Air Flow Cuts Smoke

QUESTION: I recently moved into a brand-new condominium that has a factory-built fireplace. Every time I use the fireplace, smoke fills up the room rather than flowing up the chimney. Once the fire gets going, the chimney draws properly and pulls up the smoke. Since this is a pre-engineered fireplace and chimney, I don’t understand why there’s a problem. Can you help?

ANSWER: The problem is probably caused by a negative pressure condition also called depressurization, that exists in your house. This is a fairly common in new construction. New homes are better weatherstripped, caulked and are tighter than older homes to conserve energy.

Depressurization results when more air in the house exhausts through fans and vents to the outside than flows in from infiltration. The greater the exhaust capacity and the tighter the building envelope, the more a house will depressurize.

Also, because warm indoor air is lighter than colder outside air, it tends to leak into the attic through an access door or ceiling hatch and then flows to the outside through vents and open joints.

All these factors create a suction on the lower part of the building, drawing air in. Since the building is relatively tight, the air entering the house (to equalize the pressure) will enter by the path of least resistance, in this case, the fireplace flue.

When the damper is opened there is generally an on-rush of incoming air. In some cases, the rush of air is so intense it blows ashes into the room. To eliminate this, you must equalize the air pressure between indoors and out. Do this by cracking open a window or door, preferably on the side of the house on which the wind is blowing. If you open a door or window on the opposite side, more air will be drawn out of the house, creating an even greater negative pressure.