We're installing a gas insert (QuadraFire QFI-30). The installation manual gives two ways to install it. In one, both the intake and exhaust liners run the full length of the flue, and in the other the intake liner is only 30" long and the flue is packed with non-flammable insulation (i.e. fiberglass) so intake air is drawn down the flue. Here's the diagram:
The existing chimney flue is concrete block and was constructed (in 1972) as part of a wood-burning fireplace.
My question is whether there is a significant difference between the full-length intake liner and the short version. Is one likely to be more of a maintenance headache in the future?