Thanks to everyone for their advice on my previous inquiry. We have decided to go with either the PE Summit or Enviro Boston...if we go the insert route. Both units are approved for use in ZC factory-built fireplaces. My final hurdle is safety. The steps in the plan include: A Level II inspection of both the factory-built fireplace and chimney. Reline the factory-built chimney with a 6" insulated stainless liner through the middle of our 11" factory-built chimney - making sure that the chimney cap still allows the factory-built chimney to breathe. Install a block-off plate - It may or may not be necessary with the relining but it is an added level of safety at little added cost. Install the insert per the manual. The manual installation instructions for the Summit (and literally every other approved insert) states that "the air flow within and around the fireplace must not be altered by the installation of the Insert (i.e. no blockage of louvers or cooling air inlet or outlet ports). This includes the circulating air chambers in a steel fireplace or metal heat circulator." I have a Heat-n-Glo RHW-51...a large ZC box with dimensions capable of handling almost all approved inserts. I have attached a couple of pictures showing the air vents around the fireplace opening. From speaking with Hearth & Home (parent of Heat-n-Glo), the side vents feed outside air into the fireplace via an OAK. The bottom vents allow room air to be drawn into the cooling chamber of the fireplace. Heat-n-Glo tech service person says that "An insert will not totally block off all of these holes thus allowing the unit to still breathe"...and further that "(the) unit is a RHW 51 and is a radiant model so (we) can put an insert in without a problem". All sounds great - I actually seem to have approval from both the insert and ZC manufacturer. But I still have this nagging feeling that the surround will alter the airflow...maybe not block but perhaps restrict? Am I being paranoid? Thanks for reading.