How To Make An Outdoor Fireplace Draw Better

An outdoor fireplace provides a focal point for any patio or backyard, extending the usability of the space into cooler seasons. However, the enjoyment of an outdoor fireplace hinges on its ability to "draw" properly. This refers to the fireplace's capacity to efficiently exhaust smoke and combustion gases upwards through the chimney, preventing them from billowing back into the seating area. A poorly drawing fireplace can create an unpleasant, smoky environment, rendering the outdoor space unusable. Several factors contribute to this issue, and understanding them is crucial for optimizing the fireplace's performance. This article will outline common causes of poor draft and techniques to improve the draw of an outdoor fireplace.

The term "draw" is essentially describing the airflow within the fireplace system. Ideally, heated air from the fire rises rapidly through the chimney due to its lower density compared to the surrounding cooler air. This upward movement creates a negative pressure at the fireplace opening, pulling in fresh air to fuel the combustion. When the draft is inadequate, this cycle is disrupted, and smoke spills outwards instead of being efficiently directed upwards. Addressing the underlying reasons for poor draw is essential for a safe and enjoyable outdoor fireplace experience.

Chimney Height and Diameter

The height and diameter of the chimney are fundamental factors influencing its ability to draw properly. A chimney that is too short may not generate sufficient draft to overcome downdrafts caused by surrounding structures or trees. Similarly, a chimney that is too narrow can restrict airflow, hindering the efficient removal of smoke and combustion gases. Conversely, a chimney that is excessively wide can allow the heated air to cool too rapidly, diminishing the buoyancy that drives the draft. The appropriate dimensions depend on the size of the firebox opening and the overall design of the fireplace.

As a general guideline, the chimney should extend at least two feet above any part of a structure within ten feet. This includes the roof of the house, nearby sheds, or even tall trees. This height is necessary to prevent downdrafts, which occur when wind is deflected downwards by these obstacles. The wind can then enter the chimney and force smoke back into the firebox.

The diameter, or cross-sectional area, of the chimney flue should be proportional to the firebox opening. A proper ratio ensures that enough air is drawn in to support combustion without excessive cooling of the flue gases. There are established formulas and charts available that provide recommendations for chimney flue sizing based on the firebox opening area. Consulting with a qualified chimney professional or referring to building codes is recommended to determine the optimal dimensions for a particular fireplace design.

Modifying the chimney height or diameter can be a complex and costly undertaking. Before making any structural changes, it's important to accurately assess the existing dimensions and compare them to recommended guidelines. Furthermore, any modifications should adhere to local building codes and regulations to ensure safety and compliance.

Obstructions and Blockages

Obstructions within the chimney flue can significantly impede airflow and impair the fireplace's ability to draw. These obstructions can range from simple debris accumulation to more serious structural issues. Regular inspection and cleaning of the chimney is essential to maintain optimal performance.

Common causes of obstructions include:

Bird nests: Birds frequently build nests inside chimneys, especially during nesting season. These nests can completely block the flue, preventing any upward draft.
Leaves and debris: Leaves, branches, and other debris can accumulate in the chimney, particularly in areas with numerous trees.
Creosote buildup: Creosote is a flammable byproduct of incomplete combustion that condenses on the inner walls of the chimney. Over time, creosote can build up to a significant thickness, restricting airflow and increasing the risk of chimney fires.
Animal intrusion: Small animals, such as squirrels or raccoons, may enter the chimney and become trapped, causing a blockage.
Damaged or collapsed flue liners: A damaged or collapsed flue liner can obstruct airflow and create dangerous gaps that allow combustion gases to escape into the surrounding structure.

Regular chimney cleaning is crucial to remove creosote and other debris. It is recommended that a qualified chimney sweep inspect and clean the chimney at least once a year, or more frequently if the fireplace is used regularly. In addition to professional cleaning, homeowners can take preventative measures to minimize obstructions. Installing a chimney cap with a mesh screen can prevent birds and animals from entering the chimney and reduce the accumulation of leaves and debris. Regularly inspecting the chimney for signs of damage or deterioration is also essential. If any structural issues are detected, such as cracks or missing bricks, they should be repaired promptly to prevent further damage and ensure the safety of the fireplace.

Environmental Factors and Starting the Fire

External environmental factors can also play a significant role in influencing the draw of an outdoor fireplace. These factors include atmospheric pressure, wind direction, and temperature differences. Understanding how these conditions affect the draft can help in managing the fireplace for optimal performance. Additionally, the way the fire is initially started can influence the establishment of a strong draft.

Atmospheric pressure affects the density of the air, which in turn influences the draft. On days with high atmospheric pressure, the air is denser, and the draft tends to be stronger. Conversely, on days with low atmospheric pressure, the air is less dense, and the draft may be weaker. Wind direction can also affect the draft. If the wind is blowing directly into the chimney opening, it can create a downdraft, pushing smoke back into the firebox. The difference between the temperature of the ambient air and the temperature inside the chimney is crucial for generating draft. The greater the temperature difference, the stronger the upward movement of air. This is why it can be difficult to start a fire in a cold chimney, as there is little or no initial temperature difference to create a draft.

To overcome the influence of adverse environmental factors and start a fire effectively, several techniques can be employed. Preheating the chimney flue before starting the fire can help establish a draft. This can be achieved by holding a lit torch or rolled-up newspaper inside the chimney opening for a few minutes. This heats the air inside the flue, creating an initial upward current. When building the fire, start with small, dry kindling and gradually add larger pieces of wood as the fire establishes. This allows the fire to build heat slowly and prevents it from being smothered by too much fuel. Ensuring an adequate air supply to the fire is also essential. Adjusting the damper opening or providing additional ventilation can help to promote combustion and improve the draft. Avoiding burning wet or green wood is crucial, as it produces more smoke and creosote, which can further impede the draft. Using seasoned firewood that has been properly dried ensures cleaner burning and reduces the risk of chimney obstructions.

Addressing these environmental factors and adopting proper fire-starting techniques can significantly improve the draw of an outdoor fireplace. By understanding how these conditions influence the draft, users can adjust their approach to maximize performance and create a more enjoyable outdoor experience.