Recently, a mystery popped up which confounds the chimney sweeper I consulted. After the installation of a high-efficiency condensing gas boiler, suddenly some of the smoke from the fireplace in the living room ends up in the basement below. The previous boiler's exhaust gases went up a flue in the chimney, which also has the fireplace flue in it. We believe this boiler flue is fully closed off up top and below (new boiler has its own new PVC intake and exhaust pipes). Still, the basement becomes thick with smoke when using the fireplace in the living room above it. There is adequate chimney draw as the living room remains smoke-free when using the fireplace. Thanks a lot in advance for helping me with this most vexing problem.