If you have two fireplaces with flues running up the same chimney structure, (like the picture) you are often a candidate for the problem of smoke crossover. Smoke crossover is when one chimney is breathing out smoke from the fire, and the other chimney is breathing in outside air to equalize the pressure in the home. The inhaling chimney flue is also sucking back in smoke from the other nearby chimney.
Many people assume closing the metal damper on the unused fireplace will stop this smoke crossover, but that is not the case. Dampers are not tight enough to stop this air draw. The smoky fireplace needs to be air sealed to stop the smoke. That is why a chimney balloon is needed in the unused fireplace to stop the air draw inward through the second flue.
You would measure your fireplace flue for a chimney balloon and then install that chimney balloon low and tight in the fireplace that is drawing smoke. Air sealing the smoky fireplace with the balloon forces the house to find another location (i.e. windows and doors and other envelope penetrations) to draw outside air in at where there is no smoke. This keeps the fire burning in the one fireplace and keeps the smoke outside where it belongs.
This troubleshooting article was submitted with permission by Chimney Balloon USA.