I had my fireplace professionally cleaned this summer but it still smells bad on certain days. I burn dry wood and try to keep a hot fire going. Do you have any explanations for the smoky odor and what can be done about it?

Answer

Fireplace odor typically is caused by creosote, a gummy substance that builds up in the chimney, usually near the top. Creosote will develop even with the use of high-quality firewood and good hot fires. Creosote is a fire hazard. It should not be allowed to build up more than a quarter-inch thick on the liner, so it's good that you had your chimney cleaned. Chimneys should be cleaned and inspected on a regular basis.

But your continuing odor problem after cleaning indicates that it's not a creosote problem but a pressure imbalance in the house. Pressure imbalances can cause air to flow down the chimney and into your house.

Such imbalances typically occur in well-built houses without many air leaks. When an exhaust fan in the kitchen or bathroom, or appliances such as the dryer, furnace or water heater are operating, the air they push out of the house is greater than the amount of air that can leak in through random cracks and holes. The result is that air is pulled down the chimney, which is a big, unrestricted hole in the house, to make up for the air being pushed out by the fan or appliance.

If that happens when there's a fire, you'll know it because the fire won't draft well and smoke gets into the room. But when there's no fire, your only alert is the odor some describe as an "old smoky smell." Sometimes the odor seems more pronounced if the weather is damp or foggy.

There are several remedies. Contact the Minnesota Department of Commerce Energy Information Center at 651-296-5175 or 1-800-657-3710 and ask for an energy specialist. He or she can explain them. The center also can supply you with names of house diagnosticians. It's possible that you will need to contact one to get an accurate diagnosis of what is causing the depressurization in your home, how severe it is and what should be done to fix it. The center has a list of diagnosticians and the services they supply.

The advantage of house diagnostics is that you get the details about how your house operates as a system -- how well integrated are the exhaust fans, furnace, water heater, fireplace, etc. How a house operates changes over time as new fans, furnaces, windows and siding are added. Also, in newer homes the integration of fans and appliances was rarely considered or measured. It's not uncommon for a newer home to be "out of tune" and subject to depressurization problems. A new energy code is in effect for building permit applications submitted after April 14 this year, which regulates mechanical ventilation.