Hello, seeking some help with a potential purchase of a vc defiant encore 2140. First is the current owner has it as a rear exit going into a chimney is the flue collar reversible since I need it to be a vertical chimney. The manual doesn’t show changing it and parts websites only have the one flue collar. Second the manual doesn’t show says not to use double wall pipe. Was that just at the time since it would have been never or is still a bad idea. And does using double wall if possible get rid of the need for a heat shield also per the manual?