Venting An Oil Boiler?

I live in France and the oil boiler is in the basement.
It has a 6" flexipipe coming off it and it runs up the chimney. I am trying to remove this flexipipe and use the chimney from the first floor for a woodburner.
There is a basement window to the left of the boiler and I wanted to ask if it is safe/legal to fit a new flexipe and vent it out the basement window?