Introduction

• A fireplace damper's purpose is to control the draft of air that travels through the base of the fire and up the chimney flue. If the fire is burning too hot with the damper fully open, closing it part way can calm the fire. Shutting the damper all the way when there's no fire keeps room heat from escaping up the chimney. Dampers are usually up inside the fireplace between the throat and the smoke chamber.
  
  

The System

• The principles of how a damper works are the same regardless of the individual fireplace but fireplaces are not all created equally. Even two manufactured fireplaces built to the same specifications will be unlikely to operate identically. A fireplace's behavior is dependent upon the local and regional climate and the landscape around a given house. Nearby hills and tall trees, for example, can affect air currents over the roof. This, in turn, can have a pronounced effect on operating the fireplace damper.
  The type of wood, and especially its moisture content, also affects a damper's function. Moisture generates extra smoke and this has a direct bearing on how to operate the damper.
  
  

Weather

• A fireplace damper---even with the same kind of wood with the same moisture content---can potentially behave very differently from one day to the next. This is because the fireplace draft is influenced so much by the weather. On a very cold and calm day you might be able to close the damper part way and tone a roaring fire down if it's becoming a bit too warm in the room. Try the same thing with wood of the same batch and kind and moisture content on a less cold or windy day and you might well end up with smoke in the house.
  
  

Chimney

• Old fireplaces may not have a flue lining but simply an open internal chimney. These can be quite large spaces compared to the more confined flues of modern chimneys. And it is not uncommon for these old chimneys to have more than one fireplace sharing the same internal chimney space. This would not be permitted in constructing a new chimney. In new construction each fireplace must have its own dedicated flue. All of these factors affect how a damper will work.
  
  

Damper types

• Some modern dampers that attach to the top of the chimney outside. They are arranged as a hatch that you operate with a cable from inside the fireplace. One potential problem with this type of damper is that even in the fully open position there is still a flap of metal sticking up to one side of the flue opening. This could cause an eddy of air current where the smoke is escaping from the flue and interfere with the draft, possibly causing a back draft and smoke inside the house.
  
  

Bottom line

• Whichever kind of damper and fireplace setup you have, it really takes some experimentation and trial and error (meaning, possibly, some smoke in the house) to get the feel for how your damper is going to work under different conditions. Start with it all the way open and try closing it little by little and see how the fire behaves.