Thermistor resistance reading

[jono44]

Hi everyone,
I'm getting around 18.5 k ohms on my multimeter for both dhw and heating thermistors.
Does anyone know if that's about right or should it be a bit lower - the boiler is cold, cold, cold, two days cold.
Customer getting no hot water and the heating fires and blocks out pretty well staight away.

Many thanks to any input.
Jon.
P.s It's a halstead wickes combi 102

 

[Agile]

That is ok.

You need to be gas reg to fix boilers

 

[shambolic]

That is ok.

You need to be gas reg to fix boilers

 

[jono44]

I am,
I was hoping you guys might be able to help a little.
Being an Aussie in UK I'm still trying to get my head around all of the boilers over here and am struggling a bit on occasion and don't want to fork out for wrong parts because of wrong diagnosis.
One halstead guy reckons its the thermistors and another halstead guy reckons its pump but also could be diaphram -- me, well i am not sure and trying to test everything to work it out.



Plumbing & Gas Installations

Also ARGI registered.

 

[davebgas]

Highly doubtful it's the thermistor if it's doing that. Check the pump and diaphragms as said, also check dhw microswitch is being activated when hw drawn.

 

[Agile]

I am pleased to hear that you are indeed registered. usually the drain is Pom to Aus and you are only the second I have heard of over here. The first did some very bad work and then went back though!

Could you apply with your details, name , address, reg no AND licence number on card to [email protected] to be admitted to the Combustion Chamber part of this site where trade questions like this are best asked.

Tony

 

[Temprise]

The resistance of a thermister changes depending on the temperature it is reading.

If the boiler has an NTC (negative temperature coefficient) thermister, a good starting point for a reading is 10 Kohm at 25 degrees C.

And as the temperature increases the resistance of the thermister will decrease. You can probably download a thermister comparison table from the internet somewhere so you can accurately see what resistance you should get at all temperatures.

PTC's (positive temp co) work the other way round.

Regards

 

[jono44]

Hi Tony,
Thanks for your response. I did the link from conbustion chamber a week or so ago but haven't heard anything back - will do it as you have suggested though!
I eliminated the thermistors as they read 18 k at very cold which apparently is within specs, the pump is getting 230 when called for and has resistance of about 170 ohms (OK too) so the halstead guys reckon I should change diaphram in diverter valve - so I'm going to order both sides kits and overhaul it - I hope It is the problem as I'm stuck for any thing else. The pin on the hot water side moves out but doesn't quite make contact wi flow switch. The primary side pin doesn't move at all - boiler fires and then immediately blocks out to a green flashing light on the board.

Jon.

 

[MANDATE]

This might help
http://94-193-98-45.zone7.bethere.co.uk/content/halstead-ace-ace-high-fault-finding

 

[Agile]

You seem to have two problems.

DHW diaphragm failed and the pump or boiler blocked thus preventing the flow switch being activated.

Your comment is confusing though because the boiler will not fire up unless the flow switch is made.

Tony

 

[jono44]

I agree,
the boiler only fires for a about 2 seconds on heating and then fails - no sound of pump starting just fan and burner, no movement of primary flow pin (it stays in). Hot water has a little movement on pin but doesn't quite make the flow switch and so no pump no fan/ burner.

Many thanks,
Jon.
I'm off to another job now any input will be perused a little later.

 

[Agile]

You are getting it wrong there.

The pump runs first, then it activates the pin and then the boiler can fire up.

The pump has to run FIRST !

Have you checked the pump impeller?

Tony

 

[jono44]

Hi Mandate,
That link seems to be broken and bethere.co.uk only takes me as far as a broadband supplier.
Thanks anyway,
Jon.

 

[jono44]

The pump appears to be free and turning ok when I take off the cover screw.