The Parts of a Chimney and Their Purposes
A chimney is a vital part of your home’s ventilation system. Understanding the parts of a chimney and their purposes will help you spot problems and keep your chimney working efficiently.
The chimney crown is the cement part at the top of the chimney. This should be routinely checked to ensure it doesn’t leak water into your house.
The Flue
The flue is a duct within the chimney structure that vents toxic gases, smoke and heat to the outdoor air. The duct must be properly sized and shaped for optimum draft, and it should be well-insulated to prevent the corrosive byproducts of combustion from entering your home.
The chimney’s flue lining is made of a ceramic material like terra cotta or pumice. If it cracks, breaks or has missing mortar joints between the flue tiles, the system is unsafe for use because deadly carbon monoxide could leak into the house.
A metal chimney cap is placed over the top of the flue to keep out animals, debris and precipitation. It also forces water away from the chimney’s connection with the roof. Alternatively, you can use a lintel to support the chimney’s masonry crown. The crown is a concrete surface that slopes away from the chimney’s opening and protects it from water damage.
The Throat
The throat is where smoke first enters the chimney on its path to the flue. It has sloped walls that help to compress the combustion byproducts and propel them upward into the flue. It also prevents downdrafts from interfering with this process.
The chimney liner is a pipe that goes inside the flue of a fireplace to protect it from heat and toxic fumes. It is typically made from ceramic, clay, or stainless steel. Chimney liners are often replaced when they crack or break due to age, wear and tear, or damage.
The chimney cap sits atop the chimney and acts as a barrier against rain water, sleet, debris, and animals from entering the chimney. It also helps to keep hot embers from escaping the chimney and burning the roof. Chimney caps should always be in good condition to ensure that they work properly.
The Firebox
The firebox is the inner section of your chimney system where you light and burn your fireplace fires. It is made with masonry material that can stand up to intense heat, and it is designed to be compatible with your chimney flue liner to ensure safe and effective venting.
When smoke and combustion particles exit your firebox, they travel into the expanded smoke chamber, a non-combustible section that’s typically slanted so the air will flow downwards rather than back up into the room (backdraft). If the smoke chamber isn’t slanted properly, or it’s not lined with the proper flue lining, there can be serious airflow problems in your fireplace.
Also inside the firebox, there is a metal grate that’s used to hold the burning embers. This grate is positioned in the bottom of the firebox, and it should be constructed with a solid, non-combustible material like brick to prevent sparks from igniting flammable materials on your floor.
The Smoke Chamber
Located just behind the firebox, the smoke chamber is a space that begins the process of channeling smoke and combustion gases up to the flue. It often goes unnoticed because it is not a visible part of the chimney structure. But, if the smoke chamber becomes damaged, it can lead to serious safety issues and prevent your fireplace from functioning properly.
The smoke chamber is most often constructed using a masonry technique called corbelling. This method staggers the chamber’s bricks like steps as it narrows toward the flue opening. This creates jagged edges that can collect creosote, causing it to deteriorate faster. This deterioration can cause gaps, cracks and other imperfections that allow toxic gases such as carbon monoxide to escape the chimney and into your home.
Smoke chamber parging, which is a masonry technique that smoothes the surface of the chamber, can reduce or eliminate these dangerous problems. It is important to schedule your chimney system for regular inspections and maintenance in order to ensure that the smoke chamber and smoke shelf are working properly.
The Damper
Chimneys are necessary to help heat and circulate air in a house, but they can become problematic if not installed correctly or serviced on a regular basis. Having a good understanding of the terminology associated with chimneys, vents, and connectors can help you to identify potential problems or hazards more easily and allow you to better discuss them with professionals who service these systems.
A damper is a valve that stops or regulates the flow of air inside of a duct, chimney, VAV box, or other piece of air-handling equipment. This allows you to control how quickly a fire burns and how much smoke is vented.
The Smoke Shelf
The smoke shelf, located above the damper, allows a mixture of smoke and fumes to rise into the flue. It also helps to deflect downdrafts that could otherwise blow ash and creosote into the firebox.
The surface of the smoke shelf must be smooth in order to facilitate drafting. If it becomes too rough due to age or improper construction techniques, drafting may become impaired. Additionally, the textured masonry can trap flammable creosote, increasing your risk of chimney fire.
A smoke shelf usually looks like either a flat or slightly concave area that curves up towards the chimney flue. Besides helping with the transition of large amounts of smoke into the small space of your chimney, the smoke shelf also catches rain, falling debris and ash. It can be very easy for these materials to collect on the smoke shelf, especially if it’s not cleaned after every chimney sweep.
