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Anatomy of a Masonry Chimney

Introduction

Chimneys and fireplaces are a lot more than just an opening in the wall with a tube going up to let the smoke go out. 

It’s pretty complicated. 

The reality is that there are a lot more than meets the eye. Like your car, dishwasher, or your computer, there’s a lot of moving parts (literally and figuratively) that helps them work as they should. 

That’s why we make sure all of our technicians are CSIA certified so they can fully understand everything that goes on to make your chimney safe and efficient. 

I didn’t add every single part of the chimney here, but this will give you a good idea of what’s at work.

Chimney

Sure, the whole thing is your chimney system. But this part is considered the chimney because it’s the part that sucks the smoke and gases up and pushes them out of the system. 

It consists of the cap, crown, flashing, flue, and liners. I’ve added mortar joints in this part, but it can apply to all sections.  

Chimney Cap

The chimney cap is the protective covering over your chimney. It’s used to keep rain, snow, animals, birds, and other debris out of your chimney. It’s also used to prevent downdrafts. 

Chimney Crown

Your chimney’s crown is the mortar at the top of the chimney. It seals off the air space between the outer walls of the masonry chimney and flue liner. It slopes away from the liner to help shed water. 

Flashing

Chimney flashing is a piece of sheet metal that is used to waterproof the roof valleys, hips, or angle between your chimney and your roof. 

Flue

The flue is a vertical pipe or duct that provides a safe pathway for heat, smoke, and other combustion byproducts away from the fireplace.

Flue Lining

The lining in your chimney can be clay, ceramic, or metal. It’s intended to contain the combustion products and to direct them to the outside atmosphere. The liner protects the chimney walls from heat and corrosion. 

Your chimney liner serves three main functions: 

  1. Protects the house from heat transfer to combustibles
  2. Protects the masonry from the corrosive byproducts of combustion, like creosote
  3. Provides a correctly sized flue for optimum efficiency of appliances

Mortar Joints

Mortar joints are the spaces between the bricks on your masonry chimney. Mortar is usually cement, aggregate, and water. The biggest purpose of the mortar is to act as a bonding agent between the bricks to hold everything together.

 

Smoke Chamber

The smoke chamber is located just under the chimney and above the firebox. 

The smoke chamber is used to help funnel the hot air, smoke, and gas from the firebox into the chimney. 

This section consists of the smoke chamber itself, damper, and smoke shelf. 

Smoke Chamber

The smoke chamber is the chamber in your fireplace right above the smoke shelf that extends to the base of the flue. This is designed to allow smoke to mix and rise into the flue. 

Damper

The damper in your chimney is a valve that’s used to control the flow of air or smoke. 

Smoke Shelf

This is the area at the bottom of your smoke chamber that helps prevent downdrafts. The shelf is located at the intersection of the smoke chamber and the firebox. Typically, there’s a damper on the front side of the smoke shelf.

Firebox & Base

The firebox is the part of your fireplace where your fire burns. It sits right on the base. It consists of your mantle, lintel, and firebrick.

The base of your chimney is what everything rests on. Here, you’ll find the hearth, ash pit, cleanout dor, and the foundation of your home.

Lintel

Lintels are the horizontal piece that goes across the opening of your fireplace to support the load above it. It’s usually made of masonry or steel. 

Firebrick

Firebrick is a brick that’s composed of clay and silica. It’s designed to withstand high temperatures in your firebox.

Hearth

The hearth is the floor area within the firebox of a fireplace. It’s a base that isolates the fireplace from you and any combustible items. 

Ash Pit

Also known as an ash dump, the ash pit is an opening at the bottom of your fireplace, where ashes can be dumped.

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