The word “Buizenstelsel” itself is 14 letters long.
However, depending on the context of the puzzle, there might be a few 6-letter words that could be related to a system of pipes:
- Riool (Dutch for sewer system)
- Leiding (Dutch for piping)
- Water (referring to a water pipe system)
If you can provide any additional clues from the puzzle, I might be able to give a more specific answer.