Have you tightened up the house in any way? (new
windows, doors, remodeling, new bath fan, kitchen
exhaust? It may be negative pressure is causing
downdraft from furnace flue. If so, a make-up air
intake is needed. Install a vent from outside, connect 5" insulated flex duct to vent, route it across basement ceiling, and drop it near furnace
and water heater.(if gas.) When furnace fires, it
will draw air from make-up air intake, reducing
possible negative pressure.