boiler keeps cutting out

[t3rm3y]

we moved in about 1 year ago, i have been told the boiler is not that old, but it keeps tripping and cutting out, i have to go outside, remove the cover and press a reset button.
we have Oil fuel.
The local plumber has tried a lot of things but cannot seem to find the fault. he has replaced the pipe from the oil tank to the boiler, he has replaced the control box with the reset button (twice i believe).
looked at some copper part which sometimes blocks up and cleaned that.
checked the hoses are ok.
but it still happens, and i cannot pin point when or how, if its just form having the heating on, or if the water and heating affect it, as it happens randomly and not always when we are in, if get home and not working we just press the reset button and usually it kicks in first time (kind of tiny explosion occurs and jettisons out some soot through the exhaust), sometimes it`ll take 2 or 3 presses to start..
This is frequent occurrences - weekly for past 6 weeks at least, and sometimes it'll trip a few days in a row. It does seem to then run ok for the remainder of the day after a rest / explosion of soot (it literally is small, a few specks hit me, but it sounds worse i guess as rattles through the exhaust).
im wondering if it could be some build up of burnt fuel i side the boiler (and harder for lumber ot get to ?) or even some air in the rads and this is tripping the boiler, and also the cause of the mini explosion as it kicks back in and forces air out? - but i would also have thought the plumber would have checked these first if it could be them?

any thoughts on oil boilers and weird, difficult to pinpoint, undiagnosed issues?

 

[leakydave]

Has the oil pressure and CO2 level been set correctly with a Flue Gas Analyser? Is the oil pressure steady? Is the electrode gap the correct one and is the transformer unwarped/cracked. Are the electrodes clean? Is the photocell clean? Is the fan motor running smoothly?

 

[Agile]

In other words has it been a proper boiler engineer or just a plumber more accustomed to unblocking toilets?

 

[t3rm3y]

In other words has it been a proper boiler engineer or just a plumber more accustomed to unblocking toilets?

He is a professional oil boiler engineer, regarding leakydaves answer - i was hoping more along the lines of "yes, air is in the rads, release it, rather then that reply... looks like its back to the boiler man to investigate further..

 

[leakydave]

Air in radiators would not make the boiler lock out!

 

[oilhead]

The big clue is in your statement about a small explosion and soot is expelled.
Lighting should be smooth. An explosion indicates delayed ignition which can be caused by too much air, too little air, or badly placed electrodes, or a failing nozzle, or incorrect oil pressure ( too low so imperfect atomisation, too high so struggling for enough air to ignite.
There should be no soot expelled. Soot is caused by imperfect combustion which can be caused all of the above.
First clean out all the soot and then start the checks from the burner specification from nozzle to electrodes, oil pressure and set the air with a smoke test then an analyser. If the problem still exists, then start to suspect other components such as solenoid valves or ignition transformers.
As leaky dave says, do it once..... do it right.