers insist you replace for them, coz they dont like them in the attic/kitchen etc)

 

[PlumbSafe]

so when a thermistor goes, say on a DHW HEX on combi Combi, the ressitance should be around say 10 ohms, its job is to act as a thermal cut out, is that right, so if it gets really hot the resistance goes up and stops the current to the next component, or am i way off. and if it does go it will create an open circuit and the current does not go any further. so same result as over heat.

 

[Pssst]

I used to carry a small selection of potentiometers with fly leads and croc clips to slot in when i wanted to test a faulty thermistor. I just used them to fool the control system thus demonstrating that the thermistor was faulty. Of course you might also measure its resistance at various temperatures also. Depends on boiler/situation. Thermistors can be NTC or PTC ie they can either increase or decrease in resistance with regard to temp rise...again,depends on boiler type.

 

[diamondgas]

so when a thermistor goes, say on a DHW HEX on combi Combi, the ressitance should be around say 10 ohms, its job is to act as a thermal cut out, is that right, so if it gets really hot the resistance goes up and stops the current to the next component, or am i way off. and if it does go it will create an open circuit and the current does not go any further. so same result as over heat.

They're not a fall safe component 'PlumbSafe' they're used to inform the pcb of the temperature they're reading. Differing temperatures produce differing resistances and the pcb reads these resistances and adjusts the boiler operation to suite! Depending on the boiler that could mean altering the fan speed or gas valve modureg to vary the heat input to the required demand! If one fails you usually get complaints relating to temperature output! Does that make sense?

 

[julesverne]

to make it easier to take a reading when in situ. next time you do a boiler swap keep the lead from the thermistor and use that to connect to connect the said thermistor you want to test and use the wires from it to connect to the multimeter better than messing trying to take a reading from the 2 pins on the thermistor.