Ravenheat CSI85AT - water pump running but nothing

[SueBeebies]

Hi,

My Ravenheat boiler started to cut-out and then re-do the ignition sequence. Just like a short cycle but it would do this even when just started up and the internal pipes were very cool. Now the boiler does not fire at all. However the water pump runs whether there's a call for heat or not.

I assume the boiler 'thinks' its too hot either through bad connection, faulty thermistor, or potentiometer.

Does anyone know what the electrical characteristics of the thermistor should be (base resistance and temp coefficient would be useful)? Also anyone know what the pot resistance should be?

Many thanks,

Sue.

 

[ChrisR]

Between 500 and 20k ohms. IIRC about 7k at room temp and 9k if you warm it in a personal crannie , about 37ºC.
Pot resistance are usually written on them , but they don't drift, the tracks can break, which is easy to detect.

Several other things could cause similar problems.

Where's Slug??

 

[SueBeebies]

Thanks Chris R. I'll check out the pots and thermistors tomorrow. I'm intrigued about the other causes of this problem - can you outline, I know this is a bit cheeky, but I'd like to check them out while I have the electrics in bits.

 

[Agile]

All the indications from what you have said is that its an ignition problem and nothing to do with the sensors.

By the way Chris got the restances back to front. Its an NTC.

Tony

 

[SueBeebies]

Just to finish this off in case anyone uses it for reference.

I had ruled out ignition problems as, while the fault was intermittent, the the relay supplying the switched live to the ignition board could be seen and heard operating I also confirmed that the switched live was switched at the ignition board. Indicating that the fault was before the ignition board.

The Ravenheat thermistor on the D.H.W. is the same as that on the C.H. The resistance of the DHW thermistor was 17k ohm at 18 Deg C and lowered to 16k ohm when I clutched the pipe. So it is NTC.

The CH thermistor was open circuit. To confirm this was the fault I replaced the thermistor with an 18k ohm resistor. The boiler did fire. I have of course not run the boiler the like this, as it would overheat, but this test has confirmed the fault.

A new thermistor is winging its way to me.

Sue.