Lets get the basics out of the way first. I will assume the damper was fully open when you used your fireplace and that the temperature outside is at least 40 degrees or colder. The opening of the damper is obvious to most people so I will stop there. However, not all consumers understand that a chimney works just like a tornado when a warm front meets a cold front. If the temperature outside is not cold enough, getting that tornado effect to happen is not easy, which can lead to smoke “spilling” out of the fireplace.
In most cases, many homeowners like to think that something is wrong with their chimney if it spills smoke back into the home. The truth of the matter is, most of the time there is nothing wrong with the chimney. The house pressure is causing the problem. If you have any doubts about this, open up your home - all windows and doors and start a fire in your fireplace, do you get any smoke in the home? I’ll guess you do not - its only when you close the house up and turn the furnace on, do you get the problem.
|When you heat your home in wintertime, you get two different pressure zones; warm air rising and filling up the upper part of the home with warm more buoyant air, and the lower part of the home filled up with cooler, heavier air, therefore, it’s pulling downward. (See Diagram 1)
|If you were to take this same home and have an upstairs window open, this is how the house would react. (See Diagram 2)
|If you were to take this same home and open a downstairs window, this is how the home would react. (See Diagram 3)
Note: in these diagrams that the neutral pressure point (NPP) - follows where the house “leaks” (i.e. the open window). If you open a window in the upper part of the home, the whole house basically acts like a chimney with all of the warm air rising and going out the window. In this case, any fireplace in this home will have a hard time operating without spilling. On the other hand, see how the opening of a basement window will reverse and allow a back drafting chimney to operate, due to the lowering of the NPP and the air constantly filling the home with a constant supply of airflow.
The location of the chimney/fireplace in the home is very crucial to how well it is going to draft and where the chimney was constructed on the home (i.e. inside the building envelope or outside the building envelope) is going to determine how well the fireplace will function. The building envelope is the area of the home that is heated in wintertime and cooled in summertime. As a general rule, a fireplace/chimney that is built inside of the building will experience less problems then one that is built outside of the home. Some of the problems outside fireplace/chimneys experience are, hard to start up, cold air coming down chimney when not in use, more soot build up, can smell “smoky” when the A/C is running in summertime, and smoke comes down into the basement when using the upstairs fireplace. It is recommended by us that if you are building a new home, build the fireplace within the building envelope.
Finally, what can you do about a masonry fireplace that is not drafting correctly?
1. Make sure damper is fully open.
2. Make sure the fireplace opening is not too big. Many masons have constructed fireplaces incorrectly. Typical dimensions should be 10 to 1. For every 10 square inches of a fireplace opening, there should be 1 square inch of flue opening.
3. Make sure the upper part of your home is tight as a drum!
4. If you have a ranch style house and a masonry chimney. Try removing the rain cap. Sometimes caps on ranch style houses impedes the airflow. If this works, you can have a cap installed has the mesh portion only.
5. Open one or two basement windows/doors. If this works, then you can consider having an “air to air” whole house ventilation system installed by your furnace contractor. This system will act like an open window but will do it in conjunction with your furnace so the air coming in will be heated.
6. If your fireplace does not work, you could consider having an “Exhausto” fan installed. This is a U.L. Listed fan that mounts to the top of the chimney and acts like a vacuum. Please consult us before considering doing this.
7. If your fireplace does not work, you could consider installing a sealed combustion insert. These gas, pellet, and wood inserts will work around negative pressure and give you a working fireplace that can be up to 80% efficient.
8. If you fireplace does not work, do not install an outside air intake into the fireplace. Many chimney companies will recommend installing one of these but I have never seen a case where this has solved the problem. Think of it this way. Why would air want to come into the fireplace through this small 4” round hole, and not come from the home through a typical fireplace opening of 6 square feet? Don’t waste your money on it.
If you have any questions, give our office a call at 262-797-8181.