Several factors can affect a chimney’s drafting ability, and depending on the design and size of your system, a chimney liner can either improve or reduce your fireplace’s draft. A fireplace or other fuel-burning appliance needs sufficient airflow to support efficient combustion and carry exhaust gases safely up the chimney. Poor draft can result in a cool, smoky fire, increased creosote buildup, and potentially dangerous levels of carbon monoxide inside your home.
The first step toward achieving adequate draft is ensuring a sufficient air supply. Fuel needs oxygen to burn efficiently, and that oxygen has to come from somewhere. In most homes, the necessary air comes from the room where the fuel-burning appliance is located, although some fireplaces and woodstoves are designed with dedicated air intake vents connected to the outside. Most homes allow enough air to enter through small gaps around windows and doors to supply a fire. However, an extremely well-insulated and energy-efficient home may not allow enough air to enter naturally. In this situation, slightly opening a window near the fireplace can often provide the additional airflow needed for efficient combustion.
If the inside diameter of your chimney liner or flue is too small for the size of your fireplace opening, you may experience insufficient draft. As a general guideline, the area of the fireplace opening should be no more than eight to twelve times the area inside the flue or chimney liner. If your flue area is insufficient, you can reduce the effective size of the fireplace opening by installing a wood-burning insert or fireplace doors with adjustable vents. Another option is to determine whether installing a larger-diameter chimney liner is feasible for your system.
If your chimney liner is correctly sized for your fireplace, it will often improve the overall draft of the system. The interior surface of a chimney liner is generally smoother than the interior of an unlined brick-and-mortar chimney, reducing friction and allowing exhaust gases to flow more freely. A round metal liner also promotes smoother airflow than a square or rectangular flue, where corners can create dead-air areas and disrupt the movement of gases. In addition, a chimney liner can help insulate the flue from the surrounding chimney structure, keeping combustion gases hotter as they travel upward. Hot gases rise more effectively than cooler gases, so an insulated chimney can provide stronger draft than a cold, uninsulated chimney.