"andre" wrote in message

The basement also smells of the oil, and specifically only when the
furnace has been running - not so much otherwise.

Regards
Andre

Sounds like someone is BSing you. The residual oil should have been burned
up long ago. I don't know what you mean by top flushing. When put into
service, a boiler is often boiled out with some chemicals to get rid of any
residue in the steam/water chambers. It is then flushed and closed up.
Steam boilers should be give a "blow down" on a regular basis also. This is
done when there is pressure in the system to flush out any accumulated
minerals in the water.

Yes, it is not healthy. If you can smell it, there are solids in the air
that you are breathing. The entire system may have to be flushed as
whatever was in there has been spread around. Good installation practice
would have helped.

When discussing this with anyone. you have a boiler, not a furnace. Boilers
heat water and make steam. Furnaces heat air.