How Do Chimneys Work?

When oil, wood, coal, gas, or any other fuel is combusted, it creates hot combustion byproduct gases called flue gases. These are exhausted into the ambient air through chimneys or flue-gas stacks.Chimneys

Chimneys are a vertical structure that provides ventilation for heating systems and fireplaces. Chimneys have flue sections that can be made of masonry, concrete, or stainless steel.

When your fireplace is in use, it vents the smoke and gases from the fire through a narrow opening called a flue. The flue opens when you light a fire and closes as the flames and hot gases rise through it.

The flue’s primary job is to carry smoke and dangerous vapors away from your home, but it also performs a couple of other tasks. When it’s closed, the flue keeps air warmed by your furnace or cooled by your AC from being drawn into the chimney, which reduces stress on those systems and saves energy. It also prevents animals and debris from entering the chimney.

A flue may be made of steel, clay tile, or masonry. It’s designed to withstand the high temperatures of a wood-burning fire and to provide a smooth nest surface for the smoke and gases as they move up through it. It is typically slanted slightly to the windward side, which helps prevent gusts of air from pushing back down the chimney at inconvenient times (a condition known as a “downdraft”).

A flue is usually lined with fireproof material, which helps protect the chimney and impede the buildup of creosote, a black tar-like byproduct of burning fossil fuels that can cause chimney fires if it gets too hot. The lining must be installed properly by a certified chimney sweep to ensure that it won’t leak or fail.

The Firebox

The firebox is where you build your fire. It’s also where you start your fires, and it plays a major role in the performance of your fireplace. It helps insulate the living area of your home from the heat, flames, and smoke produced by your fires. It also plays a critical role in establishing a draft, which pulls combustion gases up into the chimney and away from the fires.

The walls of the firebox are made of masonry, usually bricks or sometimes cement stone, and they are built to withstand high temperatures. They are held together by mortar that has been reinforced with high-strength fibers, known as refractory mortar. If the firebox’s walls crack, crumble, or disintegrate, it can cause a chimney leak.

Stainless steel chimney liners are commonly used in masonry chimneys to improve their performance by adding strength, rigidity, and durability. They are inserted into the chimney flue to provide a barrier between the chimney walls and the flue gases, which can reach temperatures of 1800 degrees Fahrenheit or more.

If your chimney’s flue liner is cracked or leaking, you’ll need to replace it. You can do this in a masonry chimney, or you can replace it with a prefabricated metal chimney. If you choose to do a masonry chimney, it is important that you hire a qualified, experienced professional to install the new liner. This will ensure that the refractory mortar is installed correctly and meets codes and standards.

The Damper

A damper is a metal plate that regulates airflow inside a chimney, flue, vent, or duct. Dampers are usually operated with a simple handle that latches or swings open or closed depending on how it’s turned. A damper is typically placed directly above the fireplace firebox.

It prevents cold air from escaping up the chimney when the fire isn’t burning. When this happens, it can cause your energy bills to rise as your heating and cooling systems work harder to maintain an indoor temperature. Dampers also help protect the chimney from birds, rain, animals, and other debris that could enter the chimney system.

While not all fireplaces have a damper, the majority do. It is important that you know how to operate your chimney damper correctly to keep your home warm and your chimney drafting properly.

You can check the position of your chimney damper by looking at it through the firebox or by placing your hand in the firebox to feel a draft or breeze. When the damper is closed, it will have a chain that hangs down from the fireplace wall surround and can be hooked into a notch on the chimney cap. The chain should be hanging loose when the damper is open. It’s essential to note that you shouldn’t use the fireplace when the damper is closed. If you do, dangerous smoke and gases can escape the fireplace into your home.

The Smoke Chamber

Located directly above the firebox, your chimney’s smoke chamber is like an inverted funnel that funnels smoke and gases into your flue. The smoke chamber also catches falling debris and the heat from your fire. Because of their location, smoke chambers need to be cleaned any time the chimney is swept; if left unattended, materials loosened during sweeping can fall back into your firebox and become highly flammable.

Because they’re exposed to the highest temperatures in your chimney system, smoke chambers are prone to damage, especially if they aren’t constructed from firebrick. Most older smoke chambers were built using a brick technique known as “corbelling,” which creates the funnel shape but leaves gaps in the walls. According to the CSIA, gapped smoke chambers are problematic because they can impede draft and allow heat to transfer between your fireplace and your home structure.

Chimney professionals recommend parging your smoke chamber to solve problems caused by uneven walls and gaps in the bricks. Parging involves rebuilding the walls of your smoke chamber with masonry material that can withstand high temperatures, called refractory mortar. This material has the strength to withstand the intense heat of your fireplace and also insulates your smoke chamber. Parging your smoke chamber eliminates nooks and crannies that collect acidic creosote and soot, improves draft, and helps prevent heat transfer from your fireplace to adjacent living spaces in your home.

The Smoke Shelf

Chimneys may look simple enough—a tall vertical passageway that allows smoke and gases to leave the firebox and ascend into the chimney. However, it takes several components to work together correctly for your fireplace and chimney system to function properly and safely.

The throat damper (or chimney damper) is a metal plate that can be opened or closed to regulate how quickly warm air, waste gases, and smoke exit the fireplace during a fire and to reduce heat loss up the chimney while it’s not in use. This chimney component also prevents cold air, rainwater, and insects from getting into your home through the chimney when it’s not in use.

Behind the throat damper, you’ll find a smoke chamber. This area looks like a flat or slightly concave surface that runs the full length of the throat, allowing for a more efficient transition of smoke from the firebox to the flue. A masonry fireplace also typically includes a smoke shelf built into it, which looks like a curved, shallow slope that rises towards the flue.

Since this area is right above the firebox, it’s an easy spot for ash and creosote to build up. This area must be swept to remove the accumulated debris. During a sweeping, a specialized high-micron vacuum is used to control the spread of creosote and other debris throughout the entire chimney system.

The Cap

The cap is essentially a lid that prevents any air from rising, so thunderstorms have a hard time forming. Think of a pot of boiling water with the lid on it—it’s difficult to get any steam out, right? That’s what the cap does, and it can be a challenge for meteorologists to forecast severe weather when the cap is in place.

If you hear a WFAA meteorologist talk about the cap, it’s important to understand what they’re talking about. It’s often one of the most difficult parts of the forecast because a 1- to 3-degree difference in temperature can be all that it takes for storms to form. The cap is all about a battle between the rising air and the cooler, denser air. When the air wins, storms form.