Venting Efficient Oil Boilers

mik

I'm a homeowner concerned about venting and am considering two types of efficient oil boilers, the V. Vitola VB2-33 ( AFUE = 87.2 %) and the Slant / Fin Eutectic EC - 14B (AFUE = 86 %).  I understand that both boilers condense and I'm concerned that the 6" flexible stainless steel chimney liner may not hold up.  The 100-yr-old house is a two story with an inside chimney, and the flue pipe makes two 90 degree angles before it enters the chimney.  The heat loss is about 91 000 BTU's.  Can I run these boilers safely with the existing chimney liner? Or should I insulate the flue pipe, consider an double-walled chimney liner (the existing liner is new) or look at boilers that are less likely to condense? I welcome all  suggestions.