Just had this problem presented to me from a student and thought I would share the story and see if anyone would like to comment on a solution, or what is causing the problem.
The homeowner has a fireplace insert, more like a wood stove. It doesn't have any type of blower. The homeowner uses it all winter long. I guess they think it is saving them fuel but it is not a sealed unit and gets all its combustion air from the living space.
Last fall the HVAC contractor took out their standard vented furnace and water heater and installed a 90% furnace and electric water heater. The furnace and water heater were in a separate tile liner in the same chimney with the fireplace.
Ever since they stopped using the fireplace this spring or at starting using it less, they are getting constant smells in the house and the fireplace chimney is drawing into the house. This didn't use to be a problem. When they turned on their A/C the spillage and smells got worse. They do sometimes use the fireplace when the A/C is running but that's personal preference.
To date the contractor added some outside make-up air to the duct system, tried to reduce the return air in the room and increase the supply air and the problem hasn't improved. It is an older 2-story house that is loose. Tightening the house is not an option at this time.
As an HVAC contractor, being told they caused the problem, what would you do? Hearthman, I know you understand this better than myself, but let's see if anyone else wants to think this out and figure out why this is a problem now and what could be done to alleviate the problem.
Don't use the fireplace, leave all the windows in that room open and tightening the house are not options. This is certainly not your normal summer A/C problem.