After reading a number of posts here I have number of concerns. I had a XV80 furance and a XL14i AC installed this past July. The HVAC contractor said that a chimney liner would not be necessary. My hot water heater is also vented into the same chimney as the furance. They installed a Trane BATVENT800B with the furance. The chimney is exposed on three sides to the outside. Also the furance is installed in a unfinished basement. How much efficiency am I losing with this BAYVENT installed. Should I call the heating contractor and try to have a chimney liner installed at this point and have the BAYVENT disconnected. At the time I did get bids from three other contactors to replace my system and they all had a chimney liner in there bids.