Jaguar 28KW high HW temp reading and faults

[Mhdiy]

Hi, I have a Jaguar 28KW combi.

The temp reading shows 70c, even when the system is cold. The CH will only come on if you turn the temp to max, and when requesting HW, the boiler will fire for a few seconds and then go off, then fire and go off etc and the water is cold or luke warm.

I've done some fault finding and discovered that if you disconnect the CH thermistor, then the readout still shows about 60c and there is no fault displayed. With the CH thermistor disconnected the HW works as normal. The boiler will fire when you switch the CH on, even with the CH thermistor disconnected.

I have checked the resistance of the CH thermistor and it seems ok, there is approx 4.5v to it from the PCB and I have tested the wiring which seems ok.

I'm about to go and fetch a new thermistor, but I'm suspicious it could be something else, possibly the PCB?

Has anybody come across this before?

Thanks

Mark

 

[Agile]

Hi, I have a Jaguar 28KW combi

I have checked the resistance of the CH thermistor and it seems ok, there is approx 4.5v to it from the PCB and I have tested the wiring which seems ok.

I'm about to go and fetch a new thermistor, but I'm suspicious it could be something else, possibly the PCB?

Thanks

Mark

You may have checked the resistance but why do you think that its OK?

If you tell us the temperature and resistance of each then we can judge if its likely to be OK.

Tony

 

[Mhdiy]

Hi Tony,

The resistance is 11.6k at approx 21c, I thought it was ok as it was online with the figures in the boiler manual.

What do think, is it what you would expect?

Thanks

 

[Agile]

I agree that is correct at that temperature.

But its possible that if just a little warmer it may be becoming too low resistance. I might retest at about 40C.

However, it does seem more likely that the ADC on the PCB is giving the wrong temperature value. Thast might be the ADC itself or an associated resistor.

Since the PCB is expensive, I would substitute a new NTC first just to eliminate it as I carry them but it is likely to be the PCB.

Do all the tactile switches feel correct and provide the expected changes?

Tony

 

[Beerlover]

However, it does seem more likely that the ADC on the PCB is giving the wrong temperature value. Thast might be the ADC itself or an associated resistor.

Do all the tactile switches feel correct and provide the expected changes?

Tony

What the ....

 

[Mhdiy]

Thanks Tony,
Unfortunatey I can't accurately test at higher temp as i don't have the right kit to hand, and I don't have a spare thermistor. Instead I temporarily switched the connections for the CH and HW thermistors on the pcb as It appears the HW one is reading correctly. CH still showed 70c even though everything is at room temperature, and HW (now connected to the suspect CH thermistor) showed 24c. So it looks like the suspect thermistor is ok.

All tactile switches function as expected, and feel fine.

So as you say it looks like something on the board is at fault.

Looks like I'll have to bite the bullet and get a PCB!?

 

[Agile]

What the ....

???

I do agree that a new PCB is probably going to be the only way to repair the boiler.

Depending on the price a British Gas fixed price repair might be cheaper or less risky!

Tony

 

[Mhdiy]

Thanks for the advice, I'll look into that too.

Cheers

Mark