One of the most aggravating things in a home is a smoking fireplace. Smoke billows into the air, the smoke detector is blaring and you may have a window or door open that is chilling the room. I think I would rather get poked in the eye with a sharp stick than have a smoking fireplace!
Fireplace Design Relationships
The process of exhausting smoke and toxic gases from a fire is a complex process. Out in the open, a fire burns in any fashion it desires. The smoke rises and reacts to wind changes. In your house things are quite different. You burn the fire in a controlled space. The smoke is supposed to go up. The maximum amount of heat is supposed to radiate back into the room. These things are possible so long as you understand fireplace design relationships.
The combustion process of a fire is complex. The flames of a fire go willy nilly. Smoke rolls around in the firebox. Add to that the battle of the hot air and smoke trying to get up the chimney while the cold outside air wants to fall down the chimney. If everything isn't just right, you will get smoke in your house.
Smoke-Free Fireplaces
Smoke-free fireplaces must conform to specific firebox height, width and depth, flue size and chimney height requirements. Make a mistake in any one of these areas and you can be in a smoke-filled room!
The first element of fireplace design lies in sizing the fireplace for a room. You wouldn't put a small three foot wide fireplace in an auditorium. It simply couldn't keep the room warm. A room with a square footage of say 150 square feet can be heated with a fireplace with a 24 to 30 inch width. A large room (15 feet x 28 feet) may require a fireplace with a 4 foot or larger opening.
The point is this. You must first determine the width of your fireplace opening. Once you have this width, all other dimensions can be computed.
Low Chimneys
About six weeks ago, I was hired by some people for a consultation. They had a new home with a number of problems. One of the problems involved a prefabricated fireplace that smoked. Prefabricated fireplaces are almost always designed properly. The height, width and depth of the firebox are correct. The smoke chamber and throat of the firebox are also in proportion. The only possibility for mistake lies in the flue size and chimney height.
In this case, the fireplace was located in a first floor family room. The family room was adjacent to the two story house. The chimney was due southeast of the main part of the house. Our prevailing winds here in Cincinnati are from the northwest during the winter months.
I determined that this fireplace smoked because of a chimney that was too low. The winter winds blow over the two story portion of the house and dive down toward the family room roof. On any given day, there was enough wind pressure to offset the hot air and smoke which was trying to get out of the chimney. Yes, hot air rises, but it only has so much energy.
In this case, the chimney needed to be extended to offset the diving winds.
Chimneys that are not influenced by tall surrounding structures actually benefit from wind. The wind actually creates a partial vacuum as it blows across the top of the chimney. This vacuum helps lift the smoke and gases from the firebox.
Sound Construction
If you are building a new fireplace, you need to pay particular attention to the building code and the types of materials you are using. Fireplaces are very dangerous. Houses burn down every year from defective fireplaces or chimneys. If you don't know what you are doing, you had better consult your building inspector or an experienced, qualified mason.
There are many excellent publications that go into great detail concerning fireplace construction. I could write thousands of words about proper fireplace and chimney construction. However, there is no need to reinvent the wheel. You or your bricklayer simply need to search the existing literature citations.
Fixing an Existing Problem
I know, I know, you're wondering what to do about your existing smoking fireplace. Well, the answer may not be simple. The first place to start is to make a drawing of your fireplace. Add dimensions to your drawing. Compare your drawing to the sizes given in the table shown in Fireplace Design & Dimensions column.
If your firebox dimensions and shape are OK, then maybe the problem lies in your flue size or chimney height. The easiest thing to fix is the height of your chimney. Although often expensive, it is the easiest. If you discover that your firebox, smoke chamber and/or flue size are incorrect, there is no simple solution. The only way to solve the problem may be to start over. Ouch!
Are you building a new home? If so, you must study the following table and read the literature sources I have cited. This small investment in time will allow you peace of mind and a smoke-free home.
By the way, if you have gas available, absolutely pipe a gas supply to that new fireplace. The day may come when you dislike messing with wood. The gas line with a gas valve in place will allow you the flexibility to install gas logs at some future time. In the meantime, the gas line can be connected to a nifty gas lighter kit! No more kindling wood!
Combustion and Makeup Air
Sometimes fireplaces smoke because of another problem. They are suffocating! What do I mean by that, you ask? A roaring fire consumes massive amounts of air. If you have a new air tight house, where does this air come from? Believe it or not, the fireplace may suck the required air straight down the chimney!
Model building codes have required combustion air sources on all residential fireplaces. However, if you forget to open it or it is partially closed, the fireplace may not get enough air. Old houses are susceptible as well. In our pursuit of energy conservation, we purchase new windows and doors, caulk cracks, etc. These were the places the fireplace was getting its air supply. Check to make sure your fireplace combustion air inlet is open. If you don't have one, think about installing a source of air to offset what is going up the chimney.
