A wood burning fireplace is a classic and charming fixture in many homes. The mesmerizing flicker of a fire creates a warm, inviting atmosphere. But how exactly does a wood burning fireplace work to contain the fire safely while channeling heat into a room? There is some fascinating physics and engineering behind this traditional heating method.
An Overview of Fireplace Components
A wood burning fireplace has several main parts that work together to support efficient and safe operation. These components include:
- The firebox – This is the main chamber where the fire burns. It includes the area directly in front of the fire as well.
- The damper – A movable plate that opens and closes the chimney flue. This regulates airflow.
- The throat – The opening between the firebox and the flue.
- The flue – The vertical channel that directs expelled smoke and gases up through the chimney.
- The chimney – The larger passageway that forms the upper portion of the ventilation system. It is lined with fireproof materials.
- The hearth – This is the floor in front of the fireplace, usually made of brick or stone.
- The mantel – The shelf above the fireplace.
- The surround – Decorative framing materials around the front and sides of the firebox.
How It Works Step-By-Step
Now that we know the major components, let’s look at how a fireplace operates step-by-step:
- Starting a fire – Crumpled newspaper combined with kindling is placed in the firebox, with larger firewood added on top. Starting with smaller fuels lets initial heat build slowly before larger logs are ignited. The damper is opened to allow air flow.
- Combustion occurs – As the fire ignites, the wood fuel undergoes combustion. This is an exothermic chemical reaction between the carbon and hydrogen in the wood combining with oxygen from the air. Energy is released in the form of light and heat.
- Heat radiates into the room – Fire releases radiant heat directly into the surrounding space. The fire box absorbs heat and then radiates it back into the room over time, continuing to warm the space even after the fire dies down.
- Gases and smoke rise – As combustion occurs, leftover gases and smoke rise up from the fire. Convection pulls them towards the throat, and then into the flue.
- The flue directs smoke outside – As smoke enters the flue, the narrow shape causes it to speed up thanks to the same Bernoulli’s principle that allows airplane wings to create lift. The fast-moving heated gases rise vertically, creating suction that pulls more air into the firebox, feeding oxygen to the fire.
- The damper regulates airflow – If the fireplace damper is open, smoke can exit seamlessly from the flue out through the chimney. Closing the damper slows airflow, limiting oxygen to tamp a fire down.
Factors for Optimal Efficiency and Safety
There are a few key factors that contribute to a well-functioning, efficient, and safe wood burning fireplace:
- Proper chimney height – The chimney should extend at least 3 feet above the highest point of the roof and clear any surrounding obstructions. This uses stack effect for the best draft.
- Sufficient chimney diameter – The flue should have at least 1/12th the cross-sectional area as the fireplace opening. Bigger is better to allow ample airflow.
- Smooth construction – Any rough edges or obstructions can impede draft efficiency. Chimneys lined with smooth metal materials work best.
- Dry, seasoned firewood – Drier wood burns efficiently, with less smoke. Softwood kindling helps start fires.
- Regular chimney cleaning – Creosote buildup can clog the flue over time. Annual professional cleaning clears buildup.
- Proper use – Let the fire get fully established and hot before closing the damper. Use a fireplace screen or glass doors to contain sparks.
The Science Behind Wood Burning
There is some interesting physics and chemistry that explains why wood burning fireplaces work so effectively:
Fireplaces take advantage of basic thermodynamic concepts. First, there is the endothermic chemical reaction of combustion, which releases heat into the surrounding environment. Second, convection currents carry the hot gases and smoke upwards, where less dense cold air replaces it. The firebox also radiates secondary heat into the room over time.
The venturi effect explains how the fireplace pulls in air for combustion. As hot gases rise quickly in the narrow flue, their velocity decreases the pressure. Higher pressure air from the room is pulled into the firebox to equalize, feeding the fire.
Several heat transfer processes are at work. Radiation directly warms nearby surfaces. Conduction transfers heat through the fireplace materials into bordering walls. Convection circulates air currents, spreading warmth.
The exothermic chemical reaction of wood combusting combines carbon and hydrogen compounds with oxygen to produce heat along with CO2, water vapor, and other byproducts. The controlled combustion converts chemical energy into thermal energy.
Fireplace Maintenance for Safety
Routine maintenance helps ensure your fireplace operates safely and efficiently over time:
- Annually inspect the chimney flue and firebox for any damage or cracks. Repair any issues immediately to prevent dangerous combustion gas leaks or chimney fires.
- Clear any accumulated creosote buildup from the flue. Creosote is a natural byproduct of wood fireplaces, and it can clog the flue over time. Have a chimney sweep professionally clean the flue each year.
- Check that the damper can open and close properly. Replace the damper if it does not move smoothly.
- Inspect the seal on the firebox and replace any gaskets or materials that have deteriorated. This prevents combustion gases escaping into the room.
- Use a fireplace grate or andirons to properly contain and support firewood. Burning wood directly on the firebox floor can damage the materials over time.
- Inspect the hearth in front of the firebox. Repair any cracked or damaged bricks or stones to prevent ember escape.
- Keep flammable materials like rugs away from the surrounding fireplace area. Use safe fireplace tools for handling wood and managing the fire.
Frequently Asked Questions
What materials are best for building a fireplace?
Fireplaces are constructed from non-combustible materials like brick, stone, special high-heat concrete blends, and steel. The chimney flue is usually lined with terra cotta, stainless steel, or aluminium.
What factors affect the draft in a fireplace?
The draft is the suction that pulls air into the firebox and up through the flue. It depends on the length and narrowness of the chimney, outdoor weather conditions, obstructions, and adequate supply of combustion air.
Why does smoke sometimes enter the room when starting a fire?
When a fire is just getting established and hasn’t heated up much, a weak draft results in some initial smoke spillage. Leaving the damper open fully for the first 20 minutes allows adequate air flow for the fire to heat up properly to establish the draft effect.
Is it better to close the damper when the fireplace is not in use?
Yes, keeping the damper closed when the fireplace isn’t being used prevents conditioned indoor air from escaping up the chimney. It also keeps out cold air, pests, and combustion odors.
How often should you clean the ashes out of a fireplace?
It’s best to remove cooled ashes after each fire, and at least every few fires. Accumulated ashes can impede proper air flow into the firebox. Dispose ashes in a metal container.
What are some troubleshooting tips for a sluggish fireplace draft?
Check for obstructions in the flue, buildup in the chimney, leaks around the damper, insufficient height, and room competiton for air supply. Also start with dry, finely split wood and open the damper fully when lighting.
Enjoy the Warmth and Charm of a Fire
A wood burning fireplace provides delightful ambiance and reliable warmth using scientific principles that have served us well for centuries. Following basic safety and maintenance guidelines keeps it functioning effectively and avoiding potential issues. Knowing how fireplaces harness physics and chemistry highlights the ingenuity underlying this comforting heat source.