Potterton Performa 24, CH ignition goes on and off in a loop

[picasso]

it wont have any thing to do with the new pipe, when you call for hot water the burner fires on full gas rate, on central heating it fires up on min and then modulates, if the min gas is to low the flame wont be recognised and it will attempt to reignite, its cheaper to check the gas than a new pcb you might not need.

 

[gas112]

Also fires on minimum rate initially when HW demanded not just CH. But yep its worth checking the min pressure

 

[Clint000]

Just checked the flame when in CH. When it switches on, I can see a decent flame which after 1 second or so lowers down, stays low for ~2 seconds and then stops. The process starts all over again. Would that be a sign of incorrect min gas setting?

Apparently it has got worse as it now falls into the on/off loop even when the heating is cold (before it was only affecting the heating when at hot temp).

Could anyone explain me what is the function of the air pressure switch, and whether that could cause the issue? It is the only element which I have not checked.

Thanks everybody!

 

[picasso]

check these first

View media item 31834

 

[Clint000]

I measured:
55-85V DC between 23&24 (a bit low from spec)
22-40V DC between 21&22 (within spec)

Have not been able to measure the currents as I would need to detach the connector from the PCB and wire it back up but with the multimeter in between the connector and the PCB. I can do it if absolutely needed but there is a risk of damaging the PCB when wiring it all up.

I have also noticed that for the period when the burner is on, the ignition electrode keeps producing sparks inside the chamber (I guess this is normal), and keeps of producing sparks until after 8-9 seconds when the burner switches off. Could this be to a failure in the sensing electrode? How can I test the sensing electrode?

 

[picasso]

The sparking is normal, time to get an rgi I think, you might be lucky and its just a blocked gas valve from when you had the pipes changed.

 

[Clint000]

For completeness, I get similar readings when the DHW is operated, and the DHW works fine.

Which subsystems/sensors are used in CH but not during DHW?

 

[bengasman]

..Bengasman Know what your thinking re swapping the sensors over but thes only have them on the heating unlike the 28`s and 105`s

That's one option out the window then.

 

[matt1e]

..Bengasman Know what your thinking re swapping the sensors over but thes only have them on the heating unlike the 28`s and 105`s

That's one option out the window then.

some models of 24's he have two of them
butI think a CH temp sensor problem would make the boiler anticycle (ie wait for 3mins before firing up again)
could be an bad conection or fault on any external controls
I'd link them out first
OP your boilers GC number would be useful for a start

Matt

 

[Clint000]

Hi Matt,
GC number is 47-393-06.

I noticed an interesting thing:
The DHW also goes into the on/off loop if the DHW temperature setting is close to the lowest temp.

I did not noticed this before since I always have it on max temp, but it is consistent with the issue with the CH.
When the burner has low flames (either because the DHW temp setting is low, or because the CH is on and modulates the flames) then it starts the on/off loop. This doesn't happen when there are big flames like when DHW is set to max temp setting.

Is the key here? What can cause a cut-off when the burner has low flames? Min gas pressure?

 

[Clint000]

The sparking is normal, time to get an rgi I think, you might be lucky and its just a blocked gas valve from when you had the pipes changed.

- I was thinking about this, would it be consistent with the faults I'm seeing?

- Could it be that the boiler tries to set a too low gas flow and the valve is not working properly and there is not enough gas for combustion?

- Would the boiler then auto restart if that happened?


Too many questions perhaps, but at least they have some correlation with a common fault.

 

[gas4you]

Too lazy to read through all of this, but I would go for min BP way too low.

Get someone in for a good service to sort iot out.

 

[Clint000]

Hi gas4you,
if the min BP was too low, wouldn't I get a flame failure light (i.e. PCB trying to set up flame but flame not present)? I do not get any flame failure error.

I have replaced temp sensor & thermostat, with no change in performance. Did not have much hope in it anyway, but just wanted to be able to discard those faults.

I am suspecting the PCB to be faulty, but I am open to other ideas.

 

[namsag]

SEveral people have suggest the minimum burner pressure . This needs to be checked before you spend 90 quid on a board.

 

[bengasman]

...I have replaced temp sensor & thermostat, with no change in performance.

This is a typical result of replacing parts when people working on the boiler don't know what they are doing.

I am suspecting the PCB to be faulty, but I am open to other ideas.

Get a breakdown specialist to sort the problem out for you.
It is definitely faster, definitively safer, and quite possibly cheaper than replacing random parts until the boiler works.