How Does a Chimney Work?
A high-performance chimney will have a good draft that pulls air upward, out of your home. There is a science to the operation of chimneys. If the physics are right, you will be able to enjoy your fireplace or wood stove with no issues such as smoking or an inability to easily get a fire lit.
The Stack Effect
Your entire house has a direct effect on how your chimney operates. The air in your home is in constant motion, and different factors contribute to the direction of airflow. There are factors which change constantly and that affect the air going in or out of your home and among them are:
- Wind load, the force on your home which arises from the impact of wind on the exterior that affects interior house pressure;
- Stack effect, which is detailed further below;
- Interior mechanical devices such as bathroom fans, clothes dryers, central vacuums, kitchen fans, and attic fans; and
- Fuel-burning appliances, such as wood stoves, fireplaces, water heaters, and furnaces.
Stack effect is perhaps the most complicated factor that affects the operation of your chimney. One way to explain the stack effect is to say that the movement of air in and out of chimneys and buildings is the result of air buoyancy. Temperature and moisture differences as well as differences in indoor-to-outdoor air density create buoyancy.
Negative Air Pressure
Modern homes tend to be much more airtight than in previous times, and a problem of negative air pressure can be the result. When there is negative air pressure, there is not enough air in the home to provide the needed draft in the chimney. Sometimes the use of kitchen vents and clothes dryers contributes to negative air pressure. A short-term solution is to open a window.
A chimney can operate much more efficiently if it is built within the building envelope as opposed to being on an outside wall. The chimney draft is greatly affected by the temperate of the air inside as compared to outside of the chimney. If the column of air inside the chimney is kept somewhat warm by being in the home’s interior, there is not usually a problem. If, on the other hand, the air in the chimney is approximately as cold as the air outside of it, lighting a fire can become difficult. What happens is that the cold column of air in the chimney is pushing downward, preventing a proper draft. It becomes necessary to heat the interior of the chimney in order to get the fire started; this is usually accomplished by holding a newspaper torch up through the damper once or twice.
The height of the chimney should be carefully calculated because problems result from it being too tall or too short. It helps the chimney draft to have a taller chimney, as long as it doesn’t become so tall that the air flow changes and the benefit is lost.
If a wood-burning stove or factory-built fireplace is installed, the size of the chimney flue needs to be the size recommended by the manufacturer, in order for there to be a proper draft.
The draft in the chimney is stronger when the gases in the chimney are hotter than the air outside. This is why smoldering fires are usually smoky.
For answers to questions you may have about the operation of your chimney, do not hesitate to contact our chimney professionals.
Northeastern Chimney, Inc.
37 Cody Street, West Hartford, CT 06110