Potterton Lynx High Flame/ Low Flame Problem

[lynxowner]

I am having a problem with my Lynx combi and would appreciate any help you can offer.
The boiler's GC No is 47.590.02.
I have followed the fault-finding flowcharts in the Installation and Servicing Instructions Manual but the problem doesnt quite fit any of them.
The boiler goes through the ignition sequence OK, lights up, goes to high flame and the temperature display lights show increasing temperature. As I understand it, before the display reaches the maximum, the boiler should change to low flame. The high flame goes out much too early though (usually when only two of the temperature lights are lit) and the low flame doesnt come on. The boiler then starts to cycle, making clicking noises, with lights flashing on briefly. Eventually it goes back through the ignition process, the high flame light comes back on for a short while then it goes out again and does the clicking and flashing again until I get fed up with it and switch the whole thing off. It shows the same problem on both Heating and Water. The problem seems to be temperature related as it works OK for a few minutes when starting from cold, it is when it gets hot that the problems start. It is therefore able to deliver a bit of hot water for washing etc but is useless for heating as it starts messing about way before it is getting any heat to the radiators.
I have had intermittent problems in the past which I eventually traced to bad joints on the PCBs, I have had them checked over recently and the joints resoldered so I dont think that is the problem. This problem arose briefly a week ago then went away, now it does it consistently. I have also replaced the fan recently as it packed up (not had much luck with the boiler all in all recently, getting a bit fed up with it). I suspect the modulator board but dont know how to be sure. Any ideas?

 

[Agile]

I know that its obvious but have you checked the temperature sensor ???

The symptoms imply that the boiler thinks its hotter that it really is so the sensor would be the prime suspect!

As its an NTC then that implies a lower resistance than it should be. Its not got wet has it?

Otherwise it would seem to be a mod board fault. As no details are available the best way would be to test each component ideally by comparing with a known good one!

These are getting pretty long in the tooth though!

Tony

 

[nickso]

the usual suspects when you get disco lights on these are the pcb's as you know. mod board is a particular culprit

ive also had problems with the wiring up to the airflow sensor.....yellow wires if i remember right.

 

[Agile]

Rereading his original posting he says the low heat does not come on when the full flame goes off.

That implies that its really the burner which goes off. This is likely to be the airflow sensor but could still be the PCBs.

The airflow sensor is based on an old cooker igniter and its element resistance evidently is too high to keep the boiler on. That might still be a real air flow aspect particularly as we know the fan has been changed.

I dont know the resistances to be expected on a properly functioning boiler but its something like 1.4 ohms cold at the PCB connector if I remember correctly.

Its best to first assume its OK but the leads and particularly the push on connectors are high resistance. Try removing them from the sensor and clean the spades and ensure the female is a tight fit and the leads are very low resistance.

Only as a last resort think about a new sensor as they are virtually unobtainable!

Tony

 

[baxpoti]

Only as a last resort think about a new sensor as they are virtually unobtainable!

The air flow sensor connections can be temporarily shorted together for testing purposes.

Temp sensors are interchangable, trying swapping them, or short out the DHW sensor, if all the lights illuminate replacement needed, if not
replace Mod board.

This boiler finished production over 15 years ago, probably time for something a bit newer?

 

[lynxowner]

Thanks to all those who have contributed and are therefore helping my understanding of what the problem might be. I thought about the temperature sensors and thought it unlikely that they were the cause as the problem occurs on both DHW and CH. Each has its own sensor and the chances of them both failing together seem slim to me. I will definitely pursue the airflow sensor and its connections. However, if the sensor had failed completely, it wouldnt allow the boiler to ignite, and, as I said in my post, the ignition process seems to work perfectly. Starting from cold, the boiler lights up and runs along on high flame for a few minutes. It is then hijacked by something and the burner goes out. After a few seconds of clicking and lights flashing, the burner does successfully re-ignite (only to be hijacked again, a few seconds later). The modulator board should be in control of these things, but if the airflow sensor had an intermittent fault, it could be the culprit.
Thanks again. Any further thoughts?

 

[baxpoti]

Don't think you read this in my last post.

The air flow sensor connections can be temporarily shorted together for testing purposes.

 

[namsag]

Was there not a prob on lynx with the ignition electrode metal bit slid down inside the ceramic and got too close to the burner causing it to lock out as it heated and expanded thus touching burner . Remedied by either pushing it back up only moves about 1mm or replacing it . Long time since worked on one so not sure if it caused this fault

 

[Agile]

This owner has his own idea of how his boiler works and whats wrong ( the mod PCB ) and he is not interested in listening to us!

I wonder if he works on boilers six or seven days a week as we do?

Tony

 

[baxpoti]

Was there not a prob on lynx with the ignition electrode metal bit slid down inside the ceramic and got too close to the burner causing it to lock out as it heated and expanded thus touching burner . Remedied by either pushing it back up only moves about 1mm or replacing it . Long time since worked on one so not sure if it caused this fault

You've a good memory!

I remember spending quite a long time on this fault, mine was sparking nicely but not enough to ignite the gas, few phone calls to the old pott boys got me sorted.

 

[Soggy_weetabix]

Thanks to all those who have contributed and are therefore helping my understanding of what the problem might be. I thought about the temperature sensors and thought it unlikely that they were the cause as the problem occurs on both DHW and CH.

You thought?
I guess that is the same as you 'assumed'
ummm

However, if the sensor had failed completely, it wouldnt allow the boiler to ignite,

Have you listened to the experts?

The air flow sensor connections can be temporarily shorted together for testing purposes.

An expert who you SHOULD listen too.

David

 

[namsag]

Baxpoti. had quite a few of these on contract and used to push metal bit back up or change it for the flame detection electrode . But as said long time since worked on one and about 2 months ago threw 2 complete sets of boards into the bin becaue of this ,none of mates wanted them for same reason.

 

[lynxowner]

I apologise if I have given the impression that I wasn’t paying attention to the advice I was getting. I assure you that wasn’t the case. I am grateful for all the input, I have read it and acted on it.
I took the PCBs to my local electrical repair shop and asked the helpful guy there if he would look over the connections etc. again for me. I got them back, put them in and ….. the boiler wouldn’t even ignite, the ignition light wasn’t coming on. I removed the connections from the airflow sensor, cleaned everything up and replaced the connections making sure everything was good and tight and …. It still wouldn’t ignite. I shorted the airflow sensor connections together …. but still nothing.
Feeling pretty low then. Took the two PCBs off the distribution board again and resorted to cleaning the pins on the distribution board and the slots on the PCBs into which they fit. Put them back in place and tried again. It fired up, the temperature lights lit up progressively, when 4 were lit it switched over to low flame. It has been running OK for 3 hours now!!
It seems that if there is a problem with this boiler, no matter what the symptoms are, the cause it is more often than not going to be the PCBs or the connections to them.
Thanks again to all.
One last question …. What make of boiler should I replace it with, what is reliable these days?

 

[Agile]

I dont agree about the most common faults on your model.

I would rank the faults as:-

leaks
air flow sensor
pcbs
water flow problems
diverter valve diaphragm

In about that order! In view of their age I would rate them as pretty reliable. Not many combi boilers have lasted as long apart from the Ideal sprint or Sine 18.

Tony