There are many factors that affect a chimney’s drafting ability. Depending upon a number of factors, the presence of a chimney liner can either hurt or help your fireplace’s drawing ability. It is critical that a fireplace or other fuel burning appliance draw enough air into the fire and up the chimney to support complete combustion of the fuel. If you have bad draft, you will have a cold, smoky fire, creosote buildup in your chimney, and possibly dangerous levels of carbon monoxide inside your house.
The first thing you need to ensure adequate draft is a good air supply. Your fuel requires oxygen for combustion and the oxygen has to come from somewhere. In most cases, it comes from inside the room where the fuel burning appliance is located, although some fireplaces and woodstoves are designed with air intake vents connected to the outside of the house. Most houses are sufficiently leaky to allow the fire to draw as much air as it needs through cracks around windows and doors. If you have an extremely well insulated and energy efficient house, it may not allow sufficient air to flow in to feed the fire efficiently. In this case, cracking a window near the fireplace open slightly will usually supply enough air to the fire.
If the inside diameter of your chimney liner or flue is not large enough for the area of the opening to your fireplace, you will have problems with insufficient draft. In general, the area of your fireplace opening should be no more than eight to twelve times the area inside your flue or chimney liner. If you have insufficient flue area, you can take steps to reduce the effective area of your fireplace opening, such as installing a wood burning insert or fireplace doors with adjustable vents, or you can look into the feasibility of installing a larger diameter chimney liner.
If your chimney liner size is adequate for your fireplace, then it is likely that the liner is actually improving the draft of your system. The inside of a chimney liner is smoother than the inside of an unlined brick and mortar chimney, so air flow is not restricted as much by friction. If you have a round metal liner, the round cross-section allows smoother flow of gases up the chimney than is the case with a square opening which tends to trap dead air in corners and impede flow. Also, the chimney liner will effectively insulate the flue from the outside structure of the chimney. This serves to keep the combustion gases hotter on their way out of the chimney. Hot air rises faster than warm air, so an insulated chimney will draw better than a cold, uninsulated chimney.