Oil Boiler locking out

As title says, oil boiler locking out.
Only does it first thing in a morning or on it's second run of the day at night.
What happens is it starts the fan or motor clicks as if its going to fire then locks out.
The "only" way to get it going is to press the reset button and straight away keep turning the boiler stat on and off slowly and it will catch and fire.

Its been going on for a couple of years, I have had two oil engineers to it and the jet & control box have been replaced to no effect. The only solution by the engineers was to alter the air settings.
Whats even stranger is that it never did it all summer, when the room stat was turned all the way down but now its winter & the room stat is set at say 20 it will lock out 5 out of 10 times.

Its a standard floor mounted Merlin boiler with a Sterling burner, about 5 years old.

Any ideas ?.

Sounds like you have an air leak in the oil pipe and the oil is returning
down the pipe if you have a wall mounted boiler.
Could also be for floor standing one depending on the tank layout.
Changing the control box was a waste of time.

Probably didn't have the problem in the summer as the oil is warmer and
therefore easier to lift by the pump.
Change to a two pipe system,tiger loop or non return valve should fix it.

Sticking solenoid valve is also a possibility.

Funny you should say that, I have noticed a tiny leak on the pipe from the tank at a compression joint, its so small thought it doesn't drip, just an inch of pipe either side of the compression fitting is slightly wet. Could this cause it ?.
The arrangement is, the tank is about 20 ft away from the boiler and the tank being about 2 ft elevated.

Like I said its a traditional floor standing boiler with a Sterling elect oil burner fitted.

Maxx470 said:

Funny you should say that, I have noticed a tiny leak on the pipe from the tank at a compression joint, its so small thought it doesn't drip, just an inch of pipe either side of the compression fitting is slightly wet. Could this cause it ?.
The arrangement is, the tank is about 20 ft away from the boiler and the tank being about 2 ft elevated.

Like I said its a traditional floor standing boiler with a Sterling elect oil burner fitted.

Click to expand...

Possibly blocked filter or pipe if your tank is old and steel.
Disconnect oil line flush out with compressed air.
Check oil filter and check line with a pressure test.
Sorted I would think. Still could be sticking solenoid valve though so I would replace these as they are fairly cheap.

My twopenneth worth!
It sounds like the burner is OVER AIRED, turning the burner off and on again at the point of ignition is an old engineer's trick to get a burner to fire, this action reduces the air flow at the point of ignition I would sugest the oil pressure and CO2% are checked the Sterling/Merlin should run at around 11.8% - 12.2% CO2 What size Merlin do you have, the larger ones have a Sterling Burner with an ADJUSTABLE head, if this is not correctly set then problems similar to yours can occur

Its a 65/95 with an Electro oil sterling 50 burner.

Once it has fired for the first time for that programmed run ( i.e. morning ) it will continue to run without locking out.