|
Fireplace Drafting: What Affects The Draft
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. A fireplace that drafts well will route most of these products up the chimney. Without getting technical we can simply explain the factors that influence the drafting capabilities of a fireplace. 1. The fireplace must have a chimney. You may laugh but we have seen many homes that have what appear to be a fireplace but no chimney! 2. The chimney must not be blocked off. There are many ways the chimney can be blocked off. The following are the most common.
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. 3. 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. 4. 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. To get a good idea of this you can perform this simple exercise.
5. Incorrect design. As part of number “4” above there is the incorrect design issue. In order to ensure the fireplace works the builder must strictly adhere to fireplace codes and standards. As well as a flue being too small it can also be too large. It can be so large that the heat generated by the fire is not enough to start and maintain a draft. The dimensions of the firebox itself are also important. A fireplace that is too shallow (less than 20-inches deep) may cause problems. Another example is the design of the flue. The builder must never angle any part of the flue more than 30 degrees off of straight up and down. This rule is commonly violated, especially in custom designed fireplaces. One example is the corner fireplace, where the fireplace is built into a corner of the room. In order to get the products of combustion into the chimney’s flue it has to travel more than the usual 16 or so horizontal inches. In some cases the distance can be as great as 4 feet from the top of the firebox to where the chimney flue is located outside of the house. This forces the construction of a long, angled channel that the products of combustion must travel through before reaching the vertical flue. Because this channel is angled at more than 30 degrees it may not allow the products of combustion to travel through it fast enough to provide an adequate draft and the fireplace may smoke. The above are just a few examples of incorrect design. Not all examples are listed here, as it would become too technical for the purpose of this article. 6. 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.
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. 7. 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. 8. 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 keep 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. Steps to help a fire draft properly 1. Have your system inspected by a certified, competent inspector. It is not worth risking your health and property for the relatively small fee it costs to have your system certified as safe. The peace of mind alone is worth the fee. 2. If this is the first time you have ever used the fireplace be sure it is not windy outside. Follow these steps on a calm day. 3. Make sure the damper is in the open position. 4. Make sure you have a screen to place in front of the fire to keep sparks and embers from flying into the interior of the room. If your fireplace has the “curtain” screen that hang from the lintel, close and open them before you start the fire. It is a safety hazard to burn a fire without placing the screen in front. 5. If there is a return air vent in the same room as the fireplace, be sure the furnace or air conditioner unit is turned off. 6. If your home is new, built after 1985 in the Southern California area, open a window slightly. 7. Push the fireplace metal grate back as far as it will go in the fireplace. 8. If you have a gas log lighter simply stack some dry wood, about three pieces, in the fireplace metal grate. Roll some newspaper into a “log” and place it beside the pipe with the holes in it. This pipe is the gas log lighter. Light the newspaper “log” on fire and be sure it is burning before you turn on the gas. Once you are sure the newspaper log is producing a nice flame slowly turn the gas shut-off valve on until you here a hissing sound. If the gas does not ignite after 5 seconds turn the gas off. Call a professional to inspect the gas line. Normally it will ignite and engulf the three logs with flame. Adjust the valve until the wood logs are engulfed with flame. Once the logs are burning and producing flames on their own turn the gas shut-off valve off. 9. If you do not have a gas log lighter follow these next steps. Place some kindling wood on top of a rolled up ball of newspaper under the metal fireplace grate. Your local fireplace shop can sell you a product called “Fat Wood” which works well for kindling. Once the kindling or “Fat Wood” is in place stack several small, preferably split pieces of dry wood in the metal grate. Before lighting the kindling take several sheets of newspaper and roll them into a “log”. Light the newspaper “Log” on fire and stick it up the throat as far as you comfortably can. Allow the newspaper to burn half way then drop it onto the wood. This will warm the flue up and start the warm air rising up which will aid the draft. Next light the kindling on fire. Once the kindling has started the small split wood on fire and they are burning nicely place some bigger pieces on top. REMEMBER TO PLACE THE SCREEN IN FRONT OF THE FIRE AFTER LIGHTING IT. NEVER LEAVE THE FIRE BURNING WITHOUT THE SCREEN IN PLACE. This should enable you to enjoy your home’s fireplace and obtain the maximum benefits from your investment. If you have any questions call Boston Brick & Stone at 626 296-7700. Our friendly receptionist will patch you through to one of our many competent field technicians who will be able to answer most if not all of your questions.
|
|
|
Serving the Los Angeles area.