A fireplace damper is a valve or plate that stops or regulates the flow of air through the chimney. There were earlier building codes that required dampers to have a volume opening of 90% of the flue area. The problem today is that the codes require the damper to be 100% of the flue area but does not specifically address the volume as it is opened.
There are several dampers that when opened are very close to the front face of the smoke chamber. This restricts draft, decreases airflow, and increases creosote build-up. Chimney sweeps and cap damper manufacturers are realizing that these dampers are not functioning properly, and most are converting to modern energy-efficient cap dampers. These new cap dampers allow easier access for cleaning and increase draft. This decreases creosote build-up and produces a better-functioning fireplace. Currently, the two main types of dampers are the poker damper and the rotary damper. The poker damper is more modern and will require the removal of a cotter pin to clean the chimney.
The rotary damper is also another type of damper and many times these have difficulty opening completely. Again, we always recommend replacement of all dampers that do not open 100% with energy-efficient cap dampers that will fully seal off your chimney when not in use, to prevent energy loss and moisture entry, as well as fully open to maximize your draft when the chimney is in use.