Boiler switches on for 10-12 secs then goes off again

[ditmco]

Hi, I have a potterton boiler that is cycling on/off every about 10 seconds. This happens with either CH or HW selected. Pilot comes on, the main burner fires up for 10 seconds and then this cycle continues.
I have checked to make sure radiators are all full and they are. The Pump seems to be working as when I switch between speeds there is a difference in noise. I have removed the top off the diverter valve and manually turned the valve to 'both' position. The problem still exists. Any help would be much appreciated.
Thanks

 

[geoff@cetltd]

a blockage preventing hot water leaving the heat exchanger ?
Of course, if you said what model it was, you have more of a chance of a sensible reply

 

[ditmco]

Sorry, model is a potterton primaf - 60f

 

[ChrisR]

Air Pressure Switch?
Unless it's a profile/netaheat profile/ prima type thing and it needs a new PCB? 10 secs is a bit slow for that though.

---
Ah aprima - that wasn't there when I posted!

 

[geoff@cetltd]

well, the prima pcb does have a failure mode where it cycles the fan on and off, but I don't think it would happen once the boiler has lit. it sounds like, as I said there is a blockage in the HE

 

[geoff@cetltd]

if the boiler has already lit, I wouild have thought that the APS was working. An APS problem especially cycling wouldn't let the sequence get that far

 

[ChrisR]

Hmm you do have a point . 10 seconds is perhaps quick for an overheat unless the boiler is already hot though, and would be affected by the position of the boiler stat. Does that tie up , dit?

 

[geoff@cetltd]

well, bad joints on the pcb, broken HT connector faults, wouldn't tend to be so regular.

flame sensing cutting out is not a common pcb fault

it points to o/h cutting out

 

[ditmco]

Thank you all for your comments, I have had a look at the air pressure switch and hoses which then led me to the air duct. The air duct terminal (on outside wall) has corroded and broken and the internal duct is now loose. I suspect this may be causing a problem with the air pressure switch, it is also very windy today. Does this make sense?

 

[ditmco]

Just to answer previous comment the boiler wasn't already hot so I don't think it is a overheat problem, although who am I to pass comment, thank you again

 

[geoff@cetltd]

Just to answer previous comment the boiler wasn't already hot so I don't think it is a overheat problem, although who am I to pass comment, thank you again

... you're the one in the hot seat who's best placed

So, are you sure that the main burner's coming on for a period of some seconds ?

It's an important question, because there is a common fault with the pcb (407677) where it cycles like this, but it wouldn't get to the stage of lighting the main burner

 

[ditmco]

Yes, the main burner comes on for about 10 seconds, sometimes less, sometimes a little more.

 

[HarrogateGas]

Ive had this roblem before and found the pilit passing givng a permanant pilot, this sent the boiler into the symptoms you describe. check to see if you have a permanant pilot?

 

[geoff@cetltd]

it's not a perm pilot

 

[ditmco]

No I haven't got a permanent pilot - could the flue have anything to do with this problem - ie. causing an air pressure problem ?