Having balance in your home is important, especially when it comes to fireplace ventilation. There are some tests you can perform to find out whether or not there is this in balance.
How the Pressure Works
Warmer air must rise and needs a place to escape in the higher levels of the house. Normally, at a point around halfway between the floor and the roof, there is what is known as neutral pressure. The air above and below this marker will find a way out from small holes or windows. Higher air is known as 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 properly functioning fireplace during full burning and while dying down, a certain amount of make up 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 make up air to cold air in 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 just 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, and hold it inside the fireplace. You want the smoke to travel vertically up the chimney. However if the smoke enters the room this means that there is a downward draft in the chimney. Either the chimney isn't tall enough or the room is too negatively pressurized. This downward draft can be very dangerous when the fireplace is lit and therefore must be alleviated. 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 is no fire in the fireplace and the chimney is cold.
Worried About Energy Efficiency?
Some people believe that opening a window to let in cold air while their fireplace is burning, is counter productive to cost effectiveness and energy efficiency, however, a home with a ventilation system that is properly balanced between positive and negative air pressure is safer, more comfortable, and more efficient.