How Does A Masonry Fireplace Work?
It's one thing to gather close to the firelight and enjoy the warm and cozy feelings that emanate from your fireplace. It's another thing to meet the challenges that burning a fire in your home presents. Keeping the flames in the firebox and ensuring that smoke and hot gases find their way up and out of the chimney are key in operating this heating application. First, let's begin with the components of a masonry fireplace. We'll start at the bottom and work our way up.
- Hearth. Constructed from fireproof materials, it extends beyond the fireplace as a stone ledge, or a tiled surface that is level with the floor.
- Facing, Fireplace Doors, & Mesh Screens. Also known as the decorative highlights of a fireplace. The facing serves to protect the wall around the firebox, in addition to being aesthetic and eye-catching. This surround can be part of or separate from a mantel. Fireplace doors are a beautiful way to further protect your home and loved ones from the flames and hot coals. When closed, they also help keep cold air from entering the room when your fireplace is not in use. Some homeowners install steel mesh screens for yet another layer of protection so that they can enjoy the heat from a low burning fire while staying shielded from sparks.
- Firebox. This is the open area that you gaze upon in your living space, where the fire is produced, tended, and enjoyed. The firebox also plays a role in collecting the smoke generated from a wood burning fire so that it can be pushed up and out of your chimney.
- Ash Dump. In some models, an ash dump is built into the floor. This unit is essentially a trap door that allows the homeowner to store ashes for future disposal.
- Damper. This movable covering separates the firebox from the chimney. Its main purpose is to prevent cold air from entering the house during the times when a fire is not burning.
- Smoke Shelf. Located just above the damper, it deflects downdrafts and prevents things like soot and rain from entering the fireplace. The shelf ascends into the smoke chamber, which is the connection between the fireplace and flue.
- Flue. The pathway for smoke and gases to exit the fireplace. Some flues are made from stainless steel or baked clay.
- Chimney. This is the surround for the flue. It keeps heat generated by the fire from reaching any building materials that may be flammable. Some chimneys have their own damper, which is cable-operated by the homeowner. When closed, the chimney damper stops downdrafts.
- Spark Arrester. Metal mesh that sits over the top of the flue. If any burning materials have found their way up through the flue with the exiting smoke and gases, the spark arrester prevents this burning debris from landing on the roof and causing a fire.
- Chimney Cap. Usually constructed from stainless steel, this covering prevents moisture and wind gusts from entering the flue. It is also a useful deterrent for birds and other animals.
Now that you know all the key areas of a masonry fireplace, let's dig into the mechanics of how it works. First, the reason that we have fireplaces in the home is for heat. Therefore, the most important function of a fireplace is its ability to generate air flow. Once a fire is roaring within the firebox, hot air is produced, and as we all know: hot air rises. But what many of us don't know is that while the fireplace is in use, a column of heated gases is produced inside the chimney. As this column of hot gases advances through the top of the chimney, it creates a form of energy that pulls heat from the fire along with it. This steady movement of hot gases and smoke that flow up and out of the chimney is called a draft (and better known as convection heating). And as the draft is making its way out, it allows fresh air (oxygen) to pass in the opposite direction (down) and further fuel the fire. Once the oxygen reaches the fire and begins to generate more heat, the fireplace in turn creates radiating warmth. Heat waves emanated by the fire disperse into the surrounding area and essentially “energize” the molecules in the room to move faster, promoting a warmer environment.
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