We have a Mt Vernon AE insert. Didn't run an OAK initially, but decided last fall to add one (also up the chimney). While looking into it, found that the metal plate holding the vent pipe in the chimney flue opening didn't cover the entire opening. Also, where the vert went through the damper just had some fiberglass shoved around it. Bottom line - a one foot square hole at the top, an open damper with porous insulation at the bottom - might have just as well left a window wide open! Certainly explained why the floor around the insert was always cold. The OAK plate now totally caps the top. What a difference! We even get some radiant warming from the chimney into the bedroom and library on the second floor (we keep the doors to those rooms shut as we like them cool, so it's easy to notice the difference on cold nights).