Valliant Combi VCW Sine 18 T3 overheating

[carlosg]

I'm sure one of you experts can help me sort this boiler problem. Simply because i have a second replacement boiler that i can take parts off to use to fix the faulty one.
Basically the boiler as the following problem. The hot water always works fine but the central heating is overheating. I have to watch the temp and switch off before it reaches a 100 degrees on the combi temp indicator.
I've had a few suggestions that it could be the overheat cut-out sensor but this really doesn't seem right to me as I thought this sensor triggered a cutout if it detected overheating and would require a manual reset (push button) to start the device up again. Which makes me think my problem is not caused by this sensor (Am i right?)
As mentioned above I have an old boiler which i can take parts off and am stuck as to what part to swap. Should i swap the main PCB board or just the Overheat cutout sensor or is it something else? For obvious reasons I don't want to swap both if i don't have to.
Also could it be the Room Thermo stat?
The boiler is a Valliant Combi VCW Sine 18 T3
Thanks in advance

 

[chrishutt]

If I remember correctly the main temperature sensor on this boiler is the NTC resistor, located on the flow pipe to the left I think. Very simple to unscrew and replace, once you've drained the boiler, although you can check its function (variable resistance) with a multimeter.

 

[carlosg]

Thanks Chris I see a something screwed into a pipe on the lhs it has a purple push on wire connected to it. Does this sound correct?

 

[weareleeds]

Yes the ntc resister has a single purple lead.To test when htg is on remove lead & short it to something metal (trust me it won't cause any problem) if burner then goes out the ntc is faulty

 

[carlosg]

Thanks for the NTC info i have now changed the NTC resistor and the following is now what happens.

I turn room thermostat to click (23degrees) & boiler fires up. The temp dial on the boiler hovers around 60degrees for aprox 10Mins, flicking between hi/low flame. The flame then stays on high all the way from 60-98 degrees then the boiler switches back to a low flame & the temp keeps falling all the way back down to 60ish. During this falling temp period the flame doesn't go high again not once. Only when i move the room thermostat to a higher temp. Is this normal operation as this doesn't seem quite right to me. Any ideas?