Fireplace ventilation is critical for safety, so you should check every season to make sure smoke is flowing correctly.
How the Pressure Works
Warm air rises, so the air above a fire will try to escape in the higher levels of the house. Somewhere halfway between the floor and the roof, your home's air reaches a point known as neutral pressure. The air above and below this marker will find a way out from small holes or windows. Higher air is positively pressurized, and lower air is negative pressurized. Pressure is generally measured in units called Pascals. Older houses with more leaks have a lower number of Pascals than newer homes which are better sealed.
What the Fireplace Needs
For a fireplace to work right, a certain amount of air needs to be let in near the unit or on a lower level in the house.
Many times the chimney itself is too small, or the combustion air kits designed to be built into the firebox are too small. And they can lead to other problems like creosote or ash smells and enough turbulence to cause the fireplace to smoke. You can open a window, add air to the heating system, or install an air-to-air heat exchanger. Any open fireplace will need at least one cubic foot per minute of make-up air per cubic inch of flue area to keep it from smoking as the fire dies out. It could take even more air if your fireplace is in full swing.
Testing the Pressure
You can use anything that smokes, such as incense, to test the flow of smoke before you start a full fire. Hold it inside the fireplace—you want the smoke to travel vertically up the chimney. If the smoke billows into the room, there could be a blockage or a downward draft. This can be very dangerous when the fireplace is lit.
It may be that the pressure is too low downstairs, pulling the smoke away from the chimney, so try opening a window or door on the lower floor of the house. Also, turn off fans, the furnace, and close the skylight and upstairs windows. Do these things one at a time and give the air a few minutes to turn around and reverse itself. Each time you make some change, recheck the incense smoke to see which way the air is flowing. Remember, the air must go up the chimney when there's no fire in the fireplace and the chimney is cold.
Worried About Energy Efficiency?
Letting in cold air while their fireplace is burning is counterproductive to energy efficiency. In the long run, you'll want to check your house for air leaks to ensure the fireplace directs smoke safely, while retaining the heat it produces to warm your home.