Starting and Shutting Off (banging) at specific temperature

Primoz

Hi,
I am trying to solve this issue for more than a year, and even the service guy has no clue to what it could be.

I am using Seltron for central heating and oil burner from Joannes. What happens is that at outside temperature of 11-12 degrees C the oil burner would start and immediately stop when it flames. It primes, starts the flame, flames out with a bang and repeats. Doing this over and over until I turn it off.

It would only happen at the aforementioned outside temperatures. Being colder or warmer it works without a problem either heating the water/house or in summer heating the boiler.

Suggested remedy was to replace the chimney with no specific explanations other than "the air temperature in the chimney is to high and there is no suction".

We cleaned the chimney, checked and serviced the oil burner and checked and set the correct parameters. All to spec.
We checked if the outside temp sensor might be faulty, but it is in working order.

I really don't know where to start evaluating, since I spent hundreds on technicians not providing any sort of solution or explanation.

I will appreciate the help. Thank you.