Jaguar Boiler

[Pipenut]

Hi to all luminaries.....
OK my aged Jaguar has developed a fault after being off for a few months.
The system has been flushed (even though a soft water area) all rads OK, pressure spot on (incl gas), delivers hot water fine etc
The issue is, from cold it runs up to the max CH temp of 85 quite happily and though the system gets the system hot all over the house.
The issue is that when hitting the top temp and the boiler shuts down (it would normally kick back on at 65 ) However, it re-starts immediately, ignites runs for 4 seconds, turns off for 10, and repeats until I manually stop the boiler. If I allow the system to drop into the 20's it will again run on fine until it hits the top temp.
No fault indication is shown.
Is it the flame sensor starting to fail?
Is it the thermistor?
Is it the ignition PCB?

The control panel clearly shows temprature ranges so I assume CH thermistor is functional

Any ideas? (before I rip it off the wall )

 

[Agile]

At first sight that seems to be an unusual fault.

The obvious ( to me at least ) cause would be a problem with the main control PCB.

However, before changing it I would be doing a lot of other tests!

I would also be wondering if the clip on sensor is fitted in the right place!

Tony

 

[Pipenut]

The clip sensor is as factory fitted.
This is really troubling me.
If the thermistor was failing I'm sure the controller would fail to see the recorded temperature ranges, yet it does?
I'm resisting the PCB as this is the most costly option
Without the service manual (only have the operators manual) I'm afraid I don't know the instrument ranges so I'm flying blind. I've resorted back to the factory re-set but again there is no change.
I've seen a similar issue before on an old Myson unit whereby the flame sensor once at temperature would then false read - but I'd expect to see a flame failure warning?
I'd imagine if there was a fault with the PCB it wouldn't function even from cold?
I does seem to point towards temperature recording? Am I missing something? What tests would you consider?
Cheers
Mark

 

[Agile]

I only share some diagnosting testing procedures with RGIs in the Combustion Chanber part of this site.

 

[conny]

Rip it out.
From a 'user' point of view once they start going wrong they are a pain in the backside!
As Agile will testify due to him answering a number of posts from me concerning my old one.

 

[Pipenut]

Hi Tony, many thanks, I understand.
It looks like I'll go though some trial and error testing then
I'll start with the flame sensor, thermistors then PCB - cost order lol
Cheers
Mark

 

[Pipenut]

Hi Tony, one last thing, what makes you think its the main PCB rather than the ignition PCB?
Cheers
Mark

 

[Pipenut]

I'm tempeted to rip it out - but I hate being beaten!
Your old one? got any spares lol

 

[conny]

Sorry mate, no. When I had the new boiler installed they just dumped the Jag in a skip I had outside. Said there was nothing worth salvaging.

 

[ianmcd]

most likely the thermistor your boiler is cycling on the overheat stat rather than go off with the normal temperature thermistor

 

[Pipenut]

Well guys, many thanks for the advice - it seems like I'm on the right track - I'll try the cheaper option first!!
It just so happens there are 3 scrap Jags on ebay! Thermistors and boards will no doubt be the same so it looks like I have some spares!!
I'll let you know what it was - any bets?
Cheers
Mark

 

[Agile]

Seeing three scrap boilers on Ebay at the same time might make you thin k they are unreliable?

In fact they are very reliable, just installed badly by mostly unregistered installers.

They are perceived as being difficult to repair but the reality is that they are pretty simple !

The temperature control is carried out by the main PCB in conjunction with feedback from the sensors!

Tony

 

[Agile]

Hi Tony, many thanks, I understand.
It looks like I'll go though some trial and error testing then
I'll start with the flame sensor, thermistors then PCB - cost order lol
Cheers
Mark

We dont advise DIYers to open the combustion chamber on boilers!

Nor do we give advice to any DIYer who does!

Tony

 

[Pipenut]

Tony, many thanks for the advice, I do understand your concerns about DIYers - a little knowledge etc.......but
Thanks anyhow.
Cheers
Mark

 

[lenbos]

In defence of the Jag boiler. I`v had one for 11 years never had a problem,till last week. the old no hot water! chessnut simple to fix needs new diaphragm. Biggest problem can`t get part because Plumb Center don`t do it, even though they sold it. Shop around you will get one I went to City Plumbing. You will get one on internet.