Boiler gets clogged again with soot a few days after cleaning | Solved !

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BEFORE YOU START
Access to a boiler's main chimney can be more or less difficult depending on how it was designed and the number of cleaning traps provided.
This is a job that involves cleaning light powder-like black soot. Make sure to wear old clothes, gloves, a protective respiratory mask and to use a powerful industrial vacuum cleaner to get the rid of it safely.
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Black smoke then black soot blocking a boiler despite sweeping

If you experienced a powder-like, very light and easily removable soot blocking your boiler's main chamber a few hours or days after sweeping the boiler, then this could indicate a clogged fume conduit (chimney).
Even a partially clogged conduit could lead to such a common phenomenon as it restricts the flow of flue gas (draft) generating enough back-pressure to make the burner inject the same amount of fuel for much less air which generates black smoke first followed by a complete clogging of the boiler fins. A black coat of soot will cover the whole flame chamber and the burner will stop working.
Fortunately, this black soot is not sticky and does not require a lot of efforts to be removed.
Proceed as follows:
  1. Remove as much soot as you can from the boiler's flame chamber with a powerful vacuum cleaner.
  2. You can also use a boiler sweeping brush in places where the vacuum hose cannot go
  3. Open the access traps to the main chimney, knowing that clogs always occur on the horizontal or inclined runs.
  4. Again, use your vacuum cleaner to remove the obstruction. Extend your vacuum hose as needed in order to reach far in the chimney.
  5. Re-adjust your burner.
  6. The boiler will clean itself once you start it since such soft and light soot is quickly eliminated by a good flame and a good draft
A partial obstruction of the boiler's chimney is enough to restrict the air intake of the oil burner which will generate a quick forming and light powder or granule like soot.
Cleaning the boiler and adjusting the burner to a powerful flame without removing the obstruction in the chimney causes this fast formation of light and non-sticky soot in the boiler and on the the burner.

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