Boston Brick & Stone of Pasadena, CA is continuing its public information campaign in the field of chimney safety.
This winter has been especially cold in most areas of the country. More and more of us are lighting fireplaces that haven't been used in years. Some of us are dismayed to find that the fireplace does not draft well and smoke enters back into the residence.
Fireplace drafting is how well your fireplace routes the products of combustion (smoke, hot gases, soot, sparks etc.) up the chimney and into the outside atmosphere. A fireplace that does not draft at all will route all of the products of combustion into your house.
Owner of Boston Brick & Stone, Dave Lavediere, lists seven factors that influence the drafting capabilities of a fireplace.
- The fireplace must have a chimney. You may laugh but there are many homes that have what appears to be a fireplace, but no chimney!
- The chimney must not be blocked off. There are many ways the chimney can be blocked off. The following are the most common.
a) The damper is in the closed position.
b) A chimney cap that does not fit the chimney has been installed.
c) A damper that does not fit the system has been installed.
d) An animal (or insect) has built a nest or has died in the chimney.
e) Someone has intentionally blocked the chimney off to prevent a down draft.
f) The chimney has been incorrectly repaired.
- The chimney must not be partially blocked off. All of items "a" through "f" apply here. Additionally, the most common way a chimney becomes partially blocked off is by creosote (or soot) build-up. Creosote is the residue normally left on the inside walls of the chimney after burning wood. In time it can build up to a point where it becomes hazardous. The hazard is when this build up becomes thick enough to catch on fire and start what is called a chimney fire. This is what "Chimney Sweeps" do. They sweep out this black soot so that it does not partially block off the chimney and become a chimney fire hazard. Yet another way the chimney can become blocked off is by a creosote build-up on the spark arrester, which is normally part of the chimney cap. This soot can form on the metal screen and become so thick that it will close off the small openings.
- The size of the firebox must be correct to suit the size of the chimney. The part of the chimney through which the smoke travels is called the flue. The flue and the firebox opening (that part of the fireplace that you put the wood in to burn) have a relation in size. We call it the "Drafting Ratio". It's a term that simply means the size of the firebox opening in relation to the size of the pipe (or flue). This relation in size determines whether or not the system will draft. The bigger the firebox opening the bigger the flue pipe must be. The rule of thumb ratio is 1:10. This means that for every 10 square inches of firebox opening there must be 1-square inch of flue.
- There must not be anything interfering with the pressure differences in the fireplace. Again, without getting technical, a fireplace works by hot air rising and creating a negative pressure in the flue. Basically moving air has a lower pressure than air that is not moving. This is also the principle of flight! The air pressure inside the flue is lower (because it is moving quickly up the chimney flue) than the air pressure in the room (which is normally not moving much at all). That is why air flows from the room into the fireplace and on up the chimney. What can interfere with this pressure difference? Listed below are the most common causes.
a) Wind. If the weather is windy it can create "spoils" of turbulence that can "pressurize" the top of a chimney and push the smoke back down the chimney and into the room.
b) A "return air vent" in the same room as the fireplace. A return air vent is a large vent through which the furnace or air conditioner draws air. This is the vent where you will normally find your replaceable filters. Modern furnaces and air conditioners circulate the air inside the house by drawing it from one, two and sometimes more, locations throughout the house. The unit then heats or cools that air and blows it back into the house through smaller vents. If the furnace or air conditioner is operating while there is a fire in the fireplace AND the return air vent is in the same room as the fireplace this could cause a drafting problem. By "sucking" the air out of the room the air pressure is lowered. If lowered below that of the pressure inside the chimney's flue you will get some products of combustion coming into the house.
c) A tightly sealed house. If the fireplace draws air out of the room it must be replaced. In older homes the air rushes in from between the walls and windows, the doors etc. A newer home may have these areas sealed off to comply with energy conservation laws. Simply opening a window usually remedies this particular problem.
- Insufficient heat to cause an adequate draft. As mentioned above, the fireplace operates on the principle that hot air rises. As the hot air rises it creates a lowered pressure in the chimney's flue and air rushes in from the room, complying with nature's law to create a balance. This air rushing in from the room is the vehicle that carries the products of combustion up the flue and out into the outside air. If the fire you have created does not produce enough heat to cause this to happen there will be a drafting issue. Green wood, wet wood and "pressed logs" from the supermarket will sometimes cause this to occur. Heating the flue prior to the ignition of the fire helps alleviate this problem. See "Simple steps to help stop smoke from coming back into the residence" for more tips on this. Using a gas log lighter will also correct this problem. If you don't have one, a gas log lighter can usually be installed for somewhere between $500.00 and $1200.00 depending on existing conditions.
- Fire is not in correct position in firebox. A fire can be too close to the front of the firebox. The fire has to be far enough back to utilize the air rushing in from the room. If it is too close to the front, the products of combustion may escape under this flow and drift into the room. Fireboxes that are too shallow (less than 20-inches deep) are a common cause of this problem. Ensuring that the fire grate (The metal rack that keeps the logs off of the firebox floor) is pushed back as far as possible is another easy solution to this problem. An incorrectly sized grate will keep a fire too close to the front. The fire has to be off of the firebox floor in order to provide enough oxygen to accomplish a complete combustion. If no grate is used the fire may smoke badly without producing enough heat to achieve and maintain an adequate draft.