The Chimney Sweep Online Fireplace, Woodstove, Gas Stove and Barbecue Shop
Q: We just bought a newer, tightly built house, and are grappling with a rather strange problem with our fireplace. We might expect to smell a little smoke when we have a fire going, but we don't. We notice a strong smoke smell that comes from the fireplace when we're NOT using it. We had the chimney cleaned and it didn't help (maybe our Sweep didn't do a good job?). Do you have you any idea why our fireplace smells so smokey, and what can we do about it? -Lionel
Wood-burning fireplace chimneys smell smoky whether they've just been swept or not, because no matter how thoroughly your Sweep brushes the flue, he can't possibly remove every trace of soot and soaked-in creosote. Even if he were able to sand-blast every microscopic remnant of wood smoke deposits out of the flue, the very first wood fire would deposit a fresh layer, and the pungeant, smokey odor would return. So the real question isn't why your fireplace smells smokey: the question is, why is the odor entering your house?
First, let's consider the fact that even the most tightly constructed homes have many, many openings (or air pathways) to the outside. There are intended air pathways, such as ventilation intakes, kitchen & bathroom exhaust fans, clothes dryers and combustion appliance flues, to name a few. There are also numerous unintended pathways, such as electrical, plumbing, cable and ductwork penetrations, leaky windows and doors, unsealed building cavities, ventilated flashings around vents and chimneys, etc.
Companies that perform air infiltration testing express the total of all these air pathways as if they were put together into one big hole. It is not uncommon for a supposedly "tight" house to have an air pathway total of over 400 square inches, the equivalent of an open window measuring 20 inches square! At any given moment, air is transferring out of the house through some of the pathways that make up this 'window', and replacement air is entering through whichever of the others offer the least resistance.
The biggest air pathway to the outside in most houses is the fireplace chimney. A fireplace chimney can allow airflow in both directions. When in use, a fireplace chimney is a powerful evacuating force: the chimney updraft created by an open fireplace fire can move hundreds of cubic feet of air per minute out of the house, in many cases more air than the other pathways combined can supply! This is why you don't smell the smokey odor when a fire is burning in the fireplace: it is only when the fire dies down, and the updraft diminishes to the point where evacuation from other sources overcomes it, that the airflow in the fireplace flue reverses and the odor returns.
So what other forces are evacuating air from the house, causing makeup air to be pulled in through the fireplace chimney? At any given moment, a combination of evacuating forces might be at work. Some are mechanical, as is the case with unbalanced central heating and air conditioning systems, exhaust fans and clothes dryers. Some are from natural causes, as when the wind blowing against the house creates positive pressure on the windward side and negative pressure on the leeward side. Some are thermal, like the rising exhaust gases in woodstove, furnace or water heater flues.
Another example of thermal evacuation is the so-called "stack effect". Heated air has lower density than cold air, so the warm, buoyant air in your house wants to rise through the roof, while the cold, heavy air in your unused fireplace chimney wants to flow downward into the house. If there are pathways in the upper stories or roof to allow the rising room air to escape, the warm air will flow up and out of the house and replacement air will flow down the chimney and in through the fireplace. The stack effect is more pronounced in taller, leakier houses and in houses with cold chimneys (like chimneys on outside walls, exposed to outdoor temperatures for their entire length). Rainy weather also accelerates the stack effect, because the wet air entering the chimney is heavier than the dry air in the house.
Whatever the cause, whenever air travels to the outside of the house, an equivalent amount of air attempts to enter somewhere to replace it. If the chimney offers the path of least resistance for the makeup airflow, the smokey smell of wood creosote will enter the house along with the replacement air.
What can you do to stop your house from using the fireplace chimney for makeup air? All you need to do is create enough resistance to the flow of air down the chimney so that the other air pathways will provide less resistance to nature's tendency to equalize air pressure inside and outside the house. Here's some ideas:
1) Close the fireplace damper when not in use. This will sometimes do the trick, although a damper alone may not provide sufficient flow resistance, as most fireplace dampers are pretty leaky.
2) Add a good, tight-fitting glass firescreen. This will quite often solve the problem, and will also inhibit the flow of heated air OUT of the chimney when there's a fire going.
3) Consider a top-sealing damper. These mount at the top of the chimney, and are opened and closed via a stainless steel cable running down the inside of the flue.
4) Provide a source of outside combustion air to your other combustion appliances. If your gas furnace, oil furnace, woodstove and water heater aren't siphoning air out of the house, the demand for replacement air will be reduced.
Q: Your assessment of creasote/smoke smell in the house is dead-on. We just experienced this in a newly purchased house. The source in our case,
I believe, is the radon vaccum which new builds and some mortgagers demand. The couple of pounds of negative pressure in the sub-basement
would do it. A glass screen may be our only hope if this is true. Can you confirm and pass onto readers?