thanks all for this, please understand, there was never a question that it would not be blocked off, "it is" and "will be". It's currently the "insulation of the liner" which is or is not required for correct functionality. "Recommended" states the manual. It also reads "on an exterior" ... and IMO, without illustration one could interpret that to be a masonry liner chimney rather then the outside pipe. appearently it's the norm "not" to install insulated liners for 13.5 brick exterior chimneys. you guys have got me going here ... you have no idea! keen to understand of course that it's the Osburn 1800i ULC S627, tested to S628 manual we're going by. Liner must meet ULC S635 (Canada) which building permits office already inquired about and has since been approved. "A continues 6" (152mm) stainless steel liner from the top of the chimney is the optimum system and will provide the best results, as well as compensate for poor draft situations caused by large corss-sectional chimneys. This insert will not work without a positive seal in the chimney" it also read: "Chimneys constructed outside of the home, on an exterior wall, should be avoided if possible, especially in colder climates. Outside chimneys may not draw as well and may downdraft due to the difficulty in heating them up to operating temperature. ... Draft can be increased by increasing chimney height, and reducing heat loss from the chimney through an insulated liner" now without pictures, are they referring to a chimney which is masonry part of the house or one of those chrome pipes running along the outside of the home. see where I'm getting with this ? see how it goes ... thanks all for the feedback, great to hear and not so great to hear info. stay tuned ! My building permit got red penned that the instrcutions have to be provided on site and is to be installed as per manufacturs instruction tested to ULC S628. It was already a concern by buidling permits that it read ULC S627 on the front of the manual, to which Osburn forwarded a name plate copy indicating on the unit tested to ULC 628 in a masonry fireplace. on another note, one should not forget that some may have and operate a humidex system which pulls stagnant cold air out of a basement. this would need to be turned off during fire burning ofcourse! mostly a summer item but I see people having smoking problems, and wonder about the understanding of the negative and positive of air pressure within the home. anyhow guys thats a lot of overload on information, I knew I didn't know everything! ha I'm sure all will go well and there is plenty of a learning curve here it looks like.